Effects of Nonionic Surfactants on the Interactions of

Effects of Nonionic Surfactantson the Interactions of DifferentNanoparticle Materials on Glass

Surfaces

vorgelegt von

Diplom-Chemikerin

Alejandra I. Lopez Trosell

aus Venezuela

der Fakultat II - Mathematik und Naturwissenschaften

der Technischen Universitat Berlin

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften

-Dr.rer.nat.-

genehmigte Dissertation

Promotionsausschluss:

Vorsitzender: Prof. Dr. M. Lerch

Berichter: Prof. Dr. R. Schomacker

Berichter: Prof. Dr. G. Findenegg

Tag der mundlichen Prufung: 30.03.2005

Berlin-Marz, 2005

D 83

Abstract

Adhesion or detachment of fine particles from a surface is of interest in several industrial appli-

cations such as detergency, particle filtration and oil production. Recently, adhesion on semi-

conductor surfaces and in biological systems such as cells or viruses has been center of many

researches. However, efficient particle detachment is extremely difficult because of the strong ad-

hesion forces and the nature of the particulate. Particle removal methods are generally classified

into three categories, wet chemical methods, mechanical techniques and dry processes. Unfortu-

nately, all these methods do not work effectively when the particles have a size of nanometers.

Therefore, the advantages of the first two techniques were combined to improve the removal

by applying an external force “Hydrodynamic force” and by employing a solution of “Nonionic

surfactant” to modify the interactions between the solids “particle-substrate”.

The aim of this research was to study the effects of nonionic surfactants on the interactions

between nanoparticles and glass surfaces. To reach this objective, nanoparticles of different

materials were synthesized in w/o microemulsions. This procedure has been employed in several

experiments to obtain fine particles with a good control of their sizes in nanometer ranges. A

method to coat glass surfaces with the particles was developed. The method is based on coating

the surfaces with the nanoparticles inside the micelles. Afterwards, the micellar structure is

destroyed by heat treatment. The developed method allowed to obtain coated samples with

homogenous monolayer of nanoparticles. The use of the reverse micelles for this purpose resulted

a suitable medium for controlled deposition.

In order to test the detachment of the fine particles from the surfaces a device was designed,

which was based on the application of a hydrodynamic force. The motion of a fluid inside a

cylinder causes this force. The variation in the velocity of the fluid and changes in the properties

of the surfactant solutions acts to debilitate the adhesion force between the particles and the

substrate. Finally, measurement of U.V. absorption and calculating of the attenuation coefficient

allowed the quantification of the particle detachment.

The results were discussed in terms of the physicochemical interactions that keep the parti-

cles attached on the surfaces (DLVO-Theory), the hydrodynamic force acting on the attached

particles at the glass surfaces and the influence of the surfactant on the interactions particle-

glass.

Zusammenfassung

Die Haftung feiner Partikel an Oberflachen und deren Abtrennung ist fur einige Anwendun-

gen in der Industrie von wesentlicher Bedeutung. Beispiele hierfur sind Reinigungsprozesse,

die Partikelfiltrationen und die Erdolgewinnung. Seit kurzem ist Adhasion auf Halbleiter-

oberflachen und in den biologischen Systemen wie Zellen oder Viren Mittelpunkt von vielen

Forschungsprojekten gewesen. Die dabei verwendeten Methoden zur Partikelentfernung lassen

sich in drei Gruppen einteilen: nasse chemische Methoden, mechanische Techniken und trockene

Prozesse. Bei den Untersuchungen stellte sich jedoch heraus, dass diese Methoden bei Nanopar-

tikeln nicht effektiv arbeiten, weil starke Adhasionskrafte auf die Partikel wirken und auch die

Partikelbeschaffenheit der Trennung entgegenwirkt. In dieser Arbeit sollten daher die Vorteile

der ersten beiden Techniken kombiniert werden, indem man einerseits eine hydrodynamische

Kraft auf die Partikel ausubt und anderseits nichtionische Tenside verwendet, um die Partikel

leichter von der Oberflache zu losen.

Ein weiteres Ziel war es, die Effekte der nichtionischen Tenside auf die Wechselwirkungen

zwischen Nanopartikeln und Glasoberflachen zu studieren. Hierzu wurden zunachst Nanopar-

tikel unterschiedlicher Materialien in w/o-Mikroemulsionen synthetisiert. Dieses Verfahren hat

die Herstellung von Nanopartikeln mit geringer Großenstreuung ermoglicht. Zudem wurde eine

Methode zur Beschichtung von Glasoberflachen mit Nanopartikeln entwickelt. Bei dieser Meth-

ode wurden die Nanopartikel in reversen Micellen hergestellt und auf die Oberflachen aufgetra-

gen. Anschließend wurde die mizellare Struktur durch Warmebehandlung zerstort. Es bildete

sich eine monomolekulare Schicht aus Partikeln aus. Bei Versuchen stellte sich heraus, dass das

Verfahren gut zur Abscheidung von Partikeln auf Oberflachen geeignet ist.

Um die Abtrennung der Partikel von den Oberflachen zu untersuchen, wurde außerdem eine

Vorrichtung entwickelt, mit der durch Flussigkeitsbewegung eine hydrodynamische Kraft auf die

Partikel ausgeubt werden kann. Zudem lasst sich die Tensidkonzentration variieren. Das Aus-

maßder Partikelabtrennung wurde mit Hilfe von UV-Absorption und der Berechnung des Extink-

tionskoeffizienten bestimmt. Es stellte sich heraus, dass bei Zunahme von Fließgeschwindigkeit

und Tensidkonzentration eine verstarkte Ablosung der Partikel stattfindet.

Diese Ergebnisse der Untersuchungen wurden diskutiert, indem die theoretischen Grundlagen

fur Wechselwirkungen von Partikeln und Oberflachen (DLVO-Theorie) und der hydrodynamis-

chen Kraft herangezogen wurden. Zudem wurde der Einfluss der Tenside auf die Wechselwirkung

erlautert.

iii

Acknowledgments

I would like to thank to all those who gave me the possibility to complete this thesis.

First of all I would like to express my gratitude to Prof. Dr. R. Schomacker. He has always

been extremely generous with his time and knowledge, and allowed me great freedom in this

research. It was a great pleasure for me to conduct this thesis under his supervision. I also

acknowledge Prof. Dr. G. Findenegg, who as my second supervisor provided helpful discussions

on the preliminary version of this thesis, as well as for the “Gutachtent” to extend my scolarship

each year. I wish to express my sincere thanks to Prof. Dr. M. Lerch for helping me with

the discussions concerning perovskites, and for accepting to be the “Vortsitzender” during the

examination.

I would like to give thanks to the staff of technicians at the Technical Institut of Chemistry

in TU Berlin, specially, Mr. Grimm and Mr. Knuth, without whose help this work would not

have been possible.

I am in debt to Ing. Ulrich Gernert from ZELMI (Zentraleinrichtung Elektronenmikroskopie).

Thank you kindly for your time and help with the SEM.

Specials thanks to Dr. Gerald Bode and Dr. Herry Purnama, for the great time in our

common office.

I am grateful to all of my colleagues who have collaborated with me at the TC 8. Some

of them became in valuable and close friends during this thesis Ing. Negi Devender, Dr. Ada

Chirinos, Dr. Lourdes Rodriguez and -Ing. Farhad Hafezi I appreciate for your support at all

time-.

Not only is the academic support important to conclude a thesis but also the emotional

support specially when you are so far from home. I am deeply grateful to my Venezuelan team

Ing. Camilo Cardenas, MSc. Marx Caballero, Ing. Irisay Carmona and Ing. Alejandro Arrieta

and to my german team Kirten Frank, Nicolais Wefter and George Mayer for making me feel at

home.

There are no words to thank my family for their love and support despite their the distance

from me. Thank you for your prayers that always accompany me.

I would like to thanks Ing. Vicente Mujica for being my mate of dreams, hopefulness and

striving. Thanks also for your help and unconditional support.

Finally but certainly not least important, I am infinitely grateful to my God for being my

fortitude and refuge.

Contents

1 Motivation 1

2 Surfactants 3

2.1 Physical State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Critical Micelle Concentration (CMC) . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2.1 The Hydrophobic Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.2 The Hydrophilic Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.3 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.4 Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4.1 Discrete Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4.2 Bicontinuous Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Adsorption from Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.1 Adsorption at the Liquid-Solid Interface . . . . . . . . . . . . . . . . . . . 11

2.5.2 Langmuir Adsorption Isotherm . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5.3 Frumkin Adsorption Isotherm . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.6 State of Adsorbed Surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.7 Hydrophile Lipophile Balance (HLB) . . . . . . . . . . . . . . . . . . . . . . . . . 16

3 Nanoparticles Synthesis 17

3.1 Overview of the Principal Nanoparticle Synthesis Methods . . . . . . . . . . . . 18

3.2 Synthesis of Nanoparticles in Reverse Micelles . . . . . . . . . . . . . . . . . . . 20

3.3 Model of Precipitation in Homogeneous Phase . . . . . . . . . . . . . . . . . . . 21

3.3.1 Nucleation Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

iv

CONTENTS v

3.3.2 Growth Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4 Reverse Micelles Synthesis Model . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4.1 Precipitation Model of Towey . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4.2 Precipitation Model of Hirai . . . . . . . . . . . . . . . . . . . . . . . . . 27

4 DLVO-Theory 29

4.1 London-van der Waals Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2 Structural Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3 Born Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.4 Electrostatic Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 Detachment Mechanism 38

5.1 Removal Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6 Experimental Part 44

6.1 Chemical Materials Employed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.2 The Reactor for the Synthesis of Nanoparticles . . . . . . . . . . . . . . . . . . . 46

6.3 Synthesis of Perovskite Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . 47

6.4 Synthesis of Palladium Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.5 Synthesis of YIG Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.6 Synthesis of Zirconia Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . 50

6.7 Characterization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.7.1 BET Adsorption Characterization . . . . . . . . . . . . . . . . . . . . . . 51

6.7.2 SEM Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6.7.3 X-Ray Diffraction Characterization . . . . . . . . . . . . . . . . . . . . . 53

6.8 Surface Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.9 Coating of the Surface with the Nanoparticles . . . . . . . . . . . . . . . . . . . 56

6.10 Design of the Detachment Device . . . . . . . . . . . . . . . . . . . . . . . . . . 57

6.11 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7 Results and Discussion 62

7.1 Characterization of the Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.1.1 Perovskite Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.1.2 Palladium Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

CONTENTS vi

7.1.3 Zirconia Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.1.4 Yttrium Iron Garnet Nanoparticles . . . . . . . . . . . . . . . . . . . . . . 69

7.2 Coating of the Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

7.3 Effect of the Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

7.4 Effect of the Surfactant Concentration . . . . . . . . . . . . . . . . . . . . . . . . 74

7.5 Effect of the Ethoxylation Degree . . . . . . . . . . . . . . . . . . . . . . . . . . 75

7.6 Effect of the Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

7.7 Effect of the Hydrophobic Group . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

7.8 Effect of the Particle Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

7.9 Effect of Different Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

7.10 Effect of the Hydrodynamic Parameters . . . . . . . . . . . . . . . . . . . . . . . 86

7.11 Detachment Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

8 Conclusions 89

List of Figures

2.1 Schematic Representation of a Binary Phase Diagram . . . . . . . . . . . . . . . 8

2.2 Schematic Representation of a Ternary Phase Diagram . . . . . . . . . . . . . . . 8

2.3 Schematic Representation of a Ternary Phase Diagram . . . . . . . . . . . . . . . 9

2.4 Schematic Representation of Surfactant Adsorption . . . . . . . . . . . . . . . . . 15

3.1 Schematic Representation of a Microemulsion Reaction . . . . . . . . . . . . . . . 20

3.2 Free Energy of the Nucleation Stages . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3 Formation of the Nanoparticles in Reverse Microemulsions . . . . . . . . . . . . . 26

3.4 Formation of a Particle in a Homogeneous System and Microemulsion . . . . . . 28

4.1 Impact of the Particle Size in the London-van der Waals Forces. . . . . . . . . . 30

4.2 Model Gouy-Chapman-Stern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.3 Comparison of the van der Waals and Electrostatic Forces . . . . . . . . . . . . . 36

4.4 Total Potential Energy Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.1 Hydrodynamic Flow Acting on a Attached Particle . . . . . . . . . . . . . . . . . 39

5.2 Scheme of Forces Acting on a Particle (A) . . . . . . . . . . . . . . . . . . . . . . 40

5.3 Scheme of Forces acting in a Particle (B) . . . . . . . . . . . . . . . . . . . . . . 41

5.4 Graphic of the Shear Stress vs. Particle Radius . . . . . . . . . . . . . . . . . . . 43

6.1 Scheme of the Reactor to Achieve the Synthesis . . . . . . . . . . . . . . . . . . . 46

6.2 Scheme of the SEM Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.3 Schematic Representation of the Reflection in a lattice plane. . . . . . . . . . . . 54

6.4 Diagram of the Cleaning Process . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.5 Diagram of the Coating Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.6 Schematic Diagram of the Adhesion Device . . . . . . . . . . . . . . . . . . . . . 58

vii

LIST OF FIGURES viii

6.7 Profiles of Laminar and Turbulent Flows . . . . . . . . . . . . . . . . . . . . . . . 59

6.8 Schematic Representation of the Test Process . . . . . . . . . . . . . . . . . . . . 60

7.1 Binary Phase Diagram of the Microemulsion Systems . . . . . . . . . . . . . . . . 63

7.2 SEM of the Perovskite Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.3 XRD of the Perovskite Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.4 XRD of the Palladium Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.5 SEM of the Palladium Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.6 XRD of the Zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.7 SEM of the Zirconia Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.8 XRD of the Yttrium Iron Garnet Nanoparticles . . . . . . . . . . . . . . . . . . . 70

7.9 SEM of the Yttrium Iron Garnet Nanoparticles . . . . . . . . . . . . . . . . . . . 70

7.10 SEM of Pd Nanoparticles Attached to a Glass Surface . . . . . . . . . . . . . . . 71

7.11 XRD of Pd Nanoparticles Attached to a Glass Surface . . . . . . . . . . . . . . . 71

7.12 Scheme of the Coating Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

7.13 Effect of the Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

7.14 Effect of the Surfactant Concentration . . . . . . . . . . . . . . . . . . . . . . . . 75

7.15 Effect of the Ethoxylation Degree (0.024 mol/l) . . . . . . . . . . . . . . . . . . . 76

7.16 Effect of the Ethoxylation Degree (0.036 mol/l) . . . . . . . . . . . . . . . . . . . 77

7.17 Effect of the Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

7.18 Effect of the Hydrophobic Group . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

7.19 Effect of the Particle Size (Dependence of the Volumetric Flow) . . . . . . . . . . 81

7.20 The Behavior between Flow Rate and Particle Size . . . . . . . . . . . . . . . . . 82

7.21 Effect of the Particle Size (Dependence of the Hydrodynamical Force) . . . . . . 83

7.22 The Behavior between Hydrodynamical Force and Particle Size . . . . . . . . . . 83

7.23 Effect of Different Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

7.24 Lifshitz-van der Waals Constant (A′) for different Materials . . . . . . . . . . . . 85

7.25 Shear stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

7.26 The Total Removal Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

List of Tables

2.1 Hydrophile Lipophile Balance Values . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.1 List of the employed Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.2 List of Physicochemical Properties of distilled Water at 20 oC . . . . . . . . . . . 44

6.3 List of the Chemical Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6.4 Physico-chemical Properties of the employed Surfactants . . . . . . . . . . . . . . 46

6.5 Composition of the Microemulsions for the Perovskite . . . . . . . . . . . . . . . 47

6.6 Composition of the Microemulsions for the Palladium Nanoparticles . . . . . . . 48

6.7 Composition of the microemulsion for the Yttrium Iron Garnet . . . . . . . . . . 50

6.8 Composition of the microemulsion for the Zirconia . . . . . . . . . . . . . . . . . 51

7.1 Lattice Parameters of the Perovskite . . . . . . . . . . . . . . . . . . . . . . . . . 65

7.2 Specific Surface Area SBET for Perovskite Particles . . . . . . . . . . . . . . . . . 65

7.3 Lattice Parameters of the Palladium . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.4 Palladium Average Diameter and Specific Surface Area SBET . . . . . . . . . . . 67

7.5 Lattice Parameters of the Zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . 68

7.6 Zirconia Average Diameter and Specific Surface Area SBET . . . . . . . . . . . . 68

7.7 Lattice Parameters of the Yttrium Iron Garnet . . . . . . . . . . . . . . . . . . . 69

7.8 Yttrium Iron Garnet Average Diameter and N2 Adsorption Values. . . . . . . . . 69

7.9 Average Diameter of the Particles from different Materials on Glass surface. . . . 72

ix

Chapter 1

MotivationAdhesion or detachment of fine particles from a surface is of interest in several industrial appli-

cations such as detergency [1], particle filtration [2] and oil production [3]. Recently, adhesions

on semiconductor surfaces [4] and in biological systems such as cells or viruses have been the

center of many researches [5].

There are two aspects that must be addressed in order to understand the behavior of two

solids across a fluid medium. First, the physicochemical interactions that keep the particle

attached on the surface, in other words, the nature and strength of the adhesive bond between

the particles and the surface. The major effects acting on the physicochemical interactions

are the attractive forces, which come from the dispersion of London-van der Waals and the

repulsive part or electrostatic interactions. These forces are usually comprised of electrostatic

double layer interactions, solvation and Born forces. The physicochemical interactions can be

explained in terms of the Derjaguin Landau Verwey Overbeek theory, named by its abbreviation

as DLVO-Theory, and are quite pronounced at distances of ten nanometers between solids.

The other important aspect is the hydrodynamic force acting on the attached particle to a

surface. Goldman et al. (1966) [6, 7], one of the first to study the motion of a sphere parallel

to a plane wall, developed a mathematical solution to predict the behavior between a particle

next to a wall in a laminar flow. Based on the Goldman results, O’Neill (1968) [8] found an

exact solution of the hydrodynamic force acting on a particle in contact with a plane wall

considering a laminar flow. Afterwards, Visser (1970) [9] and Claver et al (1972) [10] analyzed a

mathematical solution taking into account a similar particle-substrate system in a turbulent flow.

The theoretical works of these pioneers led to improvements and developments in investigations

in the areas of detergency, particle filtration and oil production. However, all the studies that

can be found in the literature treat with particles bigger than 0.1 µm (10−7 m), while this study

1

2

is focused in particles with a dimension in nanometers (10−9 m).

In order to compare the experimental and theoretical results, it is necessary to employ

particles with a defined form (spheres) and with a narrow average diameter. The detachment

of fine particles from a surface has several limitations. For example, there is not a variety of

available materials and the costs of such particles are considerably high. In addition, there exists

a lack of efficient techniques to coat the substrates as well as to quantify the detachment of these

particles. Therefore, one of the goals of this work is the implementation of a technique, which

permits to carry out studies of the removal of nanoparticles in a systematic way.

Efficient particle detachment is extremely difficult because of the strong adhesion forces and

the nature of the particulate. Particle removal methods are generally classified into three cate-

gories, wet chemical methods, mechanical techniques and dry processes (See chapter 5). Unfor-

tunately, all these methods do not work effectively when the particles have a size of nanometers.

Therefore, the advantages of the first two techniques were combined to improve the removal

by applying an external force “Hydrodynamic force” and by employing a solution of “Nonionic

surfactant” to modify the interactions between the solids “particle-substrate”.

To conduct this investigation, nanoparticles of different materials such as palladium, zir-

conium oxide, yttrium iron garnet and perovskite of manganese were synthesized in w/o-

microemulsion. Such reverse micelles are a suitable medium for producing nanoparticles with a

narrow average size and defined morphology.

In this work, a methodology was developed to coat cleaned surfaces with the particles.

Additionally, a device was designed to test the detachment of the particles. The instrument

is fed with a solution of nonionic surfactant, which generates the hydrodynamic force needed

to detach the particles from the surface, while the surfactant acts to modify the interactions

between the solids (particle-substrate) to favor the removal. Finally, the quantification of the

particles detachment was followed by U.V.-spectroscopy.

Chapter 2

Surfactants

Amphiphiles are molecules with a characteristic molecular structure, They consist of a lyophobic

group together with a lyophilic group. The first, has a very little attraction for the solvent,

while the last presents strong attraction for it. When the solvent is an aqueous medium the

lyophilic group is named hydrophilic and the lyophobic group is known as hydrophobic. The

term surfactant was adopted to point out those amphiphile substances that have the property

of adsorbing onto the surfaces of a system modifying their interface free energies.

Industrial surfactant applications are extremely varied and their contribution to industrial

processes cannot be left out. Some of the principal applications of surfactants are in the pa-

per industry, commercial laundering, industrial hard surface cleaning, inhibitor of corrosion,

enhanced of oil recovery, asphalt emulsion, coal transport, ore flotation between others [11],[12].

Surfactants can be classified according to their physicals properties or functionalities. The

following is the most common classification and it is based on the nature of the hydrophilic

group.

Anionic The surface-active portion of the molecule exhibits a negative charged, like as alkyl-

benzene sulfonate (RC6H4SO−3 Na

+).

Cationic The surface-active portion bears positive charge, for example a salts of a long-chain

amine, like as quaternary ammonium chloride (RN(CH3)+3 Cl

−).

Zwitterionic Both, positive and negative charges are presented in the surface-active portion,

for example a long-chain amino acid, like as sulfobetaine (RN+(CH3)2CH2CH2SO−3 ).

3

2.1. PHYSICAL STATE 4

Nonionic The surface-active portion bears no apparent ionic charge, like as polyoxyethylenated

alcohols (R(OC2H4)xOH).

Some of the most important physicochemical features of the surfactants are summarized in the

following sections. A mayor emphasis is given in those characteristics of the nonionic surfactants,

that are relevant for this investigation.

2.1 Physical State

The ionic surfactants are generally amorphous or crystalline solids and the nonionic surfactants

are liquid or pastes. Crystalline surfactants can be prepared relatively purely. They can be

polymorphic, if their structures have different unit cell, or polytypic if their structures have

one dimensional polymorphism. Amorphous solids are surfactants that have one or more chiral

centres and exist in multiple optical isomers. Liquid crystalline surfactants exhibit properties

common to crystalline and to liquid physical state. Liquid Surfactants are fundamentally amor-

phous with no long range order and are typically isotropics.

2.2 Critical Micelle Concentration (CMC)

Micelle formation or micellization is an important parameter due to a number of important

interfacial phenomena, such as detergency and solubilization, depend on the existence of micelles

in solution. Furthemore, micelles have become a subject of great interest to the organic chemistry

and the biochemistry because of their unusual catalysis of organic reactions and their similarity

to biological membranes and globular proteins [12].

The concentration of surfactant at which micellization beginns is called the critical micelle

concentration (CMC). This parameter can be determined by many different techniques. The

most popular techniques include surface tension, turbidity, self diffusion, conductivity, osmotic

pressure and solubilization. All of these methods involve plotting a measure as a function of the

logarithm of surfactant concentration. The breakpoint in the plot represents the CMC.

Several investigators have developed empirical relationships between the CMC and the struc-

tural features of surfactants. Becher [13] calculated the coefficients for the linear relationship

between the logarithm of the CMC, the number of ethylene oxide (EO) and the number of

2.2. CRITICAL MICELLE CONCENTRATION (CMC) 5

carbon atoms (C) for homologous series of linear alkyl hexaethoxylates (CnEOm)

log10 CMC = a− b C + d EO (2.1)

with a = (1.646 ± 0.082), b = (0.496 ± 0.08) and d = (0.0437 ± 0.0094).

The CMC is affected by several factors like as hydrophobic group, hydrophilic group, tem-

perature, connection of the group in the structure and addition of salts and organic solvents

[12],[14].

2.2.1 The Hydrophobic Group

In aqueous medium, the CMC decreases linearly with the increase of the carbon number in the

alkyl chain length of polyoxyethylene n-alkylalcohols. When the hydrophobic group is branched,

the carbon atoms on the branches appear to have about one-half the effect of carbon atoms on

a straight chain [12].

2.2.2 The Hydrophilic Group

In aqueous medium, ionic surfactants have much higher CMC than nonionic surfactants con-

taining equivalent hydrophobic groups. For the usual type of polyoxyethylene (in which the

hydrophobic group is a hydrocarbon residue), the CMC in aqueous medium increases with the

increment of the number of oxyethylene units in the polyoxyethylene chain.

Due to the fact that commercial polyoxyethylene (POE) nonionics are mixtures contain-

ing POE chains with different numbers of oxyethylene units, which are clustered about some

mean value, their CMCs are sightly lower than those of the single species containing the same

hydrophobic group and with oxyethylenen content corresponding to that mean value. This

is probably because the component with low oxyethylene content in the commercial material

reduces the CMC more than it is raised by those with high oxyethylene content [15].

2.2.3 Temperature

The effect of temperature on the CMC of surfactants in aqueous medium is complex Rosen [12]

pointed out that the value appearing first to decrease with the temperature to some minimum

and then to increase with further increase in temperature.

2.3. SOLUBILITY 6

The increase of the temperature causes decrease of the hydratation of the hydrophilic group,

which favors the micellization. However temperature increase also causes disruption of the

structured water surrounding of the hydrophobic group, an effect that disfavors micellization.

The relative magnitude of these two opposing effects, therefore, determines whether the CMC

increases or decreases over a particular temperature range.

From the data available in the literature, the minimum in the CMC temperature curve

appears to be around 25 oC for ionic surfactants [16] and around 50 oC for nonionic [15].

2.2.4 Salts

Addition of inert salts to an aqueous solution of surfactant usually decreases the CMC of

ionic surfactants. This effect is less pronounced when the surfactants is nonionic. Salts tent

to screen electrostatic repulsions between headgroups and make the surfactant effectively more

hydrophobic. This increases hydrophobic interactions among the surfactants cause them to

aggregate a lower concentration, thereby the CMC decreases [11].

2.3 Solubility

Nonionic surfactants which are polar covalent compounds are very good soluble in polar sol-

vents like alcohol and ketone, but less soluble in solvents of lower dielectric constant such as

hydrocarbons.

The study of the structure of aggregates of nonionic surfactants in apolar solvents has a

great interest because the application of such aggregated structures in ternary oil recovery [17],

as catalyst for chemical reactions and in the synthesis of fine colloidal particles [18]. In order

to understand the aggregation of nonionic surfactants, it is important to know their physical

properties (e.g. detergency, solubility, micelle formation and solubilization of substance) in

nonaqueous solutions.

A prediction of the solubility of nonionic surfactants in apolar solvents can be very useful to

explain their effectiveness as emulsifier. However information on nonionic surfactants solutions

is scarce, probably because of the difficulty of obtaining homogeneous samples as well as, their

relatively limited solubility in various apolar solvents. Shinoda and Arai [19] examined different

binary system (surfactant-hydrocarbon) and found that the solubility increases as function of

2.4. MICROEMULSIONS 7

decreasing the ethoxyethylene chain length of the surfactant.

An important tool to characterize ionic and nonionic surfactants are the phase diagrams, in

which the formation of micelle and other aggregated are represented. It is possible to plot three

different phases diagrams increasing the complexity of the systems.

Binary phase diagrams is a two dimensional maps of the phase domains as a function of

temperature and surfactant/solvent mole fraction (or weigh fraction). Figure 2.1 shows a

typical binary diagram.

Ternary phase diagrams represents the behavior of a water-oil-surfactants system. It shows

the domains of each aggregated structure. The figure 2.2 is a schematic representation of a

ternary phase diagram at constant temperature corresponding to a typical water/nonionic

surfactant/oil-ternary system.

Ternary phase diagrams vs. temperature is an extention of the ternary phase diagram as

a function of temperature. As it is shown in figure 2.3

2.4 Microemulsions

An important property of micelles is that they can promote solubility of compounds inside a

solvent, where they are normally insoluble, through the formation of microemulsions.

Microemulsions are defined as thermodynamically stable isotropic dispersions of two im-

miscible liquids consisting in microdomains of one of both liquids in the other. These dispersions

are stabilized by an interfacial film of surface-active molecules. The microemulsion presents a

phase that changes with temperature and it is reversible. The composition of the system is

specified by the weight fraction of oil in the mixture of oil and water α, and the weight fraction

of surfactant in the ternary mixture (γ), equations 2.2 and 2.3, respectively.

α =mo

mo +mw(2.2)

γ =ms

mo +mw +ms(2.3)


Figure 2.1: Schematic representation of a binary phase diagram. L1, L2 and L3 denote

isotropic liquid solutions, H1 is a normal hexagonal phase and Lα represents a lamellar

liquid crystalline phase.

Figure 2.2: Schematic representation of a ternary phase diagram at constant temperature.


Figure 2.3: Schematic representation of a ternary phase diagram. W , O and S represent

the water, oil and surfactant, respectively. 1, 2 and 3 are the number of co-existing phases

in the region and T is the temperature.

where mo, ms and mw represent the weight of oil, surfactant and water, respectively.

Two main general structures for the discrete microemulsion have been accepted as follows.

2.4.1 Discrete Microemulsions

They are poor in either water or oil and presenting a micellar structure. They are classified into

two categories.

Water in oil microemulsion (w/o-microemulsion) Surfactant (S) dissolved in organic sol-

vents form spheroidal aggregates called reverse micelles. They can be formed both in

the presence and in the absence of water. However if the medium is completely free of

water, the aggregates are very small and polydisperse. The presence of water is necessary

to form large surfactant aggregates. Water is readily solubilized in polar cores, forming

reverse micelles, in which the hydrophilic heads of the surfactant are absorbed in the water

microdroplets while the hydrophobic tails are oriented toward the oil.

Oil in water microemulsion (o/w-microemulsion) in which the lipophilic tails of the sur-

factant is absorbed in the oil microdroplets while the lyophobic part is oriented toward

the water.


An important parameter to describe the droplet size is the so-called “water pool”, char-

acterized by W0. This parameter is the water-surfactant molar ratio (equation 2.4), which is

considered to be proportional to the radius R of the droplet as was shown by Pileni [20].

W0 =nw

ns(2.4)

Where nw is the water mole and ns is the surfactant mole. The aggregates containing a small

amount of water (below W = 15) are reverse micelles whereas microemulsions correspond to

droplets containing a larger amount of water (above W = 15).

The spontaneous curvature of the surfactant films of reverse micelles corresponds to the ener-

getically favorable packing configuration of the surfactant molecules at the interface. Assuming

that water-in-oil droplets are spherical, the radius of the sphere is expressed as

R =3 V

S(2.5)

where R, V and S are the radius, the volume and the surface of sphere. If the volume and the

surface of the droplets are governed by the volume of the water molecules (Vw) and by the area

of surfactant molecules at the interface (σ). The water pool radius (R = Rw) can be expressed

as

Rw =3 Vw nw

σ ns(2.6)

2.4.2 Bicontinuous Microemulsions

The bicontinuous microemulsions contain similar amounts of oil and water and relatively high

amounts of surfactant and show a sponge-like structure. The figure 2.2 shows the different

domains.

Due to their physicochemical properties, microemulsions have enormous applications in pro-

cesses such as: lubrication, biotechnology (enzymatic reactions in microemulsions), pharmaceu-

2.5. ADSORPTION FROM SOLUTION 11

tics, oil recovery and extraction processes, detergency as well as in the synthesis of nanoparticles

of a desired size [21].

2.5 Adsorption from Solution

Surfactants have the property to aggregate in solution or at interfaces forming micelles in aqueous

solutions and structured films at the liquid-gas interface. This property is known as adsorption

and results in an increase in the surfactant concentration at the solid-liquid or liquid-gas interface

in comparison to the bulk concentration.

Soluble surfactants absorb to interfaces in equilibrium with their bulk. The study of such

adsorption permits to determine the energy changes (∆G), (∆H) and (∆S) in the system.

These properties provide information on the type and mechanism of any interaction between

the surfactant and the interface. The amount of adsorbed surfactant is determined as a function

of solution concentration. It can give information to deduce the mechanism from the adsorption

isotherm. Increment of bulk concentration rises the surface adsorption. The region approaching

the CMC is the zone in which increasing bulk surfactant concentration yields increasing surface

excess concentration.

At the liquid-water interface the surface excess surfactant concentration (Γ) can be calculated

using the Gibbs adsorption equation

(

∂ γ

∂ ln(c)

)

T

= − R T Γ (2.7)

The surface excess surfactant concentration Γ, can be graphically calculated from the variation

of surface tension γ with the logarithmic change in bulk concentration (C).

2.5.1 Adsorption at the Liquid-Solid Interface

The hydrophobic forces, which control surfactants aggregation at air-water interfaces, are essen-

tially the same forces that drive surfactant adsorption onto solid surfaces. However, liquid-solid

interfaces significantly differ in that the solid surface can be the source of additional chemical

forces such as electrostatic force from ionized surface groups, hydrogen bonding force between

surfactant and surface functional groups, and finally, dispersion forces between portions of the


surface and hydrophobic portion of the surfactant. In the case of nonionic surfactants, hydrogen

bonding and dispersion forces are less significantly.

The adsorption of surfactants at solid-liquid interfaces is strongly influenced by a number

of factors such as, the surface properties (particle size, porosity and chemical composition), the

molecular structure of the surfactant being adsorbed (it is a mixture or a single component

other it is ionic or nonionic, the hydrophobic group is straight or branched, long or short etc.)

and the environment of the aqueous phase (its pH, electrolyte content, etc.). All theses factors

together determine the mechanism by which adsortion of surfactant onto solid from aqueous

solution occurs, as well as, the efficiency and effectiveness of the surfactant adsortion. Rosen

[12] classified the adsorption mechanisms of ionic and nonionic surfactants as follows

Ion exchange This mechanism involves the replacement of counterions adsorbed onto the sub-

strate from solution by similarly charged surfactant ions.

Ion pairing Adsorption takes place from solution onto oppositely charged sites unoccupied by

counterions.

Hydrogen bonding Adsorption occurs by hydrogen bounding between substrate and adsor-

bate.

Adsorption by polarization of π electrons Adsorption onto solid surfaces is the result of

attractive interaction forces between electron-rich aromatic nuclei of the adsorbate and

positive sites located on the substrate.

Adsorption by van der Waals dispersion forces Adsortion by this mechanism generally

increases with the increment of the molecular weight of the adsorbate. this mechanism

may acts as a supplementary mechanism to all other types of adsorption mechanism.

Adsorption by altering hydrophobic bonding Adsorption occurs when the attractive forces

become large enough to permit them adsorb onto the solid surface by chain aggregation.

The concentration of the surface-active agent at the interface can be described by Langmuir

and Frumkin isotherms.


2.5.2 Langmuir Adsorption Isotherm

Surfactant adsorption at the liquid-solid interfaces may be driven by various combinations of the

above forces. In the limit of noninteracting adsorption (i.e, where adsorbed surfactant molecules

do not interact with each other), the adsorption may be modelled by the Langmuir Adsorption

Isotherm as follows.

Γ =b c Γs

b c + 1(2.8)

Where (Γ) is the adsorbed amount (typically µ mol/m2), (Γs) represents the saturation mono-

layer coverage, (c) is the equilibrium surfactant concentration in the solution phase, and the

equilibrium constant (b), which describes the adsorption process as

b = exp

(

−∆Gads

R T

)

(2.9)

The free energy of adsorption is assigned to (∆Gads), (R) is the gas constant, and (T ) is the

temperature. In addition, an alternative representation of the Langmuir Adsorption Isotherm

is given by

c

Γ=

1

b Γs+

c

Γs(2.10)

In this case, a plot of (c/Γ) as function of (c) yields a straight line, in which the slope repre-

sents the saturation coverage. Afterwards, the equilibrium constant can be calculated from the

interception between this line and the X axis (C).

2.5.3 Frumkin Adsorption Isotherm

The Frumkin Adsorption Isotherm considers a generalization of the Langmuir Adsorption ap-

proach. Basically, Frumkin Adsorption theory models the interaction between absorbed species

based on the interaction parameter α using the following equation.

Γ/Γs

1 − Γ/Γs= b c exp

[

2 α

(

Γ

Γs

)]

(2.11)

2.6. STATE OF ADSORBED SURFACTANT 14

It is important to note that the Langmuir Adsorption equation (eqn. 2.8) can be obtained from

the above equation when the interaction parameter ((α = 0)) is zero.

2.6 State of Adsorbed Surfactant

Sharma [22] edited a complete summary about the resolution of the molecular packing of surfac-

tant adsorbed at liquid-solid interfaces. This overview has not been confirmed by experimentally.

However, a variety of working models has been presented which have met with varied responses.

The unequivocal identification of how isolated surfactant molecules adsorb has less theoretical

support, but in this case, only several possibilities need to be postulated to cover the range of

material behavior. The end-on Adsorption Model is commonly utilized to study the isolated

adsorption. Basically, an electrostatic or chemical (bond) force aims the headgroup structure

of the surfactant towards the surface whereas the hydrophobic group adopts an axisymmetric

orientation. According to details of the headgroup structure and the type of specific interaction

with the surface, the adsoption could be end-on (a) or skewed (b) as shown figure 2.4 [11].

Straight chain cationic surfactants typically are shown as binding (fig.2.4 (a)). Figures 2.4 (c)

and (d) illustrate the molecular orientantions when the hydrophobic forces lead the adsorption

as for example the interaction between the surfactant tail portion and the hydrophobic portion

of the surface. In addition, structures such as (e) and (f) are expected when both the headgroup

and the tail interact strongly with the surface.

The situation in the case of polymeric surfactants is even more sophisticated. Many theories

have postulated in order to investigate the means by which the individual molecules surfactants

coalesce to form aggregates on surfaces. Two distinct pictures of hemimicelles are plotted in

figure 2.4 (g) and (h). The structure is a hemimicelles (fig. 2.4 (g)). It is a region of a close-

packed monolayer. A strong electrostatic interaction between the headgroup and the surface

produces the formation of such structures [11]. It would be tempting to belive that there is

hydrophobic bonding among the tail groups as an additional driving for such force adsorption,

were it is not for the fact that such configurations tend to create a high energy interface between

the tail groups and the solvent phase. More than 20 years ago, the alternative hemimicellar

structure shown in fig. 2.4 (h) was generally accepted in reasonable analogy to the model of

2.6. STATE OF ADSORBED SURFACTANT 15

Figure 2.4: Schematic representation of surfactant adsorption at the liquid-solid interface

[11]. (a-f) Isolated surfactants molecules modes of adsorption, (g-j) Two dimensional

cross-sections. (g-h) hemimicelles, (i) admicelle and (j) adsorbed micelle.

spherical micelles. In this case, the surfactant in contact with the solid surface adopts a statically

weighted variety of structures such as those illustrated in figures (a-f) and then a hemispherical

structures is completed to satisfy packing parameter constraints. Such a structure does not

create high-energy surfactant tail solvent interactions.

The admicelle structure is plotted in fig. 2.4 (i). It appears to form a bilayer showing a

topologically indistinguishable form with respect to the hemimicellar structure of figure (h), in

which an “end-cap” structure is imposed and then lateral dimension is similar to the surfactant

length. Finally, adsorbed micelles (fig. 2.4 (j)) may suffer relatively minor perturbations to their

overall molecular packing and can interact with surfaces in specific way through headgroups

without “spreading”.

2.7. HYDROPHILE LIPOPHILE BALANCE (HLB) 16

2.7 Hydrophile Lipophile Balance (HLB)

The hydrophilic lipophilic balance number (HLB) was introduced by Griffin [23], [24]. This

number is an empirical expression for the relationship of the hydrophilic and hydrophobic groups

of a surfactant. The HLB number provides a semiquantitative description of the efficacy of

surfactants with respect to emulsification of water and oil systems. This scale was introduced

to characterize nonionic surfactants using oxyethylene oligomer as hydrophilic group. The HLB

number for nonionic surfactants can be calculated through the following equation

HLB = 20

(

1 − ML

MT

)

(2.12)

where ML is the formula weight of the hydrophobic portion of the molecule and MT is the total

formula weight of the surfactant molecule. The table 2.1 lists HLB values along with typical

performance.

Table 2.1: Hydrophile Lipophile Balance (HLB) and their typical properties

HLB Property

< 10 Oil soluble

> 10 Water Soluble

4-8 Antifoaming agent

7-11 w/o-Emulsifier

12-16 o/w-Emulsifier

11-14 Wetting agent

12-15 Detergent

16-20 Stabilizer

Chapter 3

Nanoparticles Synthesis

Ultrafine particles normally present a physical dimension between 1-100 nm (such a grain size)

and a significant amount of surfaces and interfaces. Thereby, they are frequently called nanopar-

ticles. Such particles have gained a considerable interest due to their remarkable physical prop-

erties with respect to reaction bulk material.

Nanoparticles have important technological applications, such as in catalysis [25], micro-

electronic devices [26], high-performance ceramic materials [27], and more recently they have

been used in cancer treatment [28]. Therefore, the efforts to develop synthetical methods, which

permit to obtain ultrafine particles have been increased.

Section 3.1 contains a survey of the major nanoparticles synthesis methods. Afterwards,

section 3.2 presents a review about microemulsions and particle synthesis in reverse micelles.

Synthesis in w/o-microemulsion is treated with emphasis due to the nanoparticles, which were

employed in this work, were prepared using this method.

In order to understand the particle formation mechanisms, models of precipitation in homo-

geneous phase and in reverse micelles are explained in section 3.3 and section 3.4 respectively.

The mechanism in homogeneous solution is presented because of is the key to understand the

microemulsion precipitation.

17

3.1. OVERVIEW OF THE PRINCIPAL NANOPARTICLE SYNTHESIS METHODS 18

3.1 Overview of the Principal Nanoparticle Synthesis

Methods

Several techniques have been reported in the literature for the synthesis of nanoparticles to

laboratory-scale. Chemical reactions for materials synthesis can be carried out in the solid, liquid

or gaseous state (Gas-Phase Techniques [29, 30, 31, 32],Vacuum synthesis Techniques [33, 34, 35]

and Liquid-Phase Techniques [36, 37]).

The conventional solid-state synthesis is to bring the solid precursor into close contact by

grinding and mixing, and to subsequently heat treatment this mixture at high temperature to

facilitate diffusion of atoms or ions in the chemical reaction. The diffusion of atoms depends

on the temperature of the reaction and grain boundary contacts. The transport across grain

boundaries is also affected by impurities and defects located there, grain growth during high

temperature reactions leads to solids with large grain size [38].

In comparison with the to solid-state synthesis, the faster diffusion of matter in liquid or

gas phase offers several advantages with respect to solid phase. For example, the synthesis of

nanostructured materials can be achieved at lower temperatures preventing the grain growth.

Many materials can be synthesized in aqueous or nonaqueous solutions. There are three

general classes of aqueous reactions: acid/base reaction, precipitation, and reduction/oxidation

(redox). The reactants can be solids, liquids, or gases in any combination, in the form of single

elements or multi-component compounds (usually called precursor). However, due to the fact

that many advanced materials are hybrids and are prepared using multidisciplinary techniques,

clear distinction is not always possible. A brief description of the more common liquid phase

methods are presented as follows.

Aqueous Methods uses water as solvent for polar or ionic compounds. Therefore, many

chemical reactions take place in aqueous media. For example, metal powders for electronic

applications can be prepared by adding liquid reducing agents to aqueous solutions of

respective salts at adjusted pH [39].

Nonaqueous Methods are applying in the same way that aqueous methods. Many reactants

and reducing agents used in aqueous synthesis of nanoscale metal particles can also be

employed in nonaqueous solvent for the same purpose. In this method, precursor com-

pounds are dissolved or suspended in ethylene glycol or diethylene glycol. The mixture

3.1. OVERVIEW OF THE PRINCIPAL NANOPARTICLE SYNTHESIS METHODS 19

is heated to reflux. During the reaction, the precursors are reduced and metal particles

precipitate out of the solution. This synthesis is also known as the polyol process [40, 41]

Sonochemical Methods has also been used in chemical synthesis of nanostructured materials.

High energy sonochemical reactions, without any molecular coupling of the ultrasound

with the chemical species, are driven by the formation, growth and collapse of bubbles in

a liquid. This acoustic cavitation involves a localized hot spot of temperature up to 5000

K, a pressure around 1800 atm. and a subsequent cooling rate about 109K/sec, due to

implosive collapse of a bubble in the liquid. Generally, volatile precursors in low vapor

pressure solvents are used to optimized the yield [42, 43].

Hydrolysis involves the formation of an insoluble hydroxide which can then be converted to

its oxide by heat-assisted dehydration [44].

Hydrothermal uses vapor of water to achieve the reaction. The reaction mixture is heated

above the boiling point of water in an autoclave or other closed system and the sample is

exposed to steam at high pressures [45].

Sol-Gel Methods is not a new method. As early as the mid 1800s, it was reported that

silicon tetrachloride, when left standing in an open container, hydrolyzed and turned into

a gel [46]. After this time, biologists did much of their work with gels and colloid. In the

early 1930s, aerogels were discovered [47]. Since the 1950s, sol-gel techniques have been

used for phase equilibrium studies which opened up the field of ceramics [48]. Traditionally,

Sol-Gel process involves hydrolysis and condensation of metal alkoxide.

Factors that need to be considered in this method are solvent, temperature, precursors,

catalyst, pH, additives and mechanical agitation. They can influence the kinetics of hy-

drolysis and condensation in the reaction as well as the particle growth [49].

Non-Hydrolytic Sol-Gel Methods: the traditional hydrolysis and condensation reactions

are replaced by direct condensation or transesterification reactions. Non-hydrolytic re-

actions do not involve water or polar solvent. In the traditional sol-gel methods, the

condensation reaction can be reversible and special efforts are required to completely

remove water or polar solvents [50, 51].

Host-Derived Hybrid Materials: porous or layered ceramics can be used as host materials

in which nanoparticles are synthesized. The resulting hybrids may have novel properties

3.2. SYNTHESIS OF NANOPARTICLES IN REVERSE MICELLES 20

Figure 3.1: Schematic representation of a microemulsion reaction

due to the microstructure of the host. Minerals such as silicate or aluminosilicate (Zeolite)

can be used in this way [52].

3.2 Synthesis of Nanoparticles in Reverse Micelles

The microemulsion technique is based on the use of micelles as microreactors, in which chemical

reactions can be carried out , in particulate for synthesis of nanomaterials.

The method consists of mixing two microemulsion systems with the same amount of aqueous

solution, oil and surfactant but with different aqueous solution composition. For example,

figure 3.1 shows a microemulsion called “A” with a salt dissolved in the aqueous solution and a

microemulsion named “B” with the appropriate reducing agent to achieve the reaction.

w/o-Microemulsion synthesis presents some advantages over the processes mentioned in the

section 3.1, such as:

Nanoparticles control by adjusting of the microemulsion parameters like than “Wo” can be

controlled the narrow size distribution and shape.

Variability of the chemical procedure by changing of the type of reaction. It is possible

to achieve reactions via oxidation, reduction, sol-gel, etc.

Variability of the processing by changing synthesis conditions. For example, surfactants

system, temperature, concentration, addition rate, etc. This allows a control over the

reaction.

3.3. MODEL OF PRECIPITATION IN HOMOGENEOUS PHASE 21

3.3 Model of Precipitation in Homogeneous Phase

Generation of a solid-phase can be achieved by crystallization and precipitation.

Crystallization the solid is formed from a supersaturated solution, which can be controlled

by adjusting the solubility of a unreactive solute (for example temperature, evaporation

and presion) or by adding another component.

Precipitation the insoluble species are obtained by a chemical rection. In this process ag-

glomeration becomes an important growth mechanism, due to the high number density of

nuclei produced, leading to either polycrystalline or amorphous particles [53].

In general both of these methods involve the stages of nucleation and growth. After these

stages, other secondary process as agglomeration takes place. A model to explain the formation

of particles in solutions was proposed by La Mer and Dinegar [54, 55]. This model is presented

briefly in this section, because it is considered the key to understanding the process inside the

micelles.

3.3.1 Nucleation Stage

When a substance is transformed from one phase to another, the change in the molar Gibbs free

energy of the transformation (∆G), at a constant pressure and temperature, can be expressed

in terms of chemical potentials of phase 1 (µ1) and phase 2 (µ2), or in terms of saturation (S)

as shows equation 3.1.

∆G = −R T ln

(

C

Ceq

)

= −R T ln S (3.1)

The above equation assumes activity coefficients of one. R is the universal gas constant, T is the

absolute temperature, C represent the solute concentration, Ceq is the equilibrium concentration

and S denotes the supersaturation of the solute at the temperature and pressure of the system.

These thermodynamic considerations describe a driving force for crystallization, however, in

most cases nucleation and growth are controlled by kinetic.

The rate of nucleation (J) plays an important role in controlling the final particle size

distribution. Dikersen [53] broke down the nucleation process into the following three categories

Primary Homogeneous Nucleation: it occurs in absence of a solid interfaces. If it is as-

sumed that for supersaturated solutions, the solute molecules are combined by molecular


addition to produce embryos [54] (according to the equation 3.2)

mA1 Bx

Bx +A1 Bx+1

...

B(i−1) +A1 Bi (3.2)

Then the free energy of the embryo is the sum of two terms: the free energy due to the

formation of a new volume and the free energy due to the new created surface (equation

3.3).

∆G = −(

βv r3

ϑ

)

kB T ln S + γ βa r2 (3.3)

where (βv r3) is the volume and (βa r2) is the surface area of the aggregate, ϑ is the

molecular volume of the precipitated embryos, kB is Boltzmann constant and γ is the

surface free energy per unit area.

Considering the definition of the radius of a sphere and placing within equation 3.3, the

total energy of a particle size with radius r is given by

∆G = −(

(4/3) π r3

ϑ

)

kB T ln S + 4 π r2 γ (3.4)

If S ≤ 1, ∆G is positive and the formation of a new phase is not spontaneous. If S ≥ 1,

∆G has a positive maximum at a critical nuclei size r∗ as shown in the figure 3.2, this

maximum corresponds to the activation energy for nucleation. When embryos are large

than r∗, their free energy decreases and they become “stable nuclei”, which grow to form

macroscopic particles.

In physical chemistry, the term nucleus is applied to the minimum-size embryo, which is

capable of initiating spontaneous growth to produce a new phase.

In self nucleation process the critical nuclei size can be obtained by setting (d∆Gr/dr = 0)

in equation 3.3

r∗ =2 βa γ ϑ

3 βv kB T ln S(3.5)


Figure 3.2: Free energy of the nucleation stages. ∆G vs. particle radius, r∗ represents

the critical nuclei size.

This critical size corresponds to the maximum of the free energy, (∆Gmax), or the activa-

tion energy as

∆Gmax =γ βa (r∗)2

3(3.6)

Dikersen and Ring [53] found an approximation to estimate the nucleation rate (J), in

terms of the diffusion coefficient of the solute (D), the molecular diameter (d), and the

activation energy ∆Gmax as follows.

J =

(

2 D

d5

)

exp (−∆Gmax/kB T ) (3.7)

Primary Heterogeneous occurs in the presence of a solid interface of a foreign seed.

Secondary Nucleation occurs in presence of a solute-particle interface.

3.3.2 Growth Stage

In order to grow a crystal in a solution, the solute must be transported through the solution up to

the crystal surface, and arranged in conformity with the crystal structure. If it is the transport


through the solution that controls the growth velocity, both diffusion and convection may be

important. When the crystals are smaller than about 10 µm, convection can be neglected,

because the velocity of the crystal through the solution in the normal gravity field or at normal

stirring rates is very low [56].

Kinetic is discussed in terms of purely diffusion controlled growth. The mathematical for-

mulation of the diffusion problem has been treated by Reiss [57]. Chiang and Donohue [58]

compared the theory of crystal growth with experimental findings. They employed several sur-

face reaction models to explain experimentally observed growth rates by the following equation

Jgrowth = kgrowth Sp (3.8)

Where S is the saturation ratio and p is the growth rate order. Many experiments involving

spontaneous crystallization have been conducted to measure the order of the growth rate. Mc

Cabe [59], among the first to measure the crystal growth rate for copper sulfate, found p = 1. It

suggests that the diffusion was the controlling step. On the other hand, Davies and Jones [60]

found p = 2 for the growth of silver chloride.

Other experiments indicated that the over-all growth rate order is within a range between

1 and 2. The final size, to which the particles will grow and the rate at which they will grow,

depends upon three variables. The first is the number of nuclei which grow. This number is

assumed constant through the growth and equal to the number of particles present at any time.

The second variable is the total amount of diffusible solute (normally there is no other sources

of solute). This value is given by the difference between the amount of solute originally in

the mixture and the solubility of the solute in the medium. The third variable is the diffusion

coefficient (D) of solute in the medium.

In 1917 Smoluchowski [61] deduced the following theoretical equation for the diffusion coef-

ficient in a bimolecular reaction (B + C → P ) with B 6= C, kgrowth is given by

kgrowth = 4 π NA(rB + rC)(DB +DC) (3.9)

Where NA is the Number of Avogadro, rB and rC are the radius of B and C, which are considered

spheres by simplifying the equation, DB and DC are the diffusion coefficients.

The last expression can be simplified by incorporating the Stokes-Einstein relation between

the diffusion coefficient (D), and the viscosity (η) of the medium.

3.4. REVERSE MICELLES SYNTHESIS MODEL 25

DB =k T

6 π η rB; DC =

k T

6 π η rC(3.10)

By substitution of 3.10 in equation 3.9

kgrowth =

(

2 R T

3 η

)

(rB + rC)2

rB rC=

2 R T

3 η

(

2 +rBrC

+rCrB

)

(3.11)

If rB ' rC equation 3.11 is approximated as

kgrowth =8 R T

3η(3.12)

The process of particle formation can be shown schematically as in figure 3.4, which represents

the variation of the solute concentration with the time. The concentration of reactant C increases

until a critical concentration or critical supersaturation, [C]c. After this concentration the

embryos are formed and the system becomes heterogeneous by a process of self-nucleation. The

nuclei grow giving place to a descent of the critical concentration.

3.4 Reverse Micelles Synthesis Model

Several models have been proposed to explain the formation process of ultrafine particles in

reverse micelles [62, 63, 64, 65, 66, 67]. On the one hand, there are some authors as Tanori and

Pileni [68], who suggest that micelles act as “nanoreactors”, inside which the reaction take place.

They found experimentally that the particles size and form can be controlled using colloidal

assemblies as template. However this mechanism seems to be contradicted by experiments that

yielded particles large than the droplets. On the other hand, authors like Towey [64] and Hirai

[65] proposed models based on different stages of the precipitation reaction to explain the particle

formation.

3.4.1 Precipitation Model of Towey

Towey [64] studied the kinetics and mechanism of formation of cadmium sulphide particles in

w/o-microemulsions using the stopped-flow method. The author was the first in proposing a

model to explain the particles formation in reverse micelles taking into account different stages.


Droplet exchange stage occurs inside of the water cores of two microemulsions. The reac-

tants are solubilized separately and are distributed among the droplets according to a

Poisson distribution. All the reactants are inside the cores, so the reaction can only occur

on exchange between the droplets.

Figure 3.3: Schematic representation of the mechanism for formation of nanoparticles in

w/o-microemulsion proposed by Towey [64].

Reaction stage occurs on a faster time scale than droplet exchange. Therefore, Towey [64]

considered that the communication between the droplets becomes in the limiting step of

the reaction.

Nucleation stage must occur at much smaller size than in homogeneous solution, because the

prepared particles in microemulsions have a size 1− 2 nm while in homogeneous solution

the nucleation occurs in the size range 1 − 10 nm [64]. This can be explained by the

low Ksp of the product. So it is expected that the critical supersaturation inside the

microemulsion is far exceeded than in homogeneous solution.

Growth stage may occurs by a Smoluchowski rapid-coagulation mechanism. Towey [64] as-


sumed a bimolecular reaction between unchanged spheres which make contact as a result

of Brownian motion. So the nuclei grow by aggregation of monomers units to form dimers.

3.4.2 Precipitation Model of Hirai

Hirai [65] proposed an quantitative model to explain the particle formation of titanium dioxide

by hydrolysis of tetrabutoxide in AOT. In this model the reaction rate is considered to be the

limiting step of the reaction.

Droplet exchange stage occurs inside the water cores via coalition and redispersion pro-

cesses. The exchange rate is constant and reported as 106−108 M−1s−1 for the AOT/isooctane

system [69].

Reaction stage is slower than the exchange of reactants. The reactants are distributed among

the micelles according to a Poisson distribution. Thus, the reaction rate is independent

of the exchange rate of the micelles.

Nucleation stage is considered to be proportional to the number of micelles containing a

sufficient number of hydrolyzed molecules for the nucleation.

Growth stage can occur in micelles containing a nucleus or a particle, in addition to hy-

drolyzed molecules.

In order to control the particle growth and size distribution, Schmidt [67], suggested that the

micellar structure, the synthesis conditions and the kinetics of the elementary steps are important

and proposed a three-steps model (reaction, nucleation and growth) based on a combination of

Hirai and Towey [64],[65].

Reaction stage is the step determining of the reaction and occurs in a slower time scale than

the droplet exchange (like than the model of Hirai [65]). The distribution of the reactant

inside the micelles follows a Poisson distribution and the reaction rate is considered to be

independent of the exchange rate of the micelles.

Nucleation stage is described similar to the Hirai model.

Growth stage is reported to be analog to the growth by the model of Towey. This means that

nuclei growth by bimolecular reaction.


Figure 3.3 shows the possible kinetic scheme for the formation of the nanoparticles in reverse

microemulsion. It is based on the model of Towey [64] and figure 3.4 is a plot of the monomer

concentration versus time to compare the formation of a particle in a homogeneous system

and in a microemulsion, which is based on the model proposed by Schmidt [67]. This model

considers that the diffusion occurs at a slower rate than in homogeneous aqueous system, due to

the micellar structure. In this fashion, the period of nucleation is longer and the critical value

of concentration is exceeded more than in a homogeneous system. This yields a higher number

of particles with smaller diameter in comparison with aqueous solution.

Time

[C]

[C] c

Homogeneous system

Microemulsion

Nucleation

Particle growth

Critical saturation

Super saturation

Figure 3.4: Formation scheme of a particle in a homogeneous system and microemulsion.

Course of concentration of the formed monomer C vs. time in a microemulsion compared

to a homogeneous system.

Chapter 4

DLVO-Theory

Adhesion or detachment of fine particles from a surface is of interest in several industrial appli-

cations, such as detergency [1], particle filtration [2], oil production [3]. During the last years

adhesion on semiconductor surfaces [4] and in biological systems such as cells or viruses have

been center of a lot of research [5].

There are two aspects that must be addressed in order to understand the behavior of two

solids across a fluid medium. First, the physicochemical interactions that keep the particle at-

tached to the surface. In other words, the nature and strength of the adhesive forces between the

particles and the surface. Second, the hydrodynamic interactions due to the flowing fluid, which

tend to break the adhesive bond between the surfaces and detach the particle. The major effects

acting on the physicochemical interactions are the attractive forces, like than London-van der

Waals and dispersion forces or electrostatic interactions that is usually composed of electrostatic

double layer interactions, solvation or structural forces and Born forces. These interactions can

be explained in terms of the Derjaguin Landau Verwey Overbeek theory named by its abbrevi-

ation as DLVO-Theory and are quite pronounced at distances around ten manometers between

solids. These issues will be treated in this chapter and the hydrodynamic interaction in chapter

5.

Derjaguin and Landau, as well as Verwey and Overbeek developed independently a theory

(DLVO-Theory) to explain the stability of colloid. They calculated the total potential energy

based on the sum of the potential energy due to electrical double layer repulsion (DLR) and

London van der Waals attraction (LW).

29

4.1. LONDON-VAN DER WAALS FORCES 30

4.1 London-van der Waals Forces

The interaction energy per unit area of van der Waals forces for a flat-flat system is calculated

by the followed equation 4.1 [70].

VLW =−A12

12 π h2(4.1)

Where A12 is the Hamaker constant, and h is the distance of separation between surfaces. Under

some conditions A may be related to the Lifshitz-van der Waals constant A′ = (4/3) π A. In

the case of a spherical particle-flat surface system, h is a function of the radius, as was shown

by Das [71].

Sometimes the force per unit area is tremendous, and particles or surfaces can be deformed

by such forces. The additional van der Waals force due to deformation FLWdeform is a function

of the increased contact area caused by the deformation and is given by

FLWdeform =A δ2

8 π h3(4.2)

Where δ is the radius of the adhesion surfaces area.

Figure 4.1: Impact of the particle size in the London-van der Waals forces. Comparison

between FLW and FLWdeform versus particle diameter [70].

Figure 4.1 shows a comparison between FLW and FLWdeform versus particle diameter and

surface materials, which have a Lifshiftz-van der Waals constants of 8 eV and 4 eV, respectively.

4.2. STRUCTURAL FORCE 31

This constant (A’) have ranges of 0.6 eV for polymers to about 9 eV for metals such silver and

gold and is proportional to the van der Waals forces [70]. It clearly shows that deformations

around 1-5% can add tremendously to the total force of adhesion. A first look at this plot could

be incorrectly interpreted in the way that the forces of adhesion simply decrease with decreasing

particle diameter. In fact the force per unit area increases with decreasing particle size, and the

force of adhesion also increases relative to the gravitational forces acting on the particle.

4.2 Structural Force

The origin of structural forces lies in the interaction between surfaces and the fluid molecules.

This interaction may take the form of orientation of dipoles or rearrangement of the molecular

packing. Due to these effects there exists a solvation zone up to a few molecular diameters

away from the solids surfaces where the fluid density oscillates around the bulk value. As the

separation between two solids surfaces becomes less than 1 nm, their solvation zones overlap

and as a result, the mean density of the intervening medium is not longer equal to the bulk

density and depend on the molecular characteristics of the confined fluid [72].

4.3 Born Force

When the distance of separation between the solid surfaces becomes extremely small the electron

of their surface atoms tend to overlap giving rise to very repulsive forces for separation distances

of the order of the hard sphere diameter of the atoms. This repulsive force named Born repulsion

(FBorn), is derived from the 12 part of the Lennard-Jones potential and is given by

FBorn =A σ6

45 h9(4.3)

Where σ is called the collision diameter.

4.4 Electrostatic Forces

Two types of electrostatic forces may act to hold particles at surfaces. The first is due to

bulk excess charge presents at the surface and/or particles which produces a classical coulombic

4.4. ELECTROSTATIC FORCES 32

attraction known as an electrostatic image force Fi.

Fi =q2

4 π ε0 κ l2(4.4)

Where κ represents the dielectric constant of the medium between the particle and surface, ε0

is the permittivity of free space, q denotes the charge and l is the distance between charged

centres [70].

The other electrostatic force is known as electrostatic double layer force FDLR and it can be

calculated (in dyne) as

FDLR =π ε0 r ψ

2

h(4.5)

Where ψ represent the electrostatic potential. Most solid materials acquire a charge due to

preferential absorption of ions or dissociation of surface groups when immersed in a liquid

medium. These surface charges are balanced by a diffuse layer of oppositely charged counterions

present in the liquid giving rice to an electrical double layer. The repulsive force between these

two charged surfaces arises due to the mutual repulsion between counterions.

The importance of the electrostatic double layer has led to numerous studies, and many

models were proposed in the past to explain it. The earliest model of the electrical double layer

is usually attributed to Helmholtz (1879). Helmholtz treated the double layer mathematically

as a simple capacitor based on a physical model, in which a single layer of ions is adsorbed at

the surface.

Later (1910-1913) Gouy and Chapman made significant improvements by introducing a

diffuse model of the electrical double layer, in which the potential at a surface decreases expo-

nentially due to adsorbed counterions from the solution.

The charge at the surface influences the ion distribution in nearby layers of electrolyte.

The electrostatic potential (ψ) and the volume charge density (ρ) are related by the Poisson

equation 4.6, in which ρ represents the excess of charge of one type over the other.

d2ψ

dx2= − ρ

ε0 κ(4.6)

The ion distribution in the charged surface region is determined by temperature and the required


energy (Wi) to bring the ion from an infinite distance away (where ψ = 0) to the region, where

the electrostatic potential is ψ. This distribution is given by a Boltzmann equation as follows.

ni = n0i exp

( −Wi

kB T

)

(4.7)

Where n0i is the number of ions of type i per unit volume of bulk solution, Wi = Zieψ and Zi

is the valency of ion species i.

The volume charge density at electrostatic potential (ψ) is expressed as

ρ =∑

i

ni Zi e =∑

i

n0i Zi e exp

(−Zi e ψ

kB T

)

(4.8)

Thus the combination of equation 4.8 and 4.6 gives the Poisson-Boltzmann equation

d2ψ

dx2= − 1

ε0 κ

∑

i

n0i Zi e exp

(−Zi e ψ

kB T

)

(4.9)

When kB � |(Zieψ)|, the exponential can be expanded and only the two first terms are retained.

This is called the Debye-Huckel approximation

exp(x) = 1 + x+x2

2!+x3

3!+x4

4!+ .... (4.10)

henced2ψ

dx2= − 1

ε0 κ

[

∑

i

Zi e n0i −

∑

i

Z2i e

2 n0i ψ

kB T

]

(4.11)

applying the preservation of electroneutrality∑

i Zi e n0i = 0 the equation 4.11 can be written

asd2ψ

dx2=

∑

(

Z2i e

2 n0i

ε0 κ kB T

)

ψ (4.12)

d2ψ

dx2=

ψ

λ2(4.13)

ψ = ψ0 exp(−h/λ) (4.14)


Where ψ0 is the potential at the surface and λ is the Debye length or (1/λ) the Debye-Huckel

parameter (d), which are related to the double layer thickness, F is the Faraday’s constant and

Ci represents the concentration of ions.

λ2 =ε0 R T

F 2∑

Ci Z2i

(4.15)

The Gouy-Chapman model has several limitations. For example, it supposes a low potential

and a low ionic concentration. This leads to extremely low λ − values (less than the atomic

diameter). On the other hand, an ion of the diffuse layer can not approach to the surface

at a distance smaller than its ratio (as solvated ion). Under this condition the Boltzmann

distribution can not be used. Finally, this model supposes that the dielectric constant of the

bulk is the dielectric constant of the solvent, but in fact, this parameter varies with the electrolyte

concentration.

Stern modified the model by suggesting the existence of an adsorbed ions layer of thickness

X1. The mathematical treatment is essentially the same than with Gouy and Chapman but

changing the reference. In this way, the equation 4.14 can be written as 4.16.

ψ = ψ1 exp[−(x− x1)/λ] (4.16)

Figure 4.2 shows all the possible cases. The first case corresponds to the solid interface with a

Stern adsorbed layer charged negatively, which is not enough to guarantee the electroneutrality.

Therefore the diffuse layer possesses the negative charge necessary. The second case shows an

excess of negative charges at the solid surface. In this case the diffuse layer has the positive

charge. The third case, the solid interface is charged negatively, the adsorbed layer has a positive

charge, which is not enough to counteract the negative charge and the diffuse layer provides the

rest of the positive charge. The last case represent the opposite possibility than the second

case [73].

The electrostatic potential can not be measured directly. Therefore it is necessary to carry

out electrokinetic experiments that allow the calculation of the electrokinetic potential (ζ), which

is situated to a distance X2 outside of the Stern layer. Normally the value of this electrokinetic

potential, known as zeta potential, is very similar than the potential of the Stern layer.


Figure 4.2: Model Gouy-Chapman-Stern.

Figure 4.3 presents a comparison of the van der Waals forces described in previous section 4.1

and electrostatic forces of particles adhesion versus particle diameter. It is clear, at least for

these ideal calculations that the van der Waals forces dominate over electrostatic forces for very

small particles. Double layer electrostatic force also generally dominates over electrostatic image

forces for small particles [70].

When a fine particle-plate system is immersed in water, the energy of adhesion is greatly

altered, decreasing to about one-fourth of the original [74]. This diminishing effect can be

explained by a new Hamaker constant A123 for interaction of the surface 1 with the particle 2 in

the water molecules 3, which was developed by Lifshiftz [75] on the basis of certain macroscopic

properties of the interacting materials and of the fluid medium. The van der Waals forces are

too strong for such system to be counteracted by hydrodynamic forces alone. However, in order

to predict the behavior in a system of two solids immersed in a liquid, it is necessary to know

the magnitude of these forces (van der Waals and electrostatic).

Hogg et al. [76] calculated the potential energy of the interaction between dissimilar flat

double layers using Debye-Huckel or linearized approximation. The energy of interaction per

unit area is given by

VDLR =κ d

2{(ψ2

1 + ψ22)[1 − coth(d h)] + 2 ψ1 ψ2cosech(d h)} (4.17)


Figure 4.3: Van der Waals forces in comparison to electrostatic forces of adhesion as

function of particle diameter [70].

where ψ1 and ψ2 are the zeta potentials of the plate 1 and 2, respectively, h the separation

distance between the plates, κ is the dielectric constant of the solution, and d is the Debye-

Huckel parameter. The sum of the van der Waals attraction and the double layer repulsion

constitutes the classical DLVO-theory of colloid stability. This theory has been used to predict

the effect of various factors such as, ionic strength, valence of counterions, solution pH and the

stability of colloidal dispersion.

Typical plot of the total interaction energy for a surfaces-soil system (VT ) as a function of

the separation distance h, looks as shown in figure 4.4. The total interaction energy is given by

the sum of the interaction energy of van der Waals (equation 4.1) and the interaction energy of

dissimilar double layer (equation 4.17).

When the surface potentials have the same sign, it is expected to reach a maximum at a

certain distance. High zeta potentials lead to higher VTmax. Therefore, high zeta potential should

aid in the particle detachment.


Figure 4.4: Total potential energy curve for superposition of van der Waals and electro-

static force.

Chapter 5

Detachment Mechanism

As already mentioned, understanding particle removal is important for many industrial pro-

cesses. Hydrodynamic particle removal involves the application of an external force to a particle

to overcome its force of adhesion. Efficient particle detachment is extremely difficult because

of the strong adhesion forces and the nature of the particulate. Particle removal methods are

generally classified into three categories.

Wet chemical methods consists in a treatment with chemical solutions, which is carried out

in order to remove the particles. For example via surface etching.

Mechanical techniques are related to the application of a physical force, such as contact with

a brush or hydrodynamic force to remove the particle.

Dry processes include treatments with gas/vapor and plasma techniques to cause the removal

of the particle.

Improvement of the already existing removal techniques and development of other more efficient

methods requires the understanding of the force acting on a particle. The DLVO-theory predicts

the force of adhesion in a small scale taking into account the van der Waals forces and the

electrostatic forces. This issue was discussed in the previous chapter 4. However, additionally

to these forces, there are other forces, which are generated when a hydrodynamic flow acts on a

particle. These forces depend on the flow condition near the attached particle and they act to

remove the particle from the surface.

Figure 5.1 shows a general model for a particle attached to a surface in a hydrodynamic

flow. There are three forces, adhesion (FA), drag or hydrodynamic (FH) and lifting (Fl) forces.

38

5.1. REMOVAL MECHANISMS 39

In addition, there exists a torque (T ) acting on the particle in this model. Hydrodynamic

detachment experiment provides an indirect method to determine the force of adhesion since it is

proportional to the hydrodynamic force as showed by Visser [9]. However, the exact relationship

between these two quantities can only be established once the mechanism of detachment has

been identified.

Figure 5.1: Diagram of forces acting on a particle. FH is the hydrodynamic force applied

to the particle by the fluid; FA is the adhesion force, Fl and T represent the lifting force

and torque, respectively.

5.1 Removal Mechanisms

Based on the above model (see figure ) there are three potential particle removal mechanisms

lifting, sliding and rolling . The criteria for removal by the different mechanisms are derived

from force and torque balance.

Lifting criterion is derived from a force balance in the vertical direction.

FL ≥ FA (5.1)

Sliding criterion is derived from a force balance in the horizontal direction, where µf is the

static friction coefficient.

FH ≥ µf (FA + FL) (5.2)

Rolling criterion is derived from a torque balance and may be modeled most simply according

to the scheme of figure 5.2, where l is the distance between the adhesion force vector and


the point around which rolling occurs and h represent the height of roughness.

T ≥ FA l (5.3)

Figure 5.2: Scheme of forces acting on a particle (A). Diagram of forces acting on a

particle FH is the force applied to the particle by the fluid; FA is the adhesion force, FH

is the hydrodynamic force, Fl and T represent the lifting force and torque respectively. A

length l serves as a lever arm and h represent the height of roughness.

If the particle, and/or the substrate are soft the treatment becomes more complicate because

is necessary to take into account the effects of the deformation of the system. Figure 5.3 shows

the forces acting on a particle under such conditions, where l1 is the distance between the

adhesion force vector and the point around which rolling occurs, while l2 is the distance between

the lift and adhesion forces vectors and the point around which rolling occurs. In this case the

criterion must take account the deformation according to the equation 5.4. This deformation

can increase the force of adhesion such as was shown in the figure 4.1.

T + FH l1 + FL l2 ≥ FA l2 (5.4)

Hubbe [77] found a way to relate the mechanism of incipient motion and the particle radius.

He pointed out that each mode of incipient motion imply different exponential dependency of the

shear stress required for particle removal. This dependency is related to the radius of spherical

particles.

In order to understand the hydrodynamic force acting on the particle it is necessary to

distinguish between laminar and turbulent conditions. Goldman et al. described a method to

determine the force and torque acting on a fixed sphere of radius R at a distance h > R from


Figure 5.3: Scheme of forces acting in a particle (B). Diagram of forces acting on a particle

FH is the force applied to the particle by the fluid; FA is the adhesion force, FH is the

hydrodynamic force, Fl and T represent the lifting force and torque respectively. l1 is the

distance between the adhesion force vector and the point around which rolling occurs,

while l2 is the distance between the lift and adhesion forces vectors and the point around

which rolling occurs.

a plane wall in a viscous fluid, whose motion in absence of the sphere would be uniform linear

shear flow. They found an exact mathematical solution of the Stokes equation for this case [6, 7].

O’Neill based on the Goldman results presented an exact solution of the linearized Stokes flow

equation for the case when h = R [8]. These relations are given by

FH = 1.701 (6π) ηR Vx (5.5)

T = 0.944 (4π) ηR2 Vx (5.6)

where R is the particle radius, η is the fluid viscosity, T is the torque and Vx is the fluid velocity

in the x direction. Visser [9] expressed this tangential force in terms of the shear stress acting

on the wall as

FH = 32 R2 τ (5.7)

where τ is the shear stress at the wall. From hydrodynamic it is known that the shear stress is

proportional to the velocity gradient in the x-direction (dv/dx).

τ = η

(

dv

dx

)

(5.8)


For a laminar flow the velocity gradient is constant; consequently equation, 5.8 can be directly

integrated as

Vx =

(

τ

η

)

R (5.9)

Introducing equation 5.9 into equations 5.5 and 5.6 permits to obtain the relationship between

the hydrodynamic force or the torque and the shear stress, respectively.

FH = 32.06 R2 τ (5.10)

T = 11.86 R3 τ (5.11)

Cleaver and Yates [10] shown that a lift force exist due to the unsteady nature of the viscous

sublayer in the turbulent boundary layer, and proposed an equation to calculate this force.

Experiments on the latter [78] suggested that the results should be

FL = (0.304) ρ ν1/2 τ3/2 R3 (5.12)

The mechanism of incipient motion for an ideal smooth sphere attached to a substrate and

exposed to a flow can be identified from the relationship between the particle radius (R) and the

shear stress (τ). If the mechanism is sliding, the force is proportional to the force of adhesion

as follows

Fsliding = α FA (5.13)

Substitution of equation 5.7 into equation 5.13 (with FH = FA) and taking into account that

FA ∝ R as was shown by Hubbe [77] yield

τsliding =α FA

32 R2∝

R

R2∝ R−1 (5.14)

If the mechanism is assumed to be rolling then, taking the equation 5.9 and inserting it into

the equation for the torque 5.6 yield

T = 11.86 R3 τ = FA l (5.15)

the above relationship between the parameter R and l depend on the characteristics of the

experimental system (See figure 5.2). Hubbe [77] found that the relationship for smooth surface

is given by

l = b√R (5.16)


1

1E03

1E06

1E09

1E-08 1E-06 1E-04 1E-02 1

She

ar S

tres

s

Radius (m)

Rolling (soft) and LiftingRolling (hard)

Sliding

Figure 5.4: Graphic of the shear stress vs. particle radius for the different mechanisms of

incipient motion.

where b is an empirical constant and the relationship for hard surfaces is

l ∝ R3 (5.17)

The relationship between the shear stress and the particle radius for smooth and hard surfaces

is given by substituting equations 5.16 and 5.17 into equation 5.15 as follows

τhardrolling =

Fd l

11.86 R3∝

R R1/2

R3∝ R−3/2 (5.18)

τ softrolling =

Fd l

11.86 R3∝

R R2/3

R3∝ R−4/3 (5.19)

If the mechanism is assumed to be lifting, the same treatment for equation 5.12 give the rela-

tionship for the lifting shear stress and the particle radius 5.20. Figure 5.4 is a graphic of the

shear stress versus the particle radius for the different mechanisms.

τlifting ∝F

2/3d

c2/3 R2∝

R2/3

R2∝ R−4/3 (5.20)

Chapter 6

Experimental Part

6.1 Chemical Materials Employed

In order to achieve the experimental part, table 6.1 contains the employed surfactants and table

6.2 lists the principal physicochemical properties of the distilled water used in all experiments.

Table 6.1: List of the employed Surfactants

Surfactants Supplier

Marlipal O13/40 Sasol





Igepal CA-520 Aldrich

Table 6.2: List of Physicochemical Properties of distilled Water at 20 oC

η (dynes.s/cm2) ρ (g/ml) γ (dynes/cm)

water 10.09 × 10−3 0.998 71.9

In addition, the applied chemicals for the experiments are shown in table 6.3. The glass sub-

strates to be coated were supplied by Mezel-Glaser. Plexiglass was used to build the equipment

in order to test the particle detachment.

44

6.1. CHEMICAL MATERIALS EMPLOYED 45

Table 6.3: List of the Chemical MaterialsChemical Substance Formula Purity Supplier

Ammonium NH3 · H2O 25% FlukaPeroxide H2O2 30% Fluka

cyclohexane C6H6 > 99% RothStrontium acetate monohydrate Sr(CH3COO)2 · H2O > 99% FlukaCalcium acetate monohydrate Ca(CH3COO)2 · H2O > 99% FlukaMangan acetate tetrahydrate Mn(CH3COO)2 · 4H2O > 99% Fluka

Sodium hydroxide NaOH > 98% MerckPalladium(II) Chloride PdCl2 > 99% Chempure

Sodium Chloride NaCl > 99% FlukaSodium hypophosphite monohydrate NaH2PO2 · H2O > 99% Fluka

Ethanol CH3CH2OH > 99% FlukaIron(III) nitrate nonahydrate Fe(NO3)3 · 9H2O > 99% FlukaYttrium nitrate hexahydrate Y (NO3)3 · 6H2O > 99% Fluka

Ammonium carbonate (NH4)2CO3 > 99% FlukaHeptane C7H16 > 99% MerckOctane C8H18 > 99% Merck

Zirconium(IV) oxide chloride ZrOCl2 · 8H2O 99% Merckn-Butanol C4H9OH > 99% Merck

Sulfuric acid H2SO4 97% FlukaNitric acid HNO3 65% Merck

Hydrochloric acid fuming HCl 37% Fluka

Distilled water and nonionic surfactants of the alkyl poliglycolether type were employed as

eluent fluids. The commercial surfactant name is Marlipal and was supplied by Sasol Germany

GmbH. The number of carbons in the aliphatic chain as well as the ethoxylation degree are

indicated by the label after name, e.g. Marlipal O13/04. This means that the surfactant has 13

carbons in the lipophilic chain and an average ethoxylation degree of four. The physicochemical

properties of the fluids are summarized in the table 6.4. It is important to emphasize that due to

the production process, the resulting surfactants have a distribution of their ethoxylations degree

and the name only reflects an average of their ethoxylation degree. In the rest of the work, these

homologous series of surfactants are called as CnEOm, where n represents the number of carbons

in the hydrophobic chain and m is the number of oxyethylene groups (EO). The abbreviation

for the previous example is C13EO4.

6.2. THE REACTOR FOR THE SYNTHESIS OF NANOPARTICLES 46

6.2 The Reactor for the Synthesis of Nanoparticles

In order to perform the synthesis of nanoparticles, a semi-batch reactor was used. Figure 6.1

shows a scheme of the employed reactor, which consists of a tank with four baffles, it can contain

up to 200 ml of reaction mixture. The agitator is a four pitched blade turbine impeller, the feed

input was located near to the agitator and above of the liquid level. The stirring rate U (min−1)

and feed rate q (ml/s) were chosen according to the procedure shown by Schmidt [67].

Figure 6.1: Scheme of the reactor to achieve the synthesis.

The synthesis was achieved by loading 100 ml of one microemulsion “A” to 100 ml to

the reactor and adding 100 ml of microemulsion “B” at a constant feed rate of 0.30 (ml/s).

Temperature was controlled using a ultrathermostat K6 suppled by Colora Messtechnik. A

peristaltic pump (Besta E100) was utilized to the addition of one microemulsion to the other

one.

Table 6.4: Physico-chemical Properties of the employed Surfactants†Calculated using the parameters given by P. Huibers [79]

Marlipal Abreviation HLB † cmccal (M) Concentration (M)O13/40 C13EO4 9.7 2.36 × 10−5 0.002 / 0.012 / 0.024O13/60 C13EO6 12.1 2.89 × 10−5 0.002 / 0.012 / 0.024O13/70 C13EO7 12.7 3.19 × 10−5 0.002 / 0.012 / 0.024O24/70 C24EO7 9.8 1.12 × 10−10 0.002 / 0.012 / 0.024

6.3. SYNTHESIS OF PEROVSKITE NANOPARTICLES 47

6.3 Synthesis of Perovskite Nanoparticles

The synthesis of Perovskite Ca0.5Sr0.5MnO3 was carried out via co-precipitation in microemul-

sion according to the procedure shown by Lopez [80]. The reaction was carried out using sodium

hydroxide (NaOH) and peroxide (H2O2) as oxidizing agent. Two microemulsions, “A” and “B”,

with identical composition but with different aqueous phases were prepared. In microemulsion

“A” the aqueous phase was a solution of calcium acetate Ca(CH3COO)2 · H2O, strontium ac-

etate Sr(CH3COO)2 · H2O, and manganese acetate Ma(CH3COO)2 · 4H2O in stoichiometric

ratio whereas the microemulsion B was a solution containing the oxidizing agent. The reaction

can be written as the following equation

4 NaOH + Mn+2 + H2O2 � MnO+23 + 3 H2O (6.1)

The composition of the microemulsions is shown in the table 6.5. Cyclohexane and Mar-

lipal O13/70 were used as oil phase and surfactant, respectively. In order to find the best

microemulsion composition for preparing the precursor of Perovskite, the phase behavior of dif-

ferent systems were determined applying the procedure described in [81]. The detailled study

was shown in [80], where was also to synthesize the perovskite via oxidation with ammonium.

Table 6.5: Composition of the microemulsions for the perovskite nanoparticles. Mi-

croemulsion A (µEA) and microemulsion B containing the salts and oxidation agent,

respectively. Ac denotes the acetate (CH3COO−).

Surfactant Oil phase Aqueous phase

µEA Marlipal O13/70 cyclohexane Ca(Ac)2 + Sr(Ac)2 [0.12] mol/l

+ Mn(Ac)2 [0.24] mol/l

Weight (g) 15 74.38 10.62

µEB Marlipal O13/70 cyclohexane NaOH [0.96] mol/l + H2O2 [0.12] mol/l

Weight (g) 15 74.38 10.62

The reaction was carried out in the semi-batch reactor described above with a stirring rate

(1000 min−1). The precipitation was performed by adding the microemulsion containing the

salts to the microemulsion containing the oxidant such as was suggested in the reference [67].

The feed rate was 0.05 ml/s at 27oC. The amount of precharged solution in the reactor was

100 ml. The overall amount of added microemulsion was 100 ml. The reaction mixture was

6.4. SYNTHESIS OF PALLADIUM NANOPARTICLES 48

stirred for another 30 minutes. Subsequently, the particles were separated with a centrifuge at

5000 rpm for 10 min. Precipitated particles were washed three times with methanol/water and

dried at (110oC) for (10 h). Size and the morphology of the precursor particles were studied

using scanning electron microscopy (SEM). Finally, the precursor was calcined to Perovskite at

different temperatures and analyzed by X-ray diffraction.

6.4 Synthesis of Palladium Nanoparticles

The synthesis of palladium nanoparticles was carried out following the process described by

Schmidt [67]. Two microemulsions with identical composition (W0 = 9, α = 0.94 and γ = 0.175)

but with different aqueous phases were prepared. In the microemulsion A (µEA) the aqueous

phase was a solution of palladium salt and in the microemulsion B (µEB) the aqueous phase

was a solution of the reducing agent. The oil phase was cyclohexane and the surfactant Marlipal

O13/60.

Table 6.6: Composition of the microemulsion for the palladium nanoparticles. Microemul-

sions A (µEA) and B (µEB) containing the complex salts and the reduction agent.

Surfactant Oil phase Aqueous phase Wo

µEA Marlipal O13/60 cyclohexane Na2PdCl4 · 2 NaCl[0.2] mol/l 11

Weight (g) 17.5 76.54 5.96

µEB Marlipal O13/60 cyclohexane Na2H2PO2 · H2O[0.6] mol/l 11

Weight (g) 17.5 76.54 5.96


Weight (g) 17.5 79.19 3.31


Weight (g) 17.5 79.19 3.31


Weight (g) 17.5 80.51 1.99


Weight (g) 17.5 80.51 1.99

6.5. SYNTHESIS OF YIG NANOPARTICLES 49

The reaction was performed at constant temperature (27 oC), stirring rate (1200 min−1)

and feed rate (0.5 ml/s). The microemulsion composition was varied in the reaction in order to

obtain particles with different average diameter. The compositions are shown in table 6.6 for

W0 = 11, W0 = 9 and W0 = 3, respectively.

Palladium (II) chloride was employed as reactant, but due to the fact that the solubility of

this palladium salt is low in water, the reaction was achieved starting from a complex salt of

sodium tetrachloropalladate (Na2PdCl4 ·NaCl) according to the following reaction

PdCl2 + 4 NaCl −→ Na2PdCl4 · 2 NaCl (6.2)

The reduction was achieved using sodium hypophosphite monohydrate (NaH2PO2 ·H2O), which

has a potential of 0.499 V for acidic solutions and −1.565 V for alkaline solutions as shown

equations 6.3 and 6.4, respectively.

H3PO2(ac) + H2O � H3PO3(ac) + 2 H+ + 2 e− 0.499 V (6.3)

HPO2−3 + 2 H2O + 2 e− � H2PO

−2 + 3 OH− − 1.565 V (6.4)

The reaction for the palladium has a potential of 0.987 V and can be written as follows

Pd+2 + 2 e− � Pd 0.987 V (6.5)

The total reaction is given by

Pd+2 + H3PO2 + H2O � Pd + H3PO3 + 2H+ (6.6)

6.5 Synthesis of YIG Nanoparticles

Yttrium iron garnet (YIG) is a material used widely in electronic devices for the microwave

generation as well as for digital memories, which require particles with a strict control of com-

position, homogeneity, size and shape. Therefore the technique to produce such particles of YIG

is important. Vaqueiro et al. [82] synthesized YIG with the above mentioned features, which

are also desirable to study the adhesion of nanoparticles to glass surface. The method is based

on a co-precipitation of the precursors in a microemulsion according to the following reaction

5 Fe(NO3)3 + 3 Y (NO3)3 + 15 (NH4)2CO3 + 27 H2O −→

Y3Fe5O12 + 30 NH4OH + 15 CO2 + 24 HNO3 (6.7)

6.6. SYNTHESIS OF ZIRCONIA NANOPARTICLES 50

In order to achieve the reaction, a solution containing the salts of iron (III) nitrate nonahydrate

(Fe(NO3)3 · 9H2O) and yttrium (III) nitrate hexahydrate (Y (NO3)3 · 6H2O) were prepared

with a concentration of 0.2 mol/l and 0.12 mol/l, respectively. The solution containing the

precipitating agent ammonium carbonate (NH4)2CO3)) was prepared with a concentration of

0.6 mol/l. Igepal CA-520 (pentaethyleneglycol monoisonyl phenylether) was employed as surfac-

tant and heptane as oil phase to prepare the microemulsions. Table 6.7 shows the composition

of the microemulsion with W0 = 5, α = 0.87 and γ = 0.38 calculated from equations 2.2, 2.3,

and 2.4. The precipitation was carried out in the semi-batch reactor (see figure 6.1) at a constant

rate (0.05 ml/s) and stirring rate (1600 min−1).

Table 6.7: Composition of the microemulsion for YIG. Microemulsions A (µEA) and B

(µEB) containing the salts and the precipitant agent, respectively.


µEA Igepal CA-520 heptane Fe(NO3)3[0.2] mol/l + Y (NO3)3[0.12] mol/l

Weight (g) 38.13 53.87 8

µEB Igepal CA-520 heptane (NH4)2CO3[0.6] mol/l

Weight (g) 38.13 53.87 8

6.6 Synthesis of Zirconia Nanoparticles

Nanoparticles of zirconia (ZrO2) were prepared by a modification of the method developed by

Guo et al. [83]. The method described in [83] uses a mixture of AEO9 (C14EO9) surfactant and

n-butanol in a 3:2 proportion, which has a HLB of 11.2.

In the present work a mixture of Marlipal O13/80, which has a HLB around 12,5 calculated

with the equation 2.12, and n-butanol in a 3:2 ratio was used. In order to achieve the reaction,

two identical microemulsions were prepared, one containing the aqueous zirconium (IV) oxide

chloride (ZrOCl2) solution and the other one containing the aqueous precipitant ammonium

(NH3 ·H2O). Octane was used as oil phase. The composition of the microemulsions is shown

in table 6.8.

The reaction 6.8 was carried out adding the microemulion containing the precipitant to the

microemulsion containing the salt of zirconium. The temperature was kept constant (28 oC),

6.7. CHARACTERIZATION METHODS 51

the feed rate was 0.05 ml/s and the stirring rate was 1000 min−1.

ZrOCl2 · 8H2O + NH4OH −→ Zr(OH)4 + NH+4 + 2 Cl− + H+ + 6 H2O

Zr(OH)4 −→ ZrO2 + 2 H2O ↑ (6.8)

Table 6.8: Composition of the microemulsion for Zirconia. Microemulsions A (µEA) and

B (µEB) containing the zirconium salt and the reduction agent, respectively.


µEA Marlipal O13/80:n-butanol (3:2) octane ZrOCl4 · 8 H2O[0.5] mol/l

Weight (g) 30 64.83 5.17

µEB Marlipal O13/80:n-butanol (3:2) octane NH4OH[0.5] mol/l

Weight (g) 30 64.83 5.17

6.7 Characterization Methods

The received samples were characterized in this investigation by Scanning Electron Microscopy

(SEM), X-Ray Diffraction (XRD) and BET-adsorption. A brief description of these techniques

is given in further subsections.

6.7.1 BET Adsorption Characterization

Powders of the samples were analyzed by nitrogen adsortion using BET technique (Brunauer-

Emmett-Teller) to obtain their specific surface areas. In general, gas adsorption techniques

may be employed to measure the specific surface area and pore size distribution of powders

or solid materials. In order to realize the measurement, all gas has to be evacuated from a

dry sample by applying a vacuum and elevated temperature. Afterwards, the sample is cooled

to a temperature of 77K, which is the temperature of liquid nitrogen. At this temperature,

inert gases like nitrogen, argon and krypton are adsorbed on the surface of the sample. This

process can be considered to be a reversible condensation or layering of molecules on the surface,

which involves heat. In this analysis, nitrogen N2 was used as gas for measuring the surface

area in a Micromeritics Gemini with an analysis mode in equilibration, evacuation time of


1 min, equilibrium interval of 5 sec, and saturation pressure of 781 mmHg. The output of

the instrument is an isotherm, which is a plot of the volume of gas adsorbed (v cm3/g) versus

the relative pressure (i.e., sample pressure/saturation vapor pressure, P/P0). Using relative

pressure to construct the isotherm eliminates the effect of changes in pressure for small change

in temperature [84]. The relative pressure is scaled from 0 to 1. When all of the available surface

is filled with the gas (i.e. a monolayer of adsorbed gas on the surface is formed), the slope of

the isotherm becomes much smaller as a second layer is formed on the top of the first layer.

The surface area determination is carried out taking one or more data points of the adsorption

isotherm. The BET equation 6.9 is employed to obtain the volume of gas needed to form a

monolayer on the surface of the sample. The actual surface area is calculated from the size and

number of the adsorbed gas molecules, in this case N2 to 77 K has a size of 16 A2.

P

v (P0 − P )=

1

vm c+

c − 1

vm c

P

P0(6.9)

Here P0 is the vapor pressure of the adsorbed gas at the experimental temperature, vm represents

the adsorbed volume of monolayer an c is a constant. The parameter c and vm can be obtained

from the slope and the Y interception, respectively.

6.7.2 SEM Characterization

Powders of the samples and coated surfaces were characterized to obtain information about

the size and morphology, using a Hitachi S-520 Scanning Electron Microscope (SEM), which

provides topographical and elemental information about the samples.

The mean components of the device are depicted in figure 6.2 (left) while the optical system

is described in the right hand. The basic idea behind scanning electron microscopy is the

generation of a electron beam by an electron gun at the top of the column (See figure 6.2). This

beam is focused by two magnetic lenses inside the column and is directed to the sample in a

very small spot. When the electrons hit the sample, several processes take place. For example,

some of them can be absorbed, some are scattered out of the sample, etc. These processes can

be used to obtain information about the sample, if they are collected by a suitable detector.

The electron beam is not stationary, it can scan across a certain area in a raster pattern as in a

TV screen. The signal from the detector is selected to modulate the intensity on a view screen,

which is synchronously scanned with the beam in the column. Thereby an image is generated


on the viewing screen. A high brightness in the image corresponds to the areas with a strong

signal of electrons while a dark image is obtained in zones with weaker signal [85].

Vacuum System

Electron gun

Column

Sample Chanber

Power Supplies and electronics

Controllers

Screen Photo- screen

Electron gun

Condenser lens

Condenser aperture

Electron beam

Objective lens

Sample

Detectors

Figure 6.2: Scheme of the SEM instrument.

6.7.3 X-Ray Diffraction Characterization

Polycrystalline powders of the samples and coated surface were characterized using an X-ray

diffractometer Siemens D500 with Cu anode (radiation Kα of λ = 0.154 nm).

The wavelength of the X − rays (0.1 A) to (100 A) has sufficient energy to penetrate solids.

This feature permits to analyse the internal structure of solids. XRD can be used to identify

bulk phases, to monitor the kinetics of bulk transformation and to estimate particle size [86].

The instrument consists of a X − ray source, which contains a target that is bombarded

with high-energy electrons. When the target is hit by the electrons, they emit a continuous

background spectrum, which has two sharp peaks superimposed. These peaks are very narrow,

and their wavelengths are characteristic for the used target metal (for example, Cu has a Kα1

with a λ = 0.154 nm) [87].

When the X-rays incident on a crystal, the crystal can act as a lattice capable of diffracting

the radiation. The X-rays scattered by atoms in an ordered lattice interfere constructively in

directions given by Bragg’s law. Figure 6.3 illustrates how the lattice spacing can be derived

from the X-ray diffraction using the Bragg’s relation as

n λ = 2 dhkl Sinθ n = 1, 2, 3, ... (6.10)

6.8. SURFACE CLEANING PROCESS 54

where λ is the wavelength of the X-ray, dhkl is the distance between two lattice planes, θ

represents the angle between the incoming X-ray and the normal to the reflecting lattice plane,

and n is called the order of the reflexion.

The detector collects the intensity of the diffracted radiation as a function of the angle θ

between the incoming and refracted beams, which permits to calculate the distance between a

family of planes identifying the respective structure. Furthermore, the patterns can be compared

with a data base (Eva program) to identify the material. Additionally, the fitting program of

the diffrac− AT “profile V1.1” approximates the peaks to the better mathematical function

(Gauss, Lorentz, etc.) and defines the parameters of the peaks (location, height and width).

The program calculates the Full Width at the Half Maximum (FWHM) in 2θ degrees. If this

value is transformed into radians it is possible to calculate an average crystallite size < L >

applying the Debye-Scherrer’s equation 6.11.

〈 L 〉 =K λ

β Cosθ(6.11)

Where K is a constant (often taken as 1), λ represents the X-ray wavelength, β and θ are the

full width at the half maximum (in rad.) and the Bragg angle, respectively.

Figure 6.3: Schematic representation of the reflection in a lattice plane.

6.8 Surface Cleaning Process

The glass surfaces were cleaned employing the process described by Lopez [88]. This process is

based on cleaning the glass surfaces with different solutions. First, a mixture sulfuric acid and

6.8. SURFACE CLEANING PROCESS 55

Immersion of the surfaces in a solution of H 2 SO 4 (95%) and H 2 O 2 (30%) 50% v/v

during 3 days

Rinsing of the surfaces with distilled water until the pH of the solution is stable

Immersion of the surfaces in a solution of HNO 3 (5 mol/l) during 3 days

Immersion of the surfaces in a solution KOH or NaOH (4 mol/l) during 3 min. with

heating at 55˚C

Removal of the excess of KOH with distilled water Immersion of the surfaces in a solution of HNO3

(5 mol/l) during 3 days

Rinsing of the surfaces with distilled water until the pH of the solution is stable

Drying in a furnace at 60˚C for 8 h.

Figure 6.4: Diagram of the cleaning process.

hydrogen peroxide (50% v/v) followed by rinsing the surfaces with sufficient amount of distilled

water to reach a constant pH of the solution. Second a solution of nitric acid and rinsing the glass

surfaces with distilled water. Immersion of the surfaces in a solution of potassium hydroxide or

sodium hydroxide, rinsing the surface with distilled water. Third, Immersion in a solution of

nitric acid and rinsig with sufficient distilled water. Afterwards a drying process in a furnace at

60 oC for eight hours takes place. Flow Chart of the cleaning process is giving in figure 6.4.

6.9. COATING OF THE SURFACE WITH THE NANOPARTICLES 56

6.9 Coating of the Surface with the Nanoparticles

Nanostructured coatings on solid substrates have a significant importance due to the possibility

of synthesis of materials with specific physical-chemical properties such as magnetic, catalytic,

electronic, mechanical etc. These properties are attractive for several industrial applications,

therefore the efforts to prepare coated surfaces have been increased.

Coated Surface

XRD, SEM and BET Analyses

Synthesis of the Particles in w/o- microemulsions

Surface Cleaning

Coating of the Surface with the microemulsion

Centrifugation 2500 rpm for 5 min.

Drying at 110 ˚C for 10 h.

Washing with water:methanol (3:1)

Heat Treatment

Fine powders

Immersion in water:methanol (3:1)

Procedure to obtain fine powders

Procedure to obtain coated surfaces

Figure 6.5: Diagram of the coating process.

The particles for coating the surfaces were prepared according to the synthesis described in

6.10. DESIGN OF THE DETACHMENT DEVICE 57

previous sections. The method reported by Lopez et al. [89] was applied to coat the glass surfaces

with the different synthesized materials. This method used the properties of the microemulsions

to improve the deposition of the fine particles on the surfaces. The laboratory scale-method

involved five basic steps. Cleaning of the glass surfaces [88], synthesis of the nanoparticles in

w/o-microemulsions, coating, drying and calcination).

The glass surfaces were coated with a defined volume of microemulsion (0.2 ml/cm2). Af-

terwards, the surfaces were dried in a furnace at 100oC for 10 hours. The surfaces were briefly

immersed in a mixture of water methanol (3:1) to remove the excess of particles. Figure 6.5

shows an scheme of the employed procedure. The process permits to obtain powder of the

samples as well as coated surface with nanoparticles depend on the followed way.

6.10 Design of the Detachment Device

As was already mentioned in chapter 5 the particle removal methods are classified into three

categories wet chemical methods, mechanical techniques and dry processes. In this

work, a combination of the wet chemical methods and mechanical techniques was developed to

carried out the detachment of the nanoparticles from the glass surfaces. The fusion of these

two methods permit to obtain the benefits of both techniques. By controlling the chemical

composition of the cleaning solution and applying a hydrodynamic force.

It is well known that attraction between particles and surfaces are generally lower in liquids

compared to gases [90, 74]. On the other hand, particulate soil in the size range of 0.1-1 µm

is frequently unremovable by water alone, but it is removable by detergent solution. Therefore

a device was designed to test the detachment of fine particles from glass surfaces taking into

account both fluid motion and surfactant addition. A schematic diagram of the apparatus is

shown in figure 6.6. It consists of a cylinder of plexiglass with inner diameters of 3.0 cm and

length of 30 cm. The sample holder was placed in the center of the cylinder in order to avoid

the boundary conditions at the beginning and the end of the tube as well as near the walls.

Eluent flow through the cylinder was pumped from the reservoir using a submerged pump,

Neptuno TK 260. The volumetric flow Q was monitored by two flowmeter. One rotameter

measures low volumetric flows 30 (l/h) < Q < 200 (l/h), this means laminar condition with a

Reynolds number below 2300, which provides a force about 58 dyne. The other one monitors

high volumetric flows with turbulent conditions, that means 210 (l/h) < Q < 800 (l/h), 2329 <


Figure 6.6: Schematic diagram of the adhesion device. (1) Reservoir, (2) submerged

pump, (3) flowmeter for laminar conditions, (4) flowmeter for turbulent conditions and

(5) cylinder of plexiglass with sample holder.

Re < 9700 and a maximal force around 1120 dyne.

Figure 6.7 shows profiles of laminar and turbulent conditions. From the equations of volu-

metric flow 6.12 and continuity eqn. 6.13, it is known that the volumetric flow is the same at

the beginning Q0, the middle Q1 and the end of the cylinder Q2

Q =

∫

Av dA (6.12)

Q0 = Q1 = Q2 ⇒ v0 A0 = v1 A1 (6.13)

where v is the velocity and A the area of the cylinder. The velocity in laminar conditions is given

by the Stokes equation 6.14 and depends on the variation of the pressure, viscosity η length of

the cylinder L and the radius R.

v =P0 − P2

4 η L(R2 − r2) (6.14)

With introduction of equation 6.14 into equation 6.12, we obtain a relationship between the

cylinder parameters and the volumetric flow known as Poiseuilles equation 6.15 which allows

the calculation of the applied force under laminar conditions (eqn. 6.16).

Q =π (P0 − P2) R

4

8 η L(6.15)


Figure 6.7: Profiles of laminar(a) and turbulent (b) flows

∆ P =−8 Q η L

R4(6.16)

Poiseuilles equation is not valid in turbulent conditions, 2320 < Re < 10−5. Therefore, for

calculating the force acting in the cylinder under these conditions, it is necessary to use the

Darcy-Weisbach equation 6.17

∆ P =f L ρ v∗2

4 R(6.17)

where v∗2 is the mean velocity and f is a parameter that depends on the drag in the cylinder

and is given by the equation 6.18 [91]

f = 0, 3164 (Re)−1/4 (6.18)

Detachment of fine particles was carried out inside the designed device depicted in figure 6.6

and using different solutions of nonionic surfactants of the alkylpolyglycolether type as fluids.

The solutions were prepared from distilled water as solvent and commercial surfactants listed

in table 6.1 as solute. It is important to emphasize that due to the production process, the

resulting surfactants have a distribution of their ethoxylations degree and the name only reflects

an average of their ethoxylation degree. In the rest of the work, these homologous series of

surfactants are called as CnEOm, where n represents the number of carbons in the hydrophobic

chain and m is the number of oxyethylene groups (EO). The abbreviation for the previous

example is C13EO4.

6.11. TEST PROCEDURE 60

6.11 Test Procedure

It is well known that the light is attenuated by absorbed materials according to

IT = Ii exp (−α b)

T =ITIi

(6.19)

where IT and Ii are the incident and transmitted light, respectively. The sample thickness is

denoted by b. The attenuation coefficient (α) is given in equation 6.20.

Figure 6.8: Schematic representation of the test process.

α = N Cext = N Ca + N Cs (6.20)

This equation take into account the number of particles N, as well as, the extinction coeffi-

cient due to absorption Ca and the extinction coefficient due to scattering Cs. The attenuation

coefficient could be also described in terms of extinction coefficient per unit particle volume v

[92]. It is known as volume attenuation coefficient αv

αv =Cext

v(6.21)

Equation 6.19 allows to calculate the attenuation coefficient which is proportional to the

particle number N. Here particle number was not calculated but the percentage of the particles

on the substrate is given by

% Particles on Surface =αx

α0100 (6.22)

6.11. TEST PROCEDURE 61

Absorption of the coated surface was measured employing an U.V. spectrophotometer Uvicon

810. From the Transmission measurements it was possible the calculation of the attenuation

coefficient (αo), which corresponds to 100 per cent of particles at the surface. Afterwards,

the first detachment test was carried out with the lowest force that can be applied by the

device. The substrate was left inside the apparatus during a fixed period of time. After this

time, the transmition was measured again and the corresponding attenuation coefficient α1 was

determined. This process is repeated until the particles were completely removed or until the

maximum force, which can be applied with the device, is reached. Figure 6.8 shows an scheme

with the detachment test procedure.

Chapter 7

Results and Discussion

The detachment test explained in the previous chapter was carried out in order to study the

influence of the different experimental parameters (concentration, time, temperature, etc.) on

the release of fine particles from glass surfaces. The evaluation and discussion of the results are

presented in this chapter, which has been organized as follows. Section 7.1 contains the results of

the characterization of the utilized nanoparticles to investigate the detachment. Measurements

and discussions corresponding to the variation of the experimental parameters are shown between

the sections 7.3 and 7.8 as time, surfactant concentration, ethoxylation degree, temperature,

hydrophobic group, and particle size. Additionally, section 7.9 analyses the influence of the

kind of material on the detachment from the glass surface. Afterwards, section 7.10 presents

experimental results related to the hydrodynamic parameters. And finally, section 7.11 provides

a discussion about the proposed release mechanism.

7.1 Characterization of the Samples

7.1.1 Perovskite Nanoparticles

The figure 7.1 shows the behavior of the system water-oil-surfactant versus temperature. It is a

typical ternary phase diagram at a constant surfactant mass fraction (γ) (Compare with vertical

section at constant γ in fig. 2.3). The one phase region appears as a a channel extended from

the oil-rich region to the water-rich side of the phase diagram. In this region, the system is

a thermodynamically stable dispersion, which is optically transparent and has a low viscosity.

62

7.1. CHARACTERIZATION OF THE SAMPLES 63

This one-phase region is surrounded by two phase regions.

5

10

15

20

25

30

35

40

45

50

0.84 0.86 0.88 0.9 0.92 0.94 0.96

Tem

pera

ture

(ce

ntig

rade

)

alpha

CH/S/MCH/O/M

Figure 7.1: Binary phase diagram of the microemulsion systems. (CH) cyclohexane, (M)

Marlipal O13/70, (S) solution with the salts and (O) Solution with NaOH and peroxide

The cyclohexane was chosen as oil component to prepare the system due to its high solu-

bilization capacity for water in microemulsions compared to other oils such as hexane, octane

and iso-octane. As observed by Schmidt in [67]. The results offer a broad working range for

temperature and composition of the microemulsions.

It is important to emphasize that the synthesis of the perovskite is extremely affected by

the washing and drying processes, due to the wide range of oxidation states of the manganese.

Therefore it is advisable to remove the excess of hydroxide before calcination. In addition, the

calcination process does not have to exceed the temperature of 600 oC, which is an advantage in

preventing the sintering of the particles. Figure 7.2 illustrates the resulting powder of perovskite

with a mean diameter of 40 nm.

This result (40 nm) is in accordance to the measurements obtained using the equation

6.11. In figure 7.3, the diffractogram shows the peaks corresponding to the perovskite with a

pseudo-cubic structure comparable to those of the primitive perovskites. In this diffractogram

appear some peaks related to the other oxide of manganese. Table 7.1 lists the parameters of

the structure of a powder sample of perovskite. Note that the obtained distances between the

family of planes for the sample d∗hkl is in a good agreement with those for the typical perovskite

structure of the “Eva” program dᵀ

hkt.


Figure 7.2: SEM of the perovskite particles.

Figure 7.3: XRD of the perovskite particles.

It is important to note that longer calcination periods or higher temperatures result in a

higher content of an manganese oxide (MnO2) within the perovskite sample. The particles

exhibit a specific surface area in accordances with the areas reported in [93] for manganese

perovskites with a size of 35 nm (21.8m2/g). Table 7.2 summarizes the results obtained from

BET adsorption, scanning electron microscopy and XRD, as well as the “water pool” (Wo) used

to synthesize the nanoparticles of perovskite.


Table 7.1: Family of planes (hkl) in the perovskite structure and the corresponding dis-

tance between two lattice planes dhkl. (∗):experimental and (ᵀ):Theoretical. The theoret-

ical parameter are from the data base “Eva” for a typical perovskite.

hkl d∗hkl(A) dᵀ

hkl(A)

100 3.86 3.82

110 2.69 2.70

011 2.20 2.19

200 1.87 1.91

210 no obs. 1.70

211 1.55 1.55

220 1.35 1.35

Table 7.2: Specific surface area SBET Adsorption values for perovskite particles. Where

Wo is the water pool parameter, SBET is the surface area, DXRD represents the particle

diameter from XRD measurements and DSEM is particle diameter from SEM.

Wo SBET (m2/g) DXRD DSEM(nm)

20 23.11 40 40

7.1.2 Palladium Nanoparticles

Palladium particles from microemulsion with different “water pools” (Wo) were prepared in

order to obtain particle with different average diameters.

Figure 7.4 shows a typical diffractogram, which corresponds to powder of palladium with a

face-centered cubic structure. Table 7.3 lists the parameters of the mean peaks for palladium

samples and the corresponding theoretical family of planes from “Eva” program. It can be noted

the good agreement between both values.

Figure 7.5 presents a Scanning Electron Micrograph (SEM) for a powder sample of Pd

from a microemulsion with Wo = 9. Particle sizes are consistent with those obtained from the

diffractogram (See figure 7.4) using the determined parameters for the mean peak (100) in the

Scherrer‘s equation (6.11). These parameters were estimated using the profile fitting program

of the Diffrac-AT.

Additional to this, table 7.4 resumes the obtained particle size and specific surface areas for


Figure 7.4: XRD of the palladium particles.

Table 7.3: Family of planes (hkl) in the palladium structure and the corresponding dis-

tance between two lattice planes dhkl. (∗):experimental and (ᵀ):Theoretical.

hkl d∗hkl (A) dᵀ

hkl (A)

111 2.24 2.25

200 1.94 1.94

220 1.37 1.38

Figure 7.5: SEM of the palladium particles.


the different samples of Palladium comparing the SEM measurement (dSEM ) and the calculated

value from Scherrer’s equation (dScherrer). It can be observed from the data that small particles

have a bigger surface area per gram of sample, as expected.

Table 7.4: Average diameter of palladium particles calculated using Scherrer’s equation,

diameter values obtained by SEM and specific surface areas.

Wo dScherrer(nm) dSEM(nm) SBET (m2/g)

3 30 35 64,32

9 41 45 24,20

11 48 50 28,24

7.1.3 Zirconia Nanoparticles

Figure 7.6 shows the diffractogram of the synthesized zirconia nanoparticles. These particles

presented a monoclinic structure (a 6= b 6= c; α = γ = 90o; β > 90o). Table 7.5 lists the param-

eters of the principal peaks in the diffractogram for a powder sample, which are in accordance

to the theoretically expected values.

Figure 7.6: XRD of the zirconia.

From the analysis of the peaks and applying the Scherrer’s equation (eqn. 6.11) an average

particle diameter of 47 nm was calculated. This value is in concordance with the size estimated


by SEM as shown in figure 7.7. In addition, table 7.6 resumes the analysis by nitrogen adsorption

for the powder sample, as well as the particle diameters obtained by XRD and SEM.

Table 7.5: Family of planes (hkl) in the zirconia structure and the corresponding distance

between two lattice planes dhkl,∗=experimental and ᵀ=Theoretical.

hkl d∗hkl(A) dᵀ

hkl(A)

111 2.83 2.84

002 2.53 2.54

220 1.84 1.84

022 1.81 1.81

Figure 7.7: SEM of the zirconia nanoparticles.

Table 7.6: Average diameter of zirconia particles calculated using Scherrer’s equation,

diameter values obtained by SEM and specific surface areas.

W0 dScherrer (nm) dSEM (nm) SBET (m2/g)

5 47 30 64.32


7.1.4 Yttrium Iron Garnet Nanoparticles

The structure of the yttrium iron garnet was confirmed by X-ray diffraction of a powder sample,

as can be observed in figure 7.8. The table 7.7 list the principal peaks of the structure, which is

in good agreement with the values obtained by “Eva” program.

Table 7.7: Family of planes (hkl) in the yttrium iron garnet structure and the correspond-

ing distance between two lattice planes dhkl,∗=experimental and ᵀ=Theoretical.

hkl d∗hkl(A) dᵀ

hkl(A)

400 3.07 3.09

420 2.83 2.84

422 1.52 2.51

640 1.69 1.69

642 1.64 1.65

The average diameter of the crystallite size was determined from the values of the mean

peak (420), which were provided by the fitting program Diffrac-AT, and applying the Scherrer‘s

equation (eqn. 6.11). The average size of the powder sample are in concordance with the

value estimate by scanning electron microscopy. The SEM analysis (figure 7.9) shows that

nanoparticles of yttrium iron garnet (YIG) synthesized in w/o-microemulsion have uniform

size and morphology. Finally, table 7.8 lists the particle size estimated by X-ray diffraction

(dScherrer), scanning electron microscopy (dSEM ), the specific surface area (SBET ) and the

employed Wo for the synthesis of the particles in w/o-microemulsions.

Table 7.8: Average diameter of yttrium iron garnet nanoparticles calculated using Scher-

rer’s equation, diameter values obtained by SEM and specific surface areas.

W0 dScherrer (nm) dSEM (nm) SBET (m2/g)

5 20 22 4.21

7.2. COATING OF THE SURFACES 70

Figure 7.8: XRD of the yttrium iron garnet nanoparticles.

Figure 7.9: SEM of the yttrium iron garnet nanoparticles.

7.2 Coating of the Surfaces

The coating process permitted to obtain a monolayer of particles of different materials. Figure

7.10 shows a typical employed surfaces to to test the detachment. In the case of the micrograph

(7.10) the particles were palladium with an average diameter of 35 nm. The particles shown

similar morphology. This result can be compared with those calculated using Scherrer’s equation

(eqn. 6.11) and with the data from the X-ray diffractogram (30 nm) (See figure 7.11) for the

attached particles at the glass surfaces. The peaks significant smaller due to the quantity of

the particles on the substrate is lower compared with the measurements of the powder samples.

This means, that the possibility to obey the the Bragg’s (equation 6.10) law decrease.

Furthermore, they show that oxidation state of the particles are not affected by the coating


Figure 7.10: SEM of Pd nanoparticles attached to a glass surface.

Figure 7.11: XRD of Pd nanoparticles attached to a glass surface.

process. The microemulsion also appears to help in the uniform deposition of the particles

preventing agglomeration that normally takes place when the microemulsion is destroyed and

the particles are deposited from a dispersion free of surfactants. In addition, the developed

coating method appears to avoid the sintering process, because the needed heat treatment is

not very strong (compare the size of the powder sample and the size of the deposited particles

on glass surfaces).

Table 7.9 resumes the average diameter for the different monolayer of particles, which were

obtained by x-ray diffraction and scanning electron microscopy.

Figure 7.12 shows the proposed deposition mechanism. The particles inside the microemul-

sion droplets are spread on the substrate. The reverse micellar structure permits that the

particles approaches or absorb to the surfaces but keeping the particles separated. Afterwards,

when each particle has its position, the micellar structure is destroyed and the surfactant is


Table 7.9: Average diameter of the particles from different materials calculated using

Scherrer’s equation and the values obtained by SEM.

Material dScherrer (nm) dSEM (nm)

Palladium W0 = 3 30 45



Zirconia 27 28

Yttrium iron garnet 20 21

Perovskite 20 22

removed by heat treatment. The particle monolayer is obtained when the micelle structure is

eliminated by heat treatment at low temperature (110 oC) but the surfactant remains as a film

on the surface. Then, the substrate is briefly submerged in a water-methanol solution follows

by a calcination process to eliminate the surfactant. The short treatment in the water-methanol

solution appears to remove those particles that are not in direct contact with the substrate. It is

assumed that the force of adhesion of the particle-particle system with the absorbed surfactant

is much weaker than the force of adhesion between particle and substrate.

Figure 7.12: Scheme showing the possible mechanism of the particles deposition.

The detachment procedure from section 6.10 was carried out with different times interval

(5 min, 15 min and 25 min) changing the surfactant and its concentration in aqueous solution.

This series of experiment allowed to study the influence of time in the release of particles.

7.3. EFFECT OF THE TIME 73

7.3 Effect of the Time

Figure 7.13 shows the results of the different experiments studying Palladium particles with

a mean diameter of 41 nm. The percentage of the adhering particles were plotted versus the

applied flow rate for distinct analysis times (5 min, 15 min and 25 min). Diagram (A) presents

the measurements using distilled water as fluid, while graphs (B), (C) and (D) contain the results

for solutions with a concentration of [0.036] mol/l using Marlipal O13/40, O13/60 and O13/70,

respectively. The major particle detachment is observed with a low flow rate (< 100 l/h).

30

50

70

90

110

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

(A)

30

50

70

90

110

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

(B)

30

50

70

90

110

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

(C)

5 min15 min25 min

30

50

70

90

110

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

(D)

Figure 7.13: Effect of the time on the release of 41 nm Pd spheres from glass surfaces, at

20oC. Employed fluids: (A) distilled water, (B) [0.036]M Marlipal O13/40, (C) [0.036]M

Marlipal O13/60 and (D)[0.036]M Marlipal O13/70.

Afterwards, further increments of the flow (i.e. increase of the applied force) do not detach

more particles from the surface. The systems seem to reach a threshold of detachable particles.

This effect can be compared to competitive adsorption of different molecules from liquid on

surfaces. The threshold or plateau corresponds to the situation when there are no more sites

7.4. EFFECT OF THE SURFACTANT CONCENTRATION 74

available at the surfaces to adsorb more molecules of one kind. At this point the detachment

does not depend on the applied force or time, at least for the analysis conditions utilized in this

experiment. Based on this experiment the time analysis for future studies was 5 min per applied

flow and force. Another important observation is the fact that the surfactant solutions help in

the detachment of the particles. Therefore the influence of the surfactant was studied in the

following section. It is important to mention that a complete test takes a lot of time, therefore

measurement at longer times than 25 min per applied force caould be carried out.

7.4 Effect of the Surfactant Concentration

The detachment procedure was carried out using solutions of the surfactants as fluid. The

particles were palladium spheres with average diameter of 41 nm. The concentrations of the

surfactant solutions were varied between 0.002 mol/l and 0.036 mol/l. This means a surfactant

concentration above the critical micelle concentration (CMC).

Figure 7.14 shows the graphs of percentage of adhering particles versus flow rate using

aqueous solutions of Marlipal O13/40 (A), O13/60 (B) and O13/70 (C). Results using pure

water as fluid are also presented as pattern of comparison. In all cases, the percentage of the

particles on the substrate falls faster when the concentration of the surfactant is increased.

However, the major detachment is reached at low flow rates (< 100 l/h) At higher flow rates,

the detachment does not change further significantly. When distilled water without surfactant

is applied, the particles can not be removed from the glass surface.

This behavior can be explained with the observations reported in [1, 12, 14]. The authors

suggested that the forces of interaction between two surfaces are significatively affected by the

presence of surfactant molecules in the fluid.

Adsorption of surfactant monolayer at a hydrophilic surface transforms the surface into

hydrophobic (hydrophobic effect) which increases the attractive forces. This effect seems to be

opposite to the results found in this study, but this behavior takes in account only the presence

of a adsorbed monolayer at surfactant concentrations lower than the CMC. However, authors

pointed also out that if the surfactant concentration is increased, it is expected that the attractive

force again decreases due to the formation of bilayers and other complex aggregates. This

effect was confirmed by Kyraly et al. in [94]. In this case, authors studied the adsorption and

aggregation of n−octyl β−D−monoglucoside (C8G1) nonionic surfactant solution on hydrophilic

7.5. EFFECT OF THE ETHOXYLATION DEGREE 75

0

20

40

60

80

100

0 200 400 600 800

Flow Rate (l/h)

(A)

75 secs of Transition Time

0

20

40

60

80

100

Per

cent

age

Adh

erin

g

(B)

H2O[0.002]M[0.012]M[0.024]M[0.036]M

0

20

40

60

80

100

(C)

Figure 7.14: Effect of the surfactant concentration on the particle detachment of 41 nm Pd

spheres, tested under the following conditions: 20oC and 5 min. for each flow rate applied.

Fluids were (A) Marlipal O13/40, (B) Marlipal O13/60 and (C) Marlipal O13/70.

silica (CPG). They found that the adsorption isotherm is sigmoidal in shape (indicating a

cooperative adsorption mechanism), below the CMC, isolated surfactant molecules are adsorbed

weakly on polar surface sites for example by hydrogen bonding to surface silanol groups.

7.5 Effect of the Ethoxylation Degree

The effect of ethoxylation degree (EO) of the hydrophilic group on the particle detachment was

investigated considering a series of surfactants as Marlipal O13/40, O13/60 and O13/70. The

percentage of adhered palladium particles (41 nm) versus the flow rate is plotted in figures 7.15

and 7.16.

These figures show the observed results for concentrations of different surfactants solutions

[0.024] mol/l and [0.036] mol/l, respectively. These measurements demonstrated that an incre-

7.5. EFFECT OF THE ETHOXYLATION DEGREE 76

0

20

40

60

80

100

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

H2OM-O13/40M-O13/60M-O13/70

Figure 7.15: Effect of the ethoxylation degree on the particle detachment of 41 nm Pd

spheres, tested under the following conditions: 20oC and 5 min. for each applied flow

rate, [0.024]M Marlipal.

ment of the ethoxylation degree enhances the remotion of the particles.

The above results are in accordance with those presented by Harris et al. [95] and Ballun

et al. [96]. Both groups studied a series of homologous polyoxyethylen alcohols, where the

removal of protein, steric acid and fatty soils from glass surfaces improved when the number of

EO units is increased from 4 to 8 in [96] and from 5 to 10 in [95, 97], respectively. However,

the detachment decreases for ethoxylation degree higher than ten again. Jelinek et al. reported

similar results in [98] considering soil removal from cotton by polyoxyethylene alkylphenols. The

best soil removal was obtained for 10 EO units in analogy with the polyoxyethylene alcohols

followed by a decline with further increasing ethoxylation degree.

The adsorption of the surfactant onto the surface can be the key to explain these results.

Furlong et. al [99] determined the effect of the ethoxylation degree on the adsorption isotherm

for a series of homologous nonionic surfactants. Authors found S−sharped adsorption isotherms

for hydrophilic SiO2 surfaces. The initial rise of the isotherm was larger for surfactants with

higher number of oxyethylene groups. They found also an adsorption of the surfactant by

7.6. EFFECT OF THE TEMPERATURE 77

0

20

40

60

80

100

0 200 400 600 800

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

M-O13/40M-O13/60M-O13/70

Figure 7.16: Effect of the ethoxylation degree on the particle detachment of 41 nm Pd

spheres, tested under the following conditions: 20oC and 5 min. for each applied flow

rate, [0.036]M Marlipal.

their oxyethylene chains. Partyka et al. [100] demonstrated that the adsorption of nonionic

surfactants on silica gel increases with the increment of the oxyethylene chain length, which

demonstrate that the adsorption of the surfactant on the surface occurs by the head group.

7.6 Effect of the Temperature

The removal of palladium particles with the average diameter of 41 nm was tested at different

temperatures (20 oC, 25 oC and 35 oC) with Marlipal O13/70. Results are presented in figure

7.17.

These measurements do not show a pronounced difference between the temperatures. How-

ever, higher temperatures seem to support in in the detachment. Thompson [101] determined

the optimum removal conditions for a series of surfactants (ionic, nonionic and the mixtures

of ionic-nonionic). The author found a relationship between the removal and the interfacial

tension versus temperature. Results for nonionic surfactants type C12EO4 and C12EO5 showed

7.6. EFFECT OF THE TEMPERATURE 78

0

20

40

60

80

100

0 20 40 60

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

20´°C35´°C

Figure 7.17: Effect of the temperature on the particle detachment of 41 nm Pd spheres.

Conditions: 5 min for each applied flow rate and [0.036]M Marlipal O13/70.

an increment in the removal of oily soil from polyester and cotton fabric when the temperature

was increased. While interfacial tension between the fabric and the surfactant solutions for the

studied system with C12EO4 and oil had a minimum at this maximal removal. The maximum in

detachment was reached at 30 oC for C12EO4 and 45 oC for C12EO5. A similar trend is observed

in this study. However, Thompson found that further increment in the temperature above the

optimal value (30 oC for C12EO4 and 45 oC for C12EO5) again decreases the detachment and

increases the interfacial tension, as expected according to the theory described in chapter 2.

This is due to the fact that increments in temperature cause also disruption of the structured

water surrounding of the hydrophobic groups, which disfavor the micellization. These two effect

caused by the increment in temperature are competitive.

The first effect can be attributed to the decreasing water surfactant interaction due to

dehydration of the polyoxyethylene group when the temperature is raised such as in [102, 100].

The dehydration causes the adsorption and aggregation of these types of surfactants on to the

surfaces. Such adsorption decreases the van der Waals attraction between the solids.

As was pointed out in chapter 4, the DLVO-theory predicts that the total potential energy

7.7. EFFECT OF THE HYDROPHOBIC GROUP 79

of interaction VT is the sum of the potential energy of attraction (equation 4.1) and repulsion

(equation 4.5). In the expression for potential energy of attraction, which is considered to be

the more important factor for smaller particles, the Hamaker constant A must be replaced by

an effective Hamaker constant Aeff to take into account the interaction between the surfactant

solution and the solids. This new Hamaker constants is given by equation 7.1 [103].

Aeff =(

√

AP −√

AS

)2(7.1)

Where AP and AS are the Hamaker constant for the particles and solution, respectively. As

particles and solutions become more similar in nature, AP and AS come closer in magnitude and

Aeff become smaller, which results in a smaller attractive potential energy between the particle

and substrate. Furthemore, the decrease of the interfacial tension decrease also the required

work to remove the particles. This effect will be explained in section 7.11.

7.7 Effect of the Hydrophobic Group

In order to investigate the effect of the hydrophobic group on the detachment of palladium

particles (average diameter 41 nm) from glass surfaces, two surfactants, which contain the same

ethoxylation degree and concentration but different hydrophobic chains, were compared at the

same concentration as shown figure 7.18.

Since the adsorption of the surfactants onto polar surfaces (like than glass) occurs via hy-

drophilic groups [14, 100], it is expected that the hydrophobic chains have less effect on the

modification of the surfactant adsorption onto the surfaces. Therefore, the difference between

the detachment produced for both type of surfactants was not pronounced. However, in cases

where the surfactant adsorbs with the hydrophobic group to the surface, this will be the case

for Pd particles, change in the chain length cause more pronounced differences in the behavior

of the system.

7.8. EFFECT OF THE PARTICLE SIZE 80

0

20

40

60

80

100

0 20 40 60 80 100

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

M-O13/70M-O24/70

Figure 7.18: Effect of the hydrophobic group on the particle detachment of 41 nm Pd

spheres, tested at 20oC, 5 min, and [0.036]M Marlipal O13/70 and O24/70.

7.8 Effect of the Particle Size

The effect of the particle size on the detachment, process was studied with palladium particles

of different average diameters (30 nm, 41 nm and 48 nm) and considering Marlipal O13/70 as

surfactant, with a concentration of [0.036] mol/l.

The graphs in figure 7.19 represent the effect of the flow rate on the removal of palladium

particles with different sizes. Results show that small particles are more difficult to remove than

the bigger ones. This phenomenon is due to fact that the true contact area of the particles with

the substrate increases for smaller particles. This contact area represented by the parameter h

in the equation 4.1, increases as result the force of adhesion. Figure 7.20 is a plot of the particle

radius versus the flow rate necessary to reach 50% of the particle detachment. It was estimated

from the graphic 7.19 for the different particle sizes. This plot shows that the flow rate required

to obtain 50% of particle detachment decrease with increasing particle radius. This effect is the

expected behavior, as described in chapter 4.

Figure 7.21 presents a graphic of the percentage of adhered particles versus hydrodynamic

forces. The forces were calculated using the equations 6.16 for laminar conditions and 6.17 for

turbulent conditions. Figure 7.22 represents the hydrodynamic force required to remove 50%


0

20

40

60

80

100

0 20 40 60

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

30 nm41 nm48 nm

Figure 7.19: Effect of the particle size on the removal Pd spheres of different diameters.

Conditions: 20oC and 5 min for each applied flow rate, [0.036]M Marlipal O13/70.

of the particles from the surfaces. A first look at these plots (7.21 and 7.22) should not be

interpreted incorrectly in the way that the force of adhesion simply falls with decreasing particle

diameter. In fact this observed effect corresponds to a tremendous force per unit area as was

demonstrated in [70]. This study represents a similar case than figures 4.1 and 4.3. The fluid

velocity resulting from hydrodynamic flow approaches zero near the surfaces of the substrate

[6, 7, 8, 104]. Therefore small particles are only reached by the smaller velocities, which produce

not enough force to detach them, as explained in section 6.10. The hydrodynamic force for

particle detachment is calculated according to equation (5.5)

In this study was found that the force necessary to reach 50% of particle detachment was

around 1 × 10−6 dynes for palladium with an average diameter of 40 nm. If the forces of

attraction are assumed to be the most important forces keeping the particles attached to the

surfaces (according to [70]), then equation 7.2 is used for a dry system with the Hamaker constant

A = 1 × 10−19 J distance of separation h = 4 A and the particle radius (Rp) as suggested in

[70]. The adhesion force yields 0.2 m dynes ( 2 × 10−4 dynes) for a dry system.

FLW =A Rp

6 h2(7.2)


15

16

17

18

19

20

21

22

23

1.6e-06 1.8e-06 2e-06 2.2e-06 2.4e-06

Flo

w R

ate

(l/h)

Particle Radius (cm)

Figure 7.20: Flow rate vs. particle radius at 50% of the detachment for Pd spheres.

Conditions: 20oC and 5 min for each applied flow rate, [0.036]M Marlipal O13/70.

Batra [1] found a Hamaker constant of about 2.3 × 10−20 J for a system particle-substrate-

surfactant solution without hydrodynamic force. If the force of adhesion for palladium-glass

is calculated using this value, the result is 0.04 mdynes ( 4 × 10−5 dynes). This means a

decrease by a factor of about 1 × 101 dynes with respect to the dried system. The measured

value is ten time smaller than the expected value using the condition of Batra. However, the

calculation did not take into account the distance of separation between the particles and the

substrate h, which is expected to be bigger because of the adsorbed layer of surfactants. This

increasing in the separation h will produce a new decrease of the van der Waals interaction

particle-substrate. Assuming an adsorbed monolayer of thickness 2 nm the new h = 2.4 nm

and taking into account a Hamaker constant of A = 2.3 × 10−20 J . This yields a van der Waals

interaction of 1 × 10−3 mdynes, which is in good agreement with the experimental value. In

spite the values found in this study are in good agreement with the expected theoretical values.

We can not forget the fact that Hamaker constant and the separation between the particle and

the substrate are specific for each studied system and the obtaining of the true value will depend

on the particle-substrate-fluid system.


0

20

40

60

80

100

0 5e-07 1e-06 1.5e-06 2e-06 2.5e-06 3e-06

Per

cent

age

Adh

erin

g

Hydrodynamic Force (dynes)

30 nm41 nm48 nm

Figure 7.21: Effect of the particle size on the removal of Pd spheres. Conditions: 20oC

and 5 min for each applied flow rate, [0.036]M Marlipal O13/70.

0.6

0.7

0.8

0.9

1

1.1

1.2

1.5E-06 2E-06 2.5E-06

Hyd

rody

nam

ic F

orce

(m

icro

dyn

es)


Figure 7.22: Hydrodynamic force vs. particle radius at 50% of the detachment for Pd

spheres. Conditions: 20oC and 5 min for each applied flow rate, [0.036]M Marlipal O13/70.

7.9. EFFECT OF DIFFERENT MATERIALS 84

7.9 Effect of Different Materials

Nanoparticles of Yttrium iron garnet (20 nm), Perovskite (20 nm), Zirconia (27 nm) and Pal-

ladium (30 nm) were tested in order to study the influence of the material on the detachment

from glass surfaces.

0

20

40

60

80

100

0 40 80 120 160 200 240

Per

cent

age

Adh

erin

g

Flow Rate (l/h)

YIGPerovskite

ZirconiaPalladium

Figure 7.23: Effect of different materials on the particle detachment tested with the

same conditions. [0.036]M Marlipal O13/70 at 20oC. YIG (Y3Fe5O12), perovskite

(Ca0.5Sr0.5MnO3), zirconia (ZrO2) and (Pd)

Figure 7.23 is a plot of the percentage of particles adhering versus the flow rate (Q l/h). A

first look at on the results indicates that the removal of the particles decreases with the increase

of oxygen content in the material. This means that the adhesion forces between the glass surfaces

and particles are increasing within this series of oxides, because the number of hydrogen bonds

that can be formed between the particles and the substrate.

Unfortunately, it was impossible to obtain particles with exactly average diameter. There-

fore, an additional effect is included due to the sizes. Therefore particles of palladium (30 nm)

as well as Zirconia (27 nm) can be detached easier than particles of YIG (20 nm) and Perovskite

(20 nm), which can not be removed significatively.

Allen found that the Lifshitz-van der Waals constant A′, which is related to the Hamaker

7.9. EFFECT OF DIFFERENT MATERIALS 85

constant by A′ = (4/3) π A, has ranges from about 0.6 eV (1 eV = 1.6 × 10−19 J) for polymers

to about 9.0 eV for metals as silver and gold [70]. He found also that a system Ag-Ag (particle-

substrate) has 9.0 eV while a combination SiO2−SiO2 (glass-glass) has between 6.8-7.2 eV and

Au − SiO2 around 5.4 eV. The combination SiO2 − SiO2 can be related to the systems oxide

particles-glass (perovskite-glass, YIG-glass and zirconia-glass) and the combination Au− SiO2

to the system Pd-glass. The above consideration allows to estimate that the system palladium-

glass have a lower van der Waals force than the other systems. Therefore, palladium particles

on glass were easier to removes than YIG, zirconia or perovskite on glass. Figure 7.24 shows the

Lifshitz-van der Waals constant for the different systems.

0

2

4

6

8

10

A B C D E

A’ (

eV)

Materials

Figure 7.24: Lifshitz-van der Waals constant (A′) for different materials. (A) polymer −polymer, (B) Au − SiO2, (C) SiO2 − SiO2, (D) Ag − Ag and (E) Au − Au

It can be observed from the equation of Lifshitz-van der Waals constant, that the relationship

between the materials of the different systems and the Hamaker constant is the same. In this way,

it is expected that the attractive forces between particle and surface is higher for the system oxide

substrate-oxide particle (combination glass-YIG, glass-zirconia and glass-perovskite) than for

oxide substrate-metal particle system (combination glass-palladium). The above considerations

are in good qualitative agreement with the obtained results. The differences in the detachment

7.10. EFFECT OF THE HYDRODYNAMIC PARAMETERS 86

for the glass-YIG particles, glass-perovskite and glass-zirconia can be related to particle size

effects as well as, the oxygen content in the different materials.

7.10 Effect of the Hydrodynamic Parameters

Figure 7.25 plots the wall shear stress versus particle radius considering 50% of the detachment

of the palladium particles with average diameter of 41 nm. The shear stresses were calculated

using the equation 5.8. A power trend of this plot permits to identify the mechanism acting to

detach the particle as was pointed out by Hubbe [77]. These relationships have been discussed

in the chapter 5.

0

2000

4000

6000

8000

10000

12000

14000

1.6e−06 1.8e−06 2e−06 2.2e−06 2.4e−06

Wal

l She

ar S

tres

s (d

ynes

/cm

2)


0

2000

4000

6000

8000

10000

12000

14000

1.6e−06 1.8e−06 2e−06 2.2e−06 2.4e−06

Wal

l She

ar S

tres

s (d

ynes

/cm

2)


Figure 7.25: Shear stress vs. particle size at 50% of the detachment of 41 nm Pd spheres,

tested under the following conditions: 20oC and 5 min for each applied flow rate, [0.036]M

Marlipal O13/70.

The power trend was τ = 3x10−9 R−1.352p (R2 = 0.9984). This means a radius dependency

around R−4/3, which indicates that the mechanism acting to remove the particles is lifting. It

is expected that palladium particles are hard spheres.

These results seem to be contradictory with those presented in [104, 105, 106], where authors

7.11. DETACHMENT MECHANISM 87

indicate that the principal mechanism acting to detach the particles could be due to rolling

instead of lifting. However, in all these studies, the fluids were water and solutions with low

concentrations of ions. In these cases, there was no other source of vertical forces. When a

solution of surfactant is employed, we expect that its contribution is due to the adsorption of

the surfactant onto the surfaces. This should modify the van der Waals forces by increasing the

distance between the solids. Additionally, it is well know from cleaning processes that the solid

can be suspended in the bath solutions by adsorption of surfactant preventing the redeposition

by producing a steric barrier [12]. This factor acts opposite to the force of adhesion (in a diagram

of force).

7.11 Detachment Mechanism

In summary, the removal of fine particles occurs in two stages. First, the diffusion and adsorption

of the surfactant onto the surfaces, which depend on the hydrophobicity of the substrates.

The particle and the substrates can be both hydrophobic, both hydrophilic or a combination

hydrophobic-hydrophilic. The adsorption of the surfactant molecules onto the surfaces can be

improved by changing the concentration, temperature, ethoxylation, etc, as well as employing

another surfactant type (anionic-cationic) depending on the characteristics of the system.

The adsorption of surfactants onto the surfaces decreases the work required to remove the

particle, since van der Waals force is modified. The free energy per unit area involved in this

process is the work of adhesion (Wa). This work can be written as follows.

Wa = γSF + γFP + γSP (7.3)

Where γSF represents the surface tension substrate-fluid, γFP is the surface tension fluid-

particle and γSP denotes the surface tension substrate-particle. The adsorption of surfactant at

these interfaces decreases γSF and γFP producing a consequent decrease of the work required

to remove the particle.

Second, the detachment of the particles occurs by fluid action, which is referred as “hydrody-

namic detachment”. The fluid motion generates two additional forces (lifting and hydrodynamic

forces) and a torque. In this study, the major contribution on the detachment was due to the

lifting force in contrast to the suggestion in [105, 106, 104].

7.11. DETACHMENT MECHANISM 88

The impact of the hydrodynamic force seem to be less relevant than the diffusion and ad-

sorption of the surfactant onto the surface. However the application of the hydrodynamic forces

completes the detachment process. Figure 7.26 represents a scheme of the proposted mechanism.

Diffusion and adsorption of the surfactant molecules on the surfaces follows by the action of the

hydrodynamic force, which complets the removal.

Figure 7.26: The total removal mechanism acting to detach the particles from de surface.

(A) Pd on glass, (B) YIG on glass, zirconia on glass and perovskite on glass.

Chapter 8

Conclusions

The aim of this research was to study the effects of nonionic surfactants on the interactions

between nanoparticles and glass surfaces. To reach this objective, the research followed a fourfold

strategy:

Synthesis of nanoparticles Particles with a defined size and morphology were synthesized

using w/o-microemulsion. This procedure has been employed in several experiments to

obtain fine particles with a good control of their sizes in nanometer ranges.

Development of a method to coat glass substrates This method allowed to obtain coated

samples with homogenous monolayer of nanoparticles. The use of the reverse micelles

for this purpose resulted a suitable medium for controlled deposition. Additionally, the

method presented some advantage over other methods. For example the wide variety of

materials that can be synthesized in microemulsions as well as the possibility to change

the composition in order to control the deposition. The method is inexpensive and does

not require special equipment.

Design and construction of a device to test the detachment The device was based on

the application of a hydrodynamic force, which is caused by the motion of a fluid inside

the cylinder, and variation in the properties of the fluids (in this case the aqueous solutions

of surfactants), which acts to debilitate the adhesion force between the particles and the

substrate.

Implementation of a technique to quantify the detachement Quantification of the par-

ticle detachment was followed by measurements of U.V. absorption and calculating of the

89

90

attenuation coefficient. However, this method has some disadvantages. For example, the

substrates have to be transparent, which limited the study only to glass substrates and

the calculation of the attenuation coefficient does not take into account the percentage of

light that is scattered by the particles.

The effects of the time on the detachment shows that an increment of the time of analysis does

not have significant influence on the detachment of the particles, at least for the employed periods

of time. The major particle detachment is observed with a low flow velocity (< 2.3×10−1 m/s).

At higher velocities, the systems reached a threshold with no further detachment of particles.

Increments in the concentration of the surfactants caused an increment in the detachment

of the particles. This effect seems to be related to the formation of adsorbed bilayers or other

aggregates at the surface of the particles and substrates. The formation of these bilayers can

produce an increment in the distance of separation between the particles and glass surfaces,

which decreases the attractive force between the two solids.

Detachment was improved by increasing of the ethoxylation degrees of the surfactants.

Higher ethoxylation degrees increase the adsorption of the surfactants at the surface, which

can cause the formation of aggregates. This behavior is in accordance with those found in other

references [95, 96, 97] . However, it is important to emphasize that in these references it was also

found that for ethoxylations degree higher than ten the detachment again decreases. Changes

of the lipophilic chains of the surfactant have no effect on the forces of adhesion, since the

adsorption of the surfactants onto polar surfaces occurs via hydrophilic groups.

Variations in the kind of nanoparticle materials showed that the attractive forces between a

particle and surface is higher for the oxide substrate oxide particle system (combination glass-

YIG, glass-zirconia and glass-perovskite) than for oxide substrate metal particle system (com-

bination glass-palladium). This behavior is related to the Hamaker constant, which is higher

for the oxide surface oxide particles systems than for oxide surface metal particle system. The

differences in the detachment for the glass-YIG particles, glass-perovskite and glass-zirconia can

be related to particle size effects as well as the oxygen content in the different materials.

The mechanism of incipient motion was estimated from a plot of the shear stress versus the

particle radius following the procedure described by Hubbe [77]. From the radius dependence

the mechanism was assumed to be lifting. This means that the major contribution to the

detachment of the nanoparticles is a vertical force opposed to the forces of adhesion. In the

studied case there is no other source of vertical forces, so that the lifting forces were generated

91

by the adsorption of the surfactants on the surface, which modify van der Waals forces and

electrostatic forces.

The adsorption of surfactant onto the surface decrease also the work required to remove the

particles by modifying the surface tensions in the system substrate-particle-fluid. The detach-

ment occurs by diffusion and adsorption of the surfactants onto the surfaces, which debilitates

the particle-substrate adhesion, followed by the action of the hydrodynamic force.

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Erklarung

Hiermit erklare ich, dass weder fruher noch gleichzeitig eine Anmeldung der Promotion-

sabsicht oder ein Promotionsverfahren bei einer anderen Hochschule oder bei einem anderen

Fachbereich erfolg ist.

Berlin, Marz 2005

Teilen der Dissertation sind in Form von Zeitschriftenartikeln und Vortragen veroffentlicht

worden. Die folgende Liste gibt daruber Aufschluss:

Artikel in Zeitschriften:

• A. Lopez-Trosell, R. Schomacker.“Synthesis of manganese perovskite in w/o-microemulsion”,

Materials Research Bulletin (submitted).

Vortragen:

• A. Lopez-Trosell, R. Schomacker. 2005 NSTI Nanotechnology Conference, May 8-12, Ana-

heim California, U.S.A. “Coating of glass surfaces with nanoparticles of different materials

synthesized in microemulsions” (accepted)

• A. Lopez-Trosell, R. Schomacker. 2005 NSTI Nanotechnology Conference, May 8-12, Ana-

heim California, U.S.A. “Synthesis and characterization of perovskite Ca0.5 Sr0.5MnO3

nanoparticles in w/o-microemulsion” (accepted)

Eidesstattliche Versicherung

Die Arbeit wurde in der Zeit von Oktober 2001 bis Januar 2004 unter Anleitung von Herrn

Prof. Dr. R. Schomacker am Institut fur Technische Chemie an der TU Berlin angefertigt.

Hiermit versichere ich an Eides Statt, dass ich die von mir vorgelegte Dissertation selbstandig

und ohne unzulassige Hilfe angefertigt, die benutzten Quellen und Hilfsmittel vollstandig angegeben

und die Stellen der Arbeit, die anderen Werken in Wortlaut und Sinn nach entnommen sind, in

jedem Einzelfall als Entlehnung kenntlich gemacht habe.

Berlin, Marz 2005

Curriculum Vitae

B.Sc. (Chemistry). Lopez Trosell Alejandra I.

Personal details

Place of Birth Valencia- Carabobo

Nationality Venezuelan

Marital status single

Academic History

10/2001- Present Working towards the degree of Doctor in Sciences

Technische Universitat Berlin

Faculty II, Institute of Chemistry

Thesis: “Effects of Nonionic Surfactants on the Interactions of

Different Nanoparticle Materials on Glass Surfaces”

Supervisor: Dr. Schomacker Reinhard

1992-1999 Bachelor of Science (Chemistry)

Faculty of Science and Department of Physical Chemistry

Universidad Central de Venezuela

Thesis: “Experimental and Numerical Determination of Spreading

Coefficients in Silica Surfaces with Controlled Wettability”

Supervisor: Dr. Araujo Y. Carolina (PDVSA-Intevep)

Dr. Prof. Castillo Jimmy (UCV)

Department of Reservoir Exploration and Production

PDVSA-Intevep

Research Experience

05/98-02/99 Petroleum of Venezuela (PDVSA-Intevep)

Department of Reservoir Exploration and Production

El Tambor - Los Teques, 1070-A, Caracas -Venezuela

08/96-09/96 Colgate-Palmolive C.A.

Department of Development of New Products.

Av. Uslar, Zona Industrial Michelena,

Valencia -Edo. Carabobo - Venezuela.

03/94-11/97 Department of Organic Chemistry, Laboratory of

Synthesis of Natural Products,

Universidad Central de Venezuela.

Los Chaguaramos 1020-A Caracas-Venezuela

Teaching Experience

12/97-08/99 Academic Assistant Laboratory of Instrumental Analysis

(UV, IR, Chromatography and Atomic Absorption Spectroscopy)

Academic Honours

09/2000-Present Scholarship Program of German Academic Exchange Service (DAAD)

and Gran Mariscal de Ayacucho Foundation (FGMA).

07/2002 Selected by AVINA-Foundation to participate in

the 52nd. Nobel Prize Winners Meetings with Young Scientists

in Lindau on Lake Constance. 1-5 July 2002.

Publications

• A.Lopez-Trosell and R. Schomacker.“Synthesis of manganese perovskite in w/o-microemulsion”.

Materials Research Bulletin, (submitted).

• A.Lopez-Trosell and R. Schomacker, 2005 NSTI Nanotechnology Conference, May 8-12,

Anaheim California, U.S.A. “Coating of glass surfaces with nanoparticles of different ma-

terials synthesized in microemulsions” (accepted)

• A.Lopez-Trosell and R. Schomacker, 2005 NSTI Nanotechnology Conference, May 8-12,

Anaheim California, U.S.A. “Synthesis and characterization of perovskite Ca0.5 Sr0.5MnO3

nanoparticles in w/o- microemulsion” (accepted)

• A. Lopez and Y. Araujo. “Sistemas Lıquido/Lıquido/Solido”. Informe Tecnico, INT-

6226,1999. PDVSA, Intevep, 1999.

• A. Lopez and Y. Araujo. “Coeficientes de Spreading en Sistemas Lıquido/Lıquido/Solido”

2nd Congress of Physics, Cumana-Venezuela, March 2000.

Documents

Effects of Nonionic Surfactants on the Interactions of