1

DIFFUSION OF SELF ASSEMBLED STRUCTURES IN

WATER SOLUBLE POLYMER SOLUTIONS.

PROJECT REPORT

SUBMITTED FOR IASC- INSA-NASI

SUMMER RESEARCH FELLOWSHIP PROGRAMME 2014

BY

FATHIMA P.A.

COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

KERALA

UNDER THE GUIDANCE OF

Dr. P.A. HASSAN

CHEMISTRY DIVISION

BHABHA ATOMIC RESEARCH CENTRE

TROMBAY

2

CONTENTS

1. Introduction

(a) Amphiphilic assembles

(b) Classification of Surfactants

(c) Micelles

2. Experimental Section

(a) Materials

(b) Methods (Dynamic Light Scattering)

3. Results and Discussion

(a) Effect of polymer on diffusion of P123 micelle

(b) Effect of polymer on diffusion of CTAB micelle

(c) Effect of polymer on diffusion of SDS micelle

4. Conclusion

5. References

3

GOVERNMENT OF INDIA

CHEMISTRY DIVISION

BHABHA ATOMIC RESEARCH CENTRE

TROMBAY, MUMBAI-400084

SUMMER RESEARCH PROGRAMME 2014

CERTIFICATE

This is to certify that the project entitled “Diffusion of self assembled structures

in water soluble polymer solutions” submitted by FATHIMA.P.A, COCHIN

UNIVERSITY OF SCIENCE AND TECHNOLOGY, has been carried out during

08/05/2014 to 04/07/2014 as part of IASC- INSA-NASI, Summer Research

Fellowship programme 2014 at Bhabha Atomic Research Centre.

P. A. HASSAN

Place: Mumbai (Project Guide)

Date: 04/07/2014

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Acknowledgment

I would like to express my heartfelt gratitude to my guide Dr. P.A. Hassan for his invaluable help

and consistent guidance rendered to me during this project.

I am very thankful to Dr. G. Madhavan, Coordinator, Summer Research Fellowship Programme 2014

organized by Indian Academy of Science for giving me the opportunity to do this project.

I would like to express my sincere thanks to Dr. Dilip .K. Maity for coordinating our visit to BARC.

It is my pleasure to thank Suman Rana (PhD student) and other senior research scholars for their

constant support and invaluable help throughout this project.

I take this opportunity especially to honor my father, mother, sister and grandmother for their

encouragement.

Last, but not least I am so thankful to the almighty, for showering his kindness in every aspect.

FATHIMA.P.A

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1. INTRODUCTION

Transport of materials through complex fluids have attracted great deal of attention due to its

occurrence in many industrial processes, biological fluids, separation science, drug delivery

etc.(1) Water soluble polymers impart significant changes in the flow behavior of aqueous

solutions due to entanglement of the polymer network in the semi-dilute regime(2).

Understanding diffusion of colloidal particles in polymer solutions is of great relevance for both

fundamental research and industrial applications (3). The objective of this work was to examine

the transport of amphiphilic assemblies in aqueous polymer solutions. Amphiphilic assemblies

have sizes in the colloidal range and hence undergo Brownian motion in solutions. Various water

soluble polymers are used as additives in pharmaceutical formulations and knowledge of the

mobility of particles in polymer solutions or melts is of prime importance. In this work, we

investigate the diffusion of the self assembled structures formed by amphiphilic molecules in

presence of various water soluble polymers such as polyethylene glycol and polyvinyl alcohol.

The diffusion coefficients were monitored by using Dynamic Light Scattering technique. To

understand the effect of aggregate size and charge on diffusion through dilute polymer solutions,

we investigated the variation in the diffusion coefficient of cationic, nonionic and anionic

aggregates in two different polymer solutions.

(a) Amphiphilic assembles

Amphiphilic molecules have been used for diverse application in cleaning industry,

pharmaceutical and personal care products for decades. They possess distinct hydrophilic and

hydrophobic parts in the same molecule. By virtue of this nature, they have the ability to adsorb

at interfaces of different states of matter and association in selective solvents. As they tend to

accumulate at the surface and reduce the surface energy of the system, they are also known by

the name surfactants. Surfactants contain both hydrophilic ( water soluble ) head and

hydrophobic ( water insoluble ) tail. The tail of most of the surfactants are similar, consisting of

hydrocarbon chain, which can be linear, branched or aromatic. Figure 1 shows a schematic

representation of amphiphilic assembly known as micelle.

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Figure (1) shows schematic diagram of micelle

(b) CLASSIFICATION OF SURFACTANTS

Based on the charges on the hydrophilic region of the surfactant, they can be classified under

different categories. A non-ionic surfactant has no charge in its hydrophilic head (Polar head).

The head of an ionic surfactant carries a net charge. Depending on this charge surfactants are

classified into non ionic, anionic, cationic, zwitterionic.

Anionic Surfactants

Anionic surfactant contain negative charge or anionic functional group such as sulfate (-OSO3 -

),

sulphonate (-SO3 -

), phosphate on its hydrophilic head. Examples are Sodium dodecyl sulfate

(SDS), Sodium lauryl ether sulfate (SLES). They are excellent detergents and are widely used as

soap for cleaning process.

Cationic Surfactant

7

Cationic surfactants contain positive charge like quaternary ammonium (R4N+X

-), imidazonium

on its hydrophilic head. Example of cationic surfactant is cetyl trimethyl ammonium bromide

(CTAB). It is mainly used as hair conditioners and fabric softeners.

Zwitterionic Surfactant

Zwitterionic surfactants contain a head with two oppositely charged groups. The cationic part is

based on primary, secondary or tertiary amines or quaternary ammonium. The anionic part can

be more variable and include sulphonate. Examples are Cocamidopropylbetine, Lecithin,

Sulfobentaines (RN+(CH3)2CH2CH2SO3

-). Lecithin is milder on the skin and low eye-sting

effects which lead to their use in baby shampoos.

Non-ionic Surfactant

Non-ionic surfactant has no charge groups in its hydrophilic head. Examples are

polyoxyethylene glycol, polyoxypropylene glycol. Non-ionic surfactants used as low

temperature detergency and as emulsifiers.

Figure 2 shows chemical structures of the commonly used amphiphiles

Figure (2) shows surfactant molecules with hydrophobic and hydrophilic parts

8

Another important class of amphiphiles which has attracted consider attention for pharmaceutical

applications is block copolymers. A block copolymer is a polymer consisting of multiple

sequences, or blocks, of the same monomer alternating in series with different monomer blocks.

The blocks are covalently bound to each other such as AAABBBAAA fashion (A and B are

different types of monomers). Block copolymers are classified based on the number of blocks

they contain and how the blocks are arranged. For example, block copolymers with two blocks

are called diblocks; those with three blocks are triblocks; and those with more than three are

called multiblocks. A block copolymer consisting of polyethylene oxide (PEO) and

polypropylene oxide (PPO) is a well known amphiphile.

Micelles

In aqueous solution, molecules having both polar or charged groups and non polar (amphiphilic

molecule) part form aggregates called micelles. In a micelle, polar or ionic heads form an outer

shell in contact with water, while non polar tails are sequestered in the interior (4). Micelles are

widely used in industrial and biological fields for their ability to dissolve and move non polar

substances through an aqueous medium, or to carry drug which are, often, scarcely soluble in

water. The carrying ability of micelles can be altered if parameters determining their size and

shape are changed.

Micelle aggregates form only when the concentration of the amphiphilic molecule reaches a

given concentration called critical micelle concentration (CMC). That condition is monitored by

the sudden change in the chemical and physical properties of the solution. On the contrary,

below CMC micelles are completely absent. Figure 3 illustrates the adsorption of surfactants at

the interface and formation of micelles above CMC.

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Figure (3) Schematic of formation of micelles above CMC

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2.Experiment

Materials and Methods

(a) Materials

Pluronic123 (P123)

Pluronic123 are commercially available triblock copolymer based on PPO and PEO. It is a non-

ionic surfactant possessing two kinds of moieties i.e. the hydrophilic block (PEO) and

hydrophobic block (PPO). It is manufactured by BASF corporation, The chemical formula is

H(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H

Cetyl trimethyl ammonium bromide (CTAB)

Cetyl trimethyl ammonium bromide is a cationic surfactant. They carry a positive charge on the

head group due to a quaternary ammonium group. Its chemical formula is (C16H33) N (CH3)3Br.

Sodium dodecyl sulphate (SDS)

Sodium dodecyl sulphate is an anionic surfactant which means that it carry a negative charge on

the head group. The negative charge arises from dissociation of the sulphate group. Its chemical

formula is C12 H25 SO4 .

The water soluble polymer used in this experiments are :

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Polyethylene glycol (PEG)

This is a water soluble polymer. The solubility arises from hydrogen bonding interaction of

water with the oxygen atom in poly ether. In this experiment two types of polyethylene glycol is

used one having molecular weight of 4000 (PEG 4000) and other with molecular weight of 6000

(PEG 6000). Polyethylene glycol has several chemical properties that makes it especially useful

in pharmaceuticals field. Polyethylene glycol (PEG), is a hydrophilic polymer or oligomer build

with chains of monomers of ethylene glycol (-CH2-CH2-O-) with an hydroxyl group (-OH) at

both ends.

Polyvinyl alcohol (PVA)

Polyvinyl alcohol is a water soluble synthetic polymer. The solubility of PVA in water is due to

many hydroxyl groups present in the polymer backbone. This has excellent film forming abilities

and can be used to form hydrogels by cross linking with other molecules. It has the chemical

formula [CH2CH (OH)] n.

The vast majority of the polymers used in film coating are high molecular weight polyethylene

glycol, polyvinyl alcohol etc. The characteristic properties of polymers such as glass transition

temperature, melting point, solubility, molecular weight erc are considered for coating. The

coating can have several functions. It can strengthen the tablet, control its release, improve its

taste, color it, make it easier to handle and package and protect from moisture.

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(b) Methods

Dynamic Light Scattering (DLS)

Dynamic Light Scattering (DLS) is a technique used to determine the size distribution profile of

small particles present in colloidal solution (5). It can also be used to probe the behavior of

complex fluid such as concentrated polymer solution. When the light hits the small particles, the

light scatters in all directions (Rayleigh Scattering) as long as the particle are small compared to

the wavelength. If the light source is a laser, and thus is monochromatic and coherent the

scattering intensity fluctuates over time. This fluctuation is due to the fact that the small

molecules in solution are undergoing Brownian motion and so the distance between the scatters

in the solution is constantly changing. The key concept underlying in a basic DLS experiment is

the fact that time scale of these fluctuations depends on the size of the diffusing particles. Small

particles diffuse in the solution relatively rapidly resulting in a rapidly fluctuating intensity signal

in contrast to the larger particles which diffuse more slowly and hence the intensity changes slow

with time. Figure 4 shows a schematic of intensity changes with time for different size particles

Figure (4) shows Fluctuations in scattered intensity for 'large' and 'small' particles when

observed in the same time scale and the corresponding correlation functions

“small” fast moving particles

“large” slowly moving particles

AutocorrelationI(t)

t

Correlation function

t

“small” fast moving particles

“large” slowly moving particles

AutocorrelationI(t)

t

Correlation function

t

“large” slowly moving particles

AutocorrelationI(t)

t

Correlation function

t

AutocorrelationI(t)

t

I(t)

t

Correlation function

t

13

The above figure shows intensity-time profile for small and large size particles when one

observes on the same time scales for both the particles. Quantitative information from these

kinds of fluctuations in the scattered intensity has been best achieved by a mathematical

procedure known as autocorrelation. The autocorrelation function, denoted by C (), represents

the correlation or comparison between the values of the scattered intensity at a given time t and

at a later time (t+). This comparison is made at different values of t in order to get a good

statistical average. By representing the intensity at an arbitrary time as I (o) and those at a later

time () as I (), the autocorrelation function can be written as

C () = <I (o). I ()> (1)

In the extreme limit in which the sampling interval becomes very large, there should not be any

correlation between the pairs of sampled intensities and hence the above equation reduces to

C () = <I (o)>2 (2)

An autocorrelator accepts the digital photo counts from the detector which represents the light

scattering intensity I (t) and computes the second order correlation function. The detector can be

a photomultiplier tube coupled with pulse amplifier and descriminator or it can be an avalanche

photodiode. The correlation function C () is normalized with the long time correlation data, C

() that is equal to <I>2. The normalized time correlation function of the scattered intensity,

g (2) (t) can be written as

( )

( )( ) ( )2

2g

IoI

I

(3)

14

The Siegert relation is an important equation in DLS. When the scattered field is a Gaussian

process, the correlation functions g(1)(t) and g(

2)(t) are connected through the Siegert relation

g (2)(t) = 1 + | g(

1)(t) |

2 (4)

g(1)(t) and g(

2)(t) are the normalised field-field and normalised intensity-intensity

autocorrelation functions, respectively. Since the detection area cannot be zero in practice, the

scattered light is not purely coherent and an instrument parameter, b (<1), is introduced into

equation

2

)1()2()(')( gbag (5)

where a’ is the baseline and b is an adjustable parameter dependent on the scattering geometry

and independent of . The parameter is a measure of the amplitude of the normalized correlation

function.

For a suspension of monodisperse, rigid, spherical particles undergoing Brownian

diffusion, the correlation function decays exponentially and is given as

g(1)() = (b)0.5

.e-Dq2

(6)

where D is the translational diffusion coefficient.

For small, dilute, non-interacting spheres the hydrodynamic radius Rh can be obtained

from the translational diffusion coefficient using the Stokes-Einstein relationship

D= k T / (6Rh) (7)

15

Where k is the Boltzmann’s constant, is the solvent viscosity and T is the absolute temperature.

If the particle is non-spherical then Rh is often taken as the apparent hydrodynamic radius.

Figure (5) shows schematic representation of Dynamic Light Scattering instrument

16

3. Result and Discussion

Measurement of diffusion coefficient of block copolymer micelle.

First, we monitored diffusion coefficient of Pluronic (P123) micelle using DLS.

Figure (6) shows the electric field correlation function of scattered light from a suspension of

10%w/w pluronic P123 micelles,at a scattering angle of 1300. The solid line is fit to the data

using cumulants method and the obtained relaxation time is 55µs. A good fit to the correlation

function indicates the formation of micelles in the medium. In order to assess translation

diffusion of micelles, the relaxation time of the micelle is measured at different scattering angles.

The change in scattering angle leads to changes in the scattering vector, q. It is expected that for

translational diffusion, the inverse of relaxation time should vary with square of the scattering

vector.

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Fig 7. Variation of 1/T vs q2 for 5% P123 micelles in water

Figure (7) shows the variation of the inverse of relaxation time (1/T) with square of the scattering

vector, which shows linear variation indicating translational diffusion. From the slope of the plot,

the average diffusion coefficient of the micelles is calculated and is obtained as 40×10-8

cm2/s.

From this diffusion coefficient, the average size of the aggregate can be estimated using equation

(4), which yields a value of 18 nm. This is consistent with the reported diameter of Pluronic123

(P123) micelles.

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(a) Effect of polymers on diffusion of Pluronic123 (P123) micelle

Next, we investigated the effect of various water soluble polymers on diffusion of P123

micelles. Figure (8) shows the variation of diffusion coefficient of 5% w/w P123 micelle as a

function of polyvinyl alcohol concentration. It was observed that the diffusion coefficient

decreases with an increase in the concentration of polyvinyl alcohol. This decrease in diffusion

coefficient can arises from a change in viscosity of the fluid or growth of the micelles. However,

the measured decrease in diffusion coefficient is much higher than that is expected from the

changes in viscosity of the solution. This suggests that addition of PVA alters the structure of

aggregates, leading to growth of P123 micelles.

0.0 0.5 1.0 1.5 2.0 2.5

0

5

10

15

20

25

30

diff

usi

on

coe

ffic

ien

t*1

0-8 (

cm2/s

)

concentration(wt%)

Figure (8) Variation of the diffusion coefficient of PluronicP123 micelles as a function of the

concentration of polyvinyl alcohol (wt %).

To interpret the results in a semi-quantitative manner, the variation of diffusion coefficient with

concentration of PVA is fitted using the equation.

D = Do e(-)

(5)

19

where Do is the diffusion coefficient in the absence of additive, is the volume fraction of

additive and is an exponent that characterizes the extend of structural changes. Similar

approaches have been used in literature to assess the diffusion of particles in complex fluids (6,7)

The solid line in figure 8 is a fit to the data using the above equation (5).

To identify the effect of various other water soluble polymers, the diffusion coefficients were

measured in the presence of polyethylene glycol (PEG) with two different molecular weights,

say 4000 and 6000. The combined plot of variation of the diffusion coefficient in the presence of

PEG 4000 and 6000 is shown in figure (9).

0 2 4 6 8 10

5

10

15

20

25

30

35

diff

usi

on c

oeffic

ient (c

m2/s

)

concentration(wt%)

PEG4000

PEG6000

Figure (9) Variation of diffusion coefficient of 5% w/w P123 micelles with different

concentrations of water soluble polymers.

20

The combined plot of variation in diffusion coefficient with PVA and PEG, as a function of the

volume fraction of polymer are shown in figure 10. From the result, it was noted that the

polyvinyl alcohol has a profound influence in the structure of P123 micelles. The solid lines are

fit to the data using equation 5 and the best fit parameters are summarized in table 1. The

pronounced effect of PVA is clearly evident from the variation of the exponent α obtained from

fitting the diffusion coefficient data as shown in figure (10). The parameter α obtained from

fitting is summarized in table (1)

Figure (10) shows variation of normalized diffusion coefficient of P123 micelles with volume

fraction of additives. The solid lines are fit to the data using equation (5).

0.00 0.02 0.04 0.06 0.08 0.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

D/D

o

PEG4000

PEG6000

PVA

21

Table (1) Parameter obtained from fit to the curve in figure (10) using equation (5).

No. Micelles Additives D0 α

1 P123 PEG 4000 28 8.79

2 P123 PEG 6000 28 14.22

3 P123 PVA 31 78.8

(b)Effect of polymer on diffusion of CTAB micelle

Next, we investigated the effect of water soluble polymers on diffusion of CTAB

micelles. CTAB is cationic surfactant and form ionic aggregates above its CMC. To minimize

the effect of repulsive interaction between charged micelles, experiments were carried out in the

presence of 0.6M NaCl. Figure (11) shows the variation of diffusion coefficient of 5% CTAB

micelle as a function of polyvinyl alcohol concentration. It was observed that the diffusion

coefficient decreases with an increase in the concentration of polyvinyl alcohol. This decrease in

diffusion coefficient can arises from a change in viscosity of the fluid or growth of the micelles.

However, the measured decrease in diffusion coefficient is much higher than that is expected

from the changes in viscosity of the solution. This suggests that addition of PVA alters the

structure of aggregates, leading to growth of CTAB micelles.

22

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

4

6

8

10

12

14

16

18

20

22

24d

iffu

sio

n c

oe

ffic

ien

t*1

0-8 (

cm2/s

)

concentration(wt%)

Figure(11) Variation of diffusion coefficient of CTAB micelles with the addition of polyvinyl

alcohol.

To interpret the results in a semi-quantitative manner, the variation of diffusion coefficient with

concentration of PVA is fitted using the equation (5).

The combined plot showing variation of the diffusion coefficient in the presence of cationic

surfactant is shown in figure (12).

23

0 2 4 6 8

0

5

10

15

20

25

30

35diff

usi

on c

oeffic

ient*

10

-8 (

cm2/s

)

concentration(wt%)

PEG4000

PEG6000

Figure (12) Variation of diffusion coefficient of 5% w/v CTAB micelles, in the presence of 0.6M

NaCl, with different concentration of water soluble polymers, PEG 4000 and PEG 6000.

From the result, it was noted that the polyvinyl alcohol has a profound influence in the structure

of CTAB micelles. This is clearly evident from the variation of the exponent α obtained from

fitting the diffusion coefficient data as shown in figure (13). The parameter α obtained from

fitting is summerised in table (2)

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Figure (13) shows variation of normalized diffusion coefficient of CTAB micelles with volume

fraction of additives. The solid lines are fit to the data using equation (5).

Table (2) Parameter obtained from fit to the curve in figure (13) using equation (5).

No. Micelle Additives D0 α

1 CTAB PEG 4000 31 21.39

2 CTAB PEG 6000 34 33.01

3 CTAB PVA 22 42.89

0.00 0.02 0.04 0.06 0.08

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1D

/Do

PEG4000

PEG6000

PVA

25

(c) Effect of polymer on diffusion of SDS micelle

Next, we investigated the effect of water soluble polymers on diffusion of SDS micelles.

Figure (14) shows the variation of diffusion coefficient of SDS micelle as a function of polyvinyl

alcohol concentration. It was observed that the diffusion coefficient decreases with an increase in

the concentration of polyvinyl alcohol. This decrease in diffusion coefficient can arises from a

change in viscosity of the fluid or growth of the micelles. However, the measured decrease in

diffusion coefficient is much higher than that is expected from the changes in viscosity of the

solution. This suggests that addition of PVA alters the structure of aggregates, leading to growth

of SDS micelles.

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

5

10

15

20

25

diff

usi

on c

oeffic

ient *1

0-8 (

cm2 /s

)

concentration (wt%)

Figure (14) Variation of diffusion coefficient of SDS micelles with the addition of Polyvinyl

alcohol.

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4. Conclusion

In conclusion, we investigated the effect of water soluble polymers such as PVA, PEG 4000 and

PEG 6000 on various micelles. The diffusion coefficients of the micelles were investigated by

dynamic light scattering. From the variation in the diffusion coefficient of the micelles, it was

noted that addition of polymer affects the diffusion of the micelles and alters the micelle

structure/interaction. By analyzing the data, using equation D = Do e(-)

it was noted that the

polyvinyl alcohol has a profound influence in the structure of P123 and CTAB micelles. By

comparing the variation in diffusion coefficient and fitted parameters of P123 and CTAB, it was

noticed that the exponent is higher for PVA in the case of P123 micelles, while it is higher for

PEG in the case of CTAB micelles. Further studies are needed to quantify the extend of micellar

growth in the presence of additives such as PVA and PEG.

27

5. References

1. Probe Dynamics in Semidilute Polymer Solution and Gels, Wilhem Oppermann, Sebastian

Seiffert, AIP Conf. Proc. 1027, 406 (2008)

2. Self-diffusion of non-interacting hard spheres in particle gels, Jean-Christophe Gimel and

Taco Nicolai J. Phys.: Condens. Matter 23 (2011) 234115

3. Diffusion of linear macromolecules and spherical particles in semidilute polymer solution and

polymer networks. Sebastian Seieffert, Wilhem Oppermann, Polymer 49 (2008) 4115–4126 .

4. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New

York, C. Tanford, 1980.

5. Dynamic Light Scattering: Application of Photon Correlation Spectroscopy; Plenum Press:

New York, R. Pecora, 1985.

6. Physical models of diffusion for polymer solution, gel and solid, L. Masaro, X.X. Zhu, Prog.

Polym. Sci. 24 (1999) 731–775.

7. A Hydrodynamic Model for Hindered Diffusion of Proteins and Micelles in Hydrogels,

Ronald J. Phillips, Biophys J. 79, 3350 (2000).