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CHAPTER 1 INTRODUCTION 1.1 Background Recently, surfactant-based processes have been widely studied for use in environmental applications, including surfactant-based separation processes, micellar enhanced solubilization for enhanced contaminant extraction, and surfactant-modified materials for treating wastes and for landfill liners or subsurface barriers to reduce contaminant transport (Harwell and O’ Rear, 1989; Rouse et al., 1996; Sun and Jaffe, 1996; Butler and Hayes, 1998; Sabatini et al., 2000; Cheng and Sabatini, 2001). In all of these applications, surfactant adsorption onto solid surfaces is of interest. When undesirable, surfactant adsorption can render a design ineffective and significantly increase dosage requirements and thus hinder the economics of the system. Conversely, surfactant aggregates adsorbed at the solid-liquid interface can act as two-dimensional solvents, and organic solutes can partition into the adsorbed surfactants. This phenomenon, known as adsolubilization, has been widely studied in recent years (Harwell and O’ Rear, 1989). Adsolubilization has been used in many applications such as admicellar-enhanced chromatography (AEC), which is a new fixed- bed separation process based on using surfactants to induce adsorption of organic solute from the aqueous stream. Admicellar polymerization is a key process that may be used to form ultra-thin films on a substrate. Adsolubilization of pharmaceutical products by

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Page 1: Ampira Thesis Draft

CHAPTER 1

INTRODUCTION

1.1 Background

Recently, surfactant-based processes have been widely studied for use in

environmental applications, including surfactant-based separation processes, micellar

enhanced solubilization for enhanced contaminant extraction, and surfactant-modified

materials for treating wastes and for landfill liners or subsurface barriers to reduce

contaminant transport (Harwell and O’ Rear, 1989; Rouse et al., 1996; Sun and Jaffe,

1996; Butler and Hayes, 1998; Sabatini et al., 2000; Cheng and Sabatini, 2001). In all of

these applications, surfactant adsorption onto solid surfaces is of interest. When

undesirable, surfactant adsorption can render a design ineffective and significantly

increase dosage requirements and thus hinder the economics of the system.

Conversely, surfactant aggregates adsorbed at the solid-liquid interface

can act as two-dimensional solvents, and organic solutes can partition into the adsorbed

surfactants. This phenomenon, known as adsolubilization, has been widely studied in

recent years (Harwell and O’ Rear, 1989). Adsolubilization has been used in many

applications such as admicellar-enhanced chromatography (AEC), which is a new fixed-

bed separation process based on using surfactants to induce adsorption of organic solute

from the aqueous stream. Admicellar polymerization is a key process that may be used

to form ultra-thin films on a substrate. Adsolubilization of pharmaceutical products by

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food-grade surfactants can be applied in pharmaceutical formulations. Other potential

applications include water and soil remediation and surfactant-enhanced oil recovery.

In all of these applications of admicelles, this goal is to maximize surfactant

adsorption on the solid surface while minimizing the amount of surfactant required to

maintain the level of surfactant adsorption. Since the maximum surfactant adsorption

occurs at the surfactant critical micelle concentration (CMC), the goal can be achieved by

minimizing the CMC of the surfactant. Since the CMC of mixed anionic and cationic

surfactants is much lower than otherwise possible, these systems will be evaluated in the

current research.

Mixed anionic and cationic surfactant systems exhibit the greatest synergism

when it comes to reducing the CMC. The CMC of the mixed surfactant systems can be

reduced by as much as two to three orders of magnitude as compared to the single

surfactant system. Despite this exciting potential synergism, the tendency of mixed

anionic and cationic surfactant system to precipitate has limited their use in many

applications. However, a great deal of recent research studies on mixed anionic and

cationic surfactant systems has found ways to mitigate the precipitation potential (Li et

al., 1999; Marques et al., 1993). Previous research demonstrated that mixed anionic and

cationic surfactants having twin-head anionic surfactants and conventional cationic

surfactants were less susceptible to precipitation in solution due to the different structure

of each surfactant (Doan et al., 2002).

The purpose of this research is to maximize surfactant adsorption onto alumina

while minimizing the aqueous surfactant concentration by using mixed anionic and

cationic surfactants. Maximum surfactant adsorption is achieved because mixed anionic

and cationic surfactants increase the adsolubilization capacity of organic solutes of the

mixed adsorbed surfactant aggregates onto alumina. These results can be useful in many

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fields including surfactant-enhanced contaminant remediation, surfactant modification to

surfaces, nano-templating and surfactant-enhanced oil recovery.

1.2 Objectives

The main objective of this study is to investigate the adsorption characteristics of

mixed anionic and cationic surfactants, twin-head anionic surfactant and conventional

cationic surfactant system, onto positively charged alumina in batch equilibrium

experiments. The electrolyte concentration, solution pH, and temperature are fixed as

constant parameters. The specific objectives of this study are:

1. To investigate how the mole ratio of anionic and cationic surfactants in solution

affects the adsorption of the anionic and cationic surfactants onto alumina.

2. Identify the critical micelle concentrations (CMCs) of mixed anionic and cationic

surfactants onto alumina surface through establishing the point of plateau

adsorption in batch studies.

3. To investigate the solubilization and adsolubilization of organic solutes with

polar and nonpolar organic solutes by mixed anionic and cationic surfactant

system

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CHAPTER 2

THEORETICAL ASPECTS

AND LITERATURE REVIEWS

2.1 Surfactant phenomena

Surfactants, which are commonly known as soaps or detergents, are called

amphiphiles because of their unique and interesting chemical characteristics.

Surfactants are amphiphile molecules because they have both polar, hydrophilic head

groups (water-like) and nonpolar, hydrophobic tail groups (oil-like) in the same molecule.

Because of their amphiphile nature, surfactants will accumulate in interfacial regions (e.g.;

water-oil, water-air, liquid-solid interfaces) and as a consequence will reduce the

interfacial energy (Rosen, 1989). Surfactants are classified according to the nature of the

hydrophilic portion of the molecule: anionic surfactants (negatively charged head

groups), cationic surfactants (positively charged head groups), zwitterionic surfactants

(negatively and positively charged head groups) and nonionic surfactants (non-charged

head groups).

Depending on surfactant concentration in aqueous solution, surfactants are

capable of forming many different types of aggregates. At low concentration, surfactants

exist independent of one another in the solution phase and are called monomers.

Surfactant monomers will aggregate at interfaces that are present in the system. When

the surfactant concentration exceeds a certain level, surfactant monomers self-aggregate

into spherical aggregates known as micelles. In a micelle, the individual monomers are

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oriented with their hydrophilic head group facing the water or aqueous phase and their

hydrophobic tail group oriented into the interior of the spherical aggregates. Micelles

form when the surfactant concentration first exceeds the critical micelle concentration

(CMC), a value which varies for every surfactant. As additional surfactants are added

above the CMC, the incremental surfactants go to form additional micelles (West and

Harwell, 1992). Figure 2.1 shows the micelle formation of surfactants.

Figure 2. 1 Example of surfactant micellization

When a solid phase is added to the surfactant solution, the surfactants will adsorb

at the solid-liquid interface. At low surfactant concentrations, the surfactant begins to

adsorb and form micelle-like structures called hemimicelles and admicelles, depending

on whether the aggregates have one or two surfactant layers. Once the CMC is reached,

additional surfactants do not increase the amount of adsorbed surfactants, but rather

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increase the concentration of micelles in aqueous solution. Surfactant micelles, with

hydrophilic head groups (polar moieties) at the exterior and hydrophobic tail groups

(non-polar moieties) in the interior, exhibit certain unique properties. The polar exterior

makes a micelle highly soluble in water, while the non-polar interior provides a

hydrophobic sink for organic compounds, which can effectively increase the solubility of

organic compounds. Therefore the solubility of organic contaminants increases with

increasing micelle concentration in the solution, i.e., adding surfactant above the CMC.

2.2 Mixed anionic and cationic surfactants

Recently there has been a growing interest in research of mixed anionic and

cationic surfactant systems, including the synergistic effects of mixed micelle formation,

microemulsions, solubilization, and precipitation (Rosen et al., 1994; Shiau et al., 1994; Li

et al., 1999; Li and Kunieda, 2000; Doan et al., 2002). Mixed anionic and cationic

surfactant systems have many unique physicochemical properties that arise from the

strong electrostatic interactions between the oppositely charged head groups. Mixed

anionic and cationic surfactants exhibit the largest synergistic effect between surfactants

such as lower CMC and surface tension relative to single surfactant systems (Bergstrom,

2000; Kang et al., 2001; Chen et al., 2002).

2.2.1 Precipitation of mixed anionic and cationic surfactants

While mixtures of anionic and cationic surfactants exhibit the greatest

synergism, their potential to precipitate and form liquid crystal phases has limited

their use (Stellner et al., 1988; Meherteab, 1999). Figure 2.2 shows the

equilibrium present in solution containing anionic and cationic surfactants under

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conditions where the anionic and cationic surfactant form a precipitate with each

other and micelles are present in solution. The precipitation phenomenon of

mixed anionic and cationic surfactant systems has been studied by Scamehorn

and co-workers (Stellner et al., 1988; Scamehorn and Harwell, 1992). They

evaluated the precipitation phase boundaries of mixed anionic and cationic

surfactants over a wide range of concentrations by considering regular solution

theory and solubility relationships and developed a model to predict their results.

Shiau et al. (1994) considered the effects of sodium chloride (NaCl)

concentration and counterion binding on charged micelles in an effort to predict

precipitation of the anionic surfactants by calcium.

Figure 2. 2 Precipitation of anionic and cationic surfactants (Adapted from Scamehorn and Harwell, 1992)

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Since anionic and cationic surfactants tend to precipitate, many researcher

have pursued ways of avoiding the precipitation of these mixtures. Li et al.

(2000) investigated the solubilization and phase behavior of microemulsions with

mixtures of anionic and cationic surfactants and alcohols. They found that

alcohol addition was necessary to avoid liquid crystal formation, thereby allowing

formation of middle phase microemulsions. However, alcohol addition is

undesirable in environmental systems and consumer products. Thus recent

research has attempted to find methods capable of forming alcohol-free

microemulsions with mixed anionic and cationic surfactant systems, and

evaluated the use of mixed anionic and cationic surfactants in environmental

applications such as non-aqueous phase liquid (NAPL) removal in the

subsurface. Doan et al. (2002) investigated the role of surfactant selection in

designing alcohol-free microemulsion using mixed anionic and cationic surfactant

microemulsions. They found that twin-head group anionic surfactants were less

susceptible to precipitation in solution than single head group anionic surfactants

due to increased solubility and steric constraints (as shown in Appendix A).

2.3 Aluminum oxide surface structure

The crystal structure of alpha alumina oxide (Al2O3) is made up of hexagonally

packed oxygen atoms stacked on top of each other in an offset manner, with aluminum

ions packed between the oxygen layers as shown in Figure 2.3. Upon contact with water,

the crystal surface forms a layer of hydroxyl ions by a two-step process involving the

chemical adsorption of a monolayer of water and its dissociation. Since the alumina

surfaces are covered with hydroxyl groups, hydrogen and hydroxyl ions are the potential

determining ions for alumina. There is also a physically adsorbed layer of water

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molecules on top of the layer of hydroxyl ions. Therefore, the solution pH is critical for

adsorption of ionic surfactants because it controls the charged of the alumina surface.

Figure 2. 3 Schematic of crystal structure and surface layer of alpha aluminum oxide

The pH at which alumina has a net surface charge density of zero is called the

point of zero charge (PZC). At a solution pH below the PZC, the alumina surfaces are

positively charged; on the other hand, the alumina surfaces are negatively charged when

the solution pH is above the PZC. The PZC of aluminum oxide at 25oC has been

reported to be pH 9.1 (Sun and Jaffe, 1996). Alumina has been extensively studied as a

positively charged adsorbent for anionic surfactants and mixed anionic and nonionic

surfactants (Scamehorn et al., 1981; Lopata, 1988) and as a negatively charged adsorbent

for cationic surfactants at solution pH of 10 and ionic strength 0.03 M NaCl (Fan et al.,

1997)

2.4 Adsorption of ionic surfactants onto metal oxide surfaces

The adsorption of surfactants onto a solid interface is of great technological and

scientific interest because of its potential use in commercial applications and

environmental remediation. Examples of such applications are detergency, surfactant-

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g

ai,bi,i W

)VC(CG

−=

enhanced oil recovery, surfactant-enhanced subsurface remediation, surfactant-based

separation processes, and surfactant-modified materials. In addition, the adsorption

phenomenon is fundamentally important in understanding the solution and interfacial

behavior of surfactants.

Surfactant adsorption onto metal oxide surfaces such as alumina is a complex

process since solutes may adsorbed by ion exchange, ion pairing, and hydrophobic

bonding mechanisms. Adsorption of surfactants onto metal oxide surfaces has been

extensively studied including anionic surfactants onto positively charged surfaces

(Scamehorn et al., 1981; Harwell et al., 1985) and cationic surfactants onto negatively

charged surfaces (Goloub and Koopal, 1997)

The adsorption of surfactants at liquid-solid interfaces is usually characterized by

adsorption isotherms. A plot of surfactant adsorption onto a solid surface versus the

aqueous surfactant concentration at constant temperature is known as an adsorption

isotherm. The surfactant concentration before and after adsorption is quantified to

determine the amount of each species lost by adsorption.

(2.1)

where:

Γi = Adsorption density of surfactant i (mole/g)

V = Volume of sample (liter)

Ci,b = Concentration of surfactant at initial (mole/liter)

Ci,a = Concentration of surfactant at equilibrium (mole/liter)

Wg = Weight of aluminum oxide (g)

Equation 2.1 can be used to calculate the adsorption of the surfactant on the

mineral oxide surface. In this equation, the adsorption of water or salt is assumed to be

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negligible and the adsorption of the surfactant is assumed to have no effect on solution

density (Lopata, 1988).

The adsorption isotherm of ionic surfactants onto metal oxide surfaces is

typically an S-shaped isotherm (Somasudaran and Fuerstenau, 1966; Scamehorn et al.,

1981). Normally the S-shaped isotherm can be divided into four regions, as shown in

Figure 2.4. The designations for regions I, II and III first appeared in the work of

Somansudaran and Fuerstuenau (1966).

Figure 2. 4 Schematic presentation of typical surfactant adsorption isotherm

Region I corresponds to low surfactant concentration and low surfactant

adsorption. This region is commonly referred to as the Henry's Law region because in

this region, monoisomeric surfactants are generally adsorbed in a linear manner. In the

Henry's Law region, surfactants are adsorbed mainly by ion exchange, with the

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hydrophilic surfactant head groups adsorbing onto the solid surface. Adsorbed

surfactants in this region are shown as being adsorbed alone and not forming any

surfactant aggregates.

Region II is characterized by a sharply increased isotherm slope relative to the

slope in the Region I, which is a general indication of the onset of cooperative effects

between adsorbed surfactants: as the surface coverage increases due to tail-tail

interactions the tendency of surfactants to adsorb also increases. This increase in slope

indicates the beginning of lateral interactions between surfactant molecules, resulting

from interaction of the hydrophobic chains of oncoming surfactants with those of

previous adsorbed surfactants, and with themselves. This aggregation, which occurs at

concentrations well below the critical micelle concentration (CMC) of the surfactant, are

called admicelles or hemimicelles, depending on whether their structures are formed

as being local bilayers or local monolayers, respectively. The admicelle is considered as a

bilayer with the lower layer of head groups adsorb onto the solid surface and the upper

layer of head groups are facing to the solution. The hemimicelle is considered as a

monolayer with the head group of surfactant adsorbs onto the solid surface while the tail

group contacted with the solution.

Region III is characterized by a decrease in the isotherm slope relative to the

slope in Region II, because adsorption now must overcome electrostatic repulsion

between adjacent ions and the similarly charged solid surface or the beginning of

admicelle formation on lower energy surface patches.

Region IV is the plateau adsorption region for increasing surfactant

concentration. Generally, the equilibrium surfactant concentration at the transition

point from Region III to Region IV is approximately at the CMC of the surfactant. In

some systems, the Region III/Region IV transition point can be reached when the

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surface of the adsorbent becomes saturated with adsorbed surfactants. For the

adsorption of surfactants from the aqueous solution, this will correspond to bilayer

completion for ionic surfactants adsorbed on oppositely charged surfaces or to

monolayer completion for adsorption on hydrophobic surfaces. The psuedophase

separation model for surfactant adsorption of isomerically pure surfactants is shown in

the work of Harwell (1985).

Recently a new class of surfactants, known as twin-head surfactants (two-head

surfactants with one-tail group) such as sodium hexadecyl diphenyloxide disulfonate.

SHDPDS (as used in this study), have been widely studied for contaminant remediation

(Rouse et al., 1993; Sun and Jaffe, 1996; Carter et al., 1998; Deshpande et al., 2000; Doan

et al., 2002). Neupane and Park (1999) investigated the adsorption of gemini anionic

surfactant, which has two heads and two tails surfactants (dialkylated disulfonated

diphenyloxide, DADS-C12) and conventional anionic surfactants (sodium dodecylbezene

sulfonate, SDDBS) onto positively charged alumina. They found that the adsorption of

gemini surfactants is higher than the conventional surfactants. They also studied in the

partitioning of naphthalene by these surfactants onto the alumina for mobilization of

organic contaminant in subsurface through a batch experiment study.

2.4.1 Parameters affecting surfactant adsorption

The adsorption of surfactants at solid-liquid interfaces is strongly

influenced by a number of parameters: 1) the nature of the structural groups of

the solid surface; 2) the molecular structure of surfactant being adsorbed; and 3)

the environment of the aqueous solution e.g., pH, electrolyte content, and

temperature. Together these parameters determined the mechanism, by which

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the adsorption occurs, and the efficiency and effectiveness of surfactant

adsorption (Rosen, 1989).

The nature of the structural group of the solid surface (aluminum oxide

was used in this study) has already been mentioned above (Chapter 2.3). The

other parameters that affect surfactant adsorption are equilibrium pH, ionic

strength, and temperature.

2.4.1.1 Equilibrium pH

pH usually causes marked changes in the adsorption of ionic surfactants

onto charged solid surfaces. As the pH of the aqueous solution is lowered, the

alumina surface becomes more positive or less negative because of additional

protons adsorbing from the solution phase. This consequently increases the

adsorption of anionic surfactants and decreases the adsorption of cationic

surfactants (Rosen, 1989). When anionic surfactants adsorb onto alumina, the

equilibrium pH is usually higher than the initial solution pH because the anionic

surfactants exchange the ions with the adsorbed conterions and hydroxyl ions on

the alumina surface. Therefore, the equilibrium pH and the surfactant

adsorption are closely related to surfactant adsorption.

2.4.1.2 Ionic strength

Counter ions that are present in the surfactant solution are also present in

the surfactant admicelles at the solid-liquid interface. Counter ions can affect the

adsorbed surfactant by reducing electrostatic repulsion between ionic surfactant

head groups. When counter ions are present in the system, the admicelles are

formed more easily because of the repulsion between ionic surfactants is lower.

Admicelle patches with the complete bilayer are also capable of forming a larger

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aggregation number of surfactant molecule. As concentration of counterions in

solution increases, the maximum surfactant adsorption increases.

2.4.1.3 Temperature

Temperature increases generally causes a decrease in the efficiency and

effectiveness of ionic surfactant adsorption. The effect of temperature is

relatively small compared to that of pH. However, a rise in temperature usually

results in an increase in the adsorption of non-ionic surfactants containing a

polyelectrolyte chain as the hydrophobic group.

2.5 Mixed anionic and cationic surfactant adsorption

The solution properties of surfactant mixtures with oppositely charged head

groups usually experiences deviation from ideal mixing (e.g., mixtures of anionic

surfactants). Mixed anionic and cationic surfactant systems are expected to exhibit the

greatest diversion from ideal mixing behavior (Harwell and Scamehorn, 1992). There

have been few studies on the adsorption of mixed anionic and cationic surfactants onto

solid surface (Huang et al., 1989 and Capovilla et al., 1991). The one reason for a small

number of studies of the mixed anionic and cationic surfactants is their tendency to

precipitate.

Huang et al. (1989) studied the adsorption of mixed anionic and cationic

surfactants onto silica gel. They found that the adsorption of cationic surfactants was

enhanced by the amount of anionic surfactants present in the system and the adsorption

of cationic surfactants with addition of anionic surfactants to the system exactly equaled

to adsorbed anionic surfactants. They suggested that each adsorbed surfactant can co-

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adsorb with a cationic surfactant as an ion-pair onto uncharged silica gel through Van der

Waals interactions.

2.5.1 The effect to micelle formation of surfactant mixtures

The CMC of the single surfactant system is the aqueous surfactant

concentration in equilibrium with the maximum surfactant adsorption. For

mixed surfactant systems, when the surfactant solution is a mixture of surfactant

molecules, the CMC of the mixed surfactants does not correspond to the care of

either individual surfactant component. The CMC of mixed surfactant systems

can be predicted by the pseudophase separation model. If the micelles are

treated as a pseudophase and the formation of mixed micelles is treated with

either ideal solution theory (for surfactant systems with similar head groups) or

non-ideal solution theory (for surfactants with different head groups), the

concentration of the surfactant monomer of different surfactant components can

be predicted as a function of overall surfactant concentration. For a binary

surfactant system at constant weight fraction of surfactant 1 to surfactant 2, and

as the total surfactant concentration increases, the individual surfactant

concentration does not remain constant but changes continuously (Harwell and

Scamehorn, 1992). This is important to the application for surfactant adsorption

of mixed surfactant systems.

Capovilla et al. (1995) investigated the formation of mixed anionic and

cationic surfactant bilayers on laponite clay suspensions through the adsorption

of anionic surfactants by aqueous flocculated suspensions of laponite clay that

had been cationic-exchanged by cationic surfactant. The results from their

experiments showed that anionic surfactants favor tail-tail adsorption through

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Van der Waals interactions with a monolayer of adsorbed cationic surfactants

onto laponite clay. The schematic representation of anionic and cationic

surfactant bilayers at laponite clay interfaces shows that the lower layer cationic

surfactant head groups adsorb onto the negatively charged clay and the head

group of the upper layer of anionic surfactant is in contact with the solution, as

shown in Appendix A (Figure 1.2). They also found that ionic strength and the

structure of cationic surfactants affect the maximum adsorption and the stability

of mixed surfactant bilayers.

2.6 Solubilization

In pump-and-treat remediation, the amount of groundwater removed from the

subsurface to extract the contaminant depends on the aqueous solubility of the

contaminant. When surfactant is added into the aqueous phase, the organic interior of

micelle acts as an organic pseudophase into which the organic contaminants can be

partitioned. This process is known as solubilization.

In the aqueous system, the extent to which a solute will concentrate in a micelle

can be related to the octanol-water partitioning coefficient (Kow) of the organic solutes

(Edwards et al., 1991). In general, the larger the Kow (hydrophobicity) of an organic

solute the greater the tendency of the compound to concentrate in the micelle. Thus,

micelles in an aqueous solution represent an increased solubiliztion capacity of the

mobile aqueous phase for the organic solute over pure water.

Carter et al. (1998) evaluated various methods of increasing the solubility

enhancement of Dowfax components (alkylated diphenyloxide disulfonate and can be

mono- or disulfonated), such as using a co-surfactant, adding an electrolyte, and forming

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middle phase microemulsions. The result showed that the surfactant alkyl chain length

increased with increases in the solubility enhancement and the middle- phase

microemulsions greatly increased the solubility enhancement.

2.7 Adsolubilization

Effective utilization of adsorbed surfactant aggregates for processes such as

admicellar polymerization, admicellar chromatography, and ultra thin film formation,

necessitate a more complete understanding of the internal structure and capabilities of

these adsorbed layers.

Figure 2. 5 Phenomena of solubilization and adsolubilization

The hydrophobic core of an admicelle provides an ideal site for solubilizing

organic solutes. This process is known as adsolubilization. Normally, adsolubilization

is the partition of organic solutes into the interior of adsorbed surfactant aggregates.

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This phenomenon is the surface analog of solubilization, where the adsorbed surfactant

bilayer plays the role of the micelle. The phenomena of solubilization and

adsolubilization are shown in Figure 2.5.

Similar in nature to a micelle, the admicelle is characterized into three-regions.

The outer region has the most polar or ionic nature, which consists of the surfactant

head group. The inner region or the core region is non-polar in nature, which consist of

the hydrocarbon chain of surfactant tail groups. The palisade region is the region

between surfactant head groups and the core region. This region is intermediate in

polarity and consists of the carbon near head groups, and is also characterized by water

molecules that have penetrated the admicelle. The bilayer structure of surfactant is

admicelles shown in Figure 2.6.

Figure 2. 6 The bilayer structure of surfactant admicelles at the solid-liquid interface

Many studies have investigated organic solute partitioning into regions of the

admicelles. O’ Haver et al. (1989) studied the adsolubilization of alkane and alcohol into

surfactant admicelles on alumina surfaces. For alcohol systems, the ratios of alcohol to

surfactant admicelles were very high at low surfactant coverage; the adsolubilization of

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alcohol increased up to the CMC, and slightly decreased in the plateau adsorption. They

also found that the surfactant adsorption increased with decreasing ratio of alcohol to

surfactant admicelles to a value that similar to the ratio of alcohol to surfactant molecule

in micelles. For alkanes, the adsolubilization into surfactant admicelles was very high. In

addition, surfactant adsorption increased with increasing adsolubilization of alkane.

From this result, they predicted that the adsolubilization of alkane is approximately the

same as the solubilization of alkane into surfactant micelles. This indicated that the

interior of admicelles is similar to the interior of micelles.

Lee et al. (1990) showed that the adsolubilization and solubilization of alkane was

very similar. They explained the result of alcohol adsolubilization by a two-site model.

The model assumed that adsolubilization of a polar solute such as alcohol occurs both in

the palisade region and in the hydrophobic perimeter of disk-like admicelles (which are

not present in surfactant micelle). Since the fraction of adsolubilized alcohols at the

perimeter of admicelles can be significant at low surfactant coverage, the ratios of

adsolubilized alcohols to adsorbed surfactant were very high at low surfactant

adsorption. For adsolubilization of non-polar alcohol, they found that as the surfactant

coverage increased, the availability of the hydrophobic perimeter surface decreased along

with the admicellar partitioning coefficient (Kadm), thereby approaching the micellar

partitioning coefficient (Kmic).

As mentioned, it has been suggested that the admicellar partition coefficient can

be used to elucidate the locus of solubilization in the surfactant micelles (Edwards et al.,

1991; Rouse et al., 1993; Nayyar et al., 1994). Due to the analogy between micelles and

admicelles, the partition coefficient of solubilized micelles can be applied to adsolubilized

admicelles. Through the solubilization and the partition coefficients, the following

trends have been observed: 1) If the solute partitions primarily to the core, the partition

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coefficient increases with increasing mole fraction of the solute solubilization, 2) If the

solute partitions to the palisade layer, the partition coefficient decreases with increasing

mole fraction of the solute solubilization, and 3) If the solute partition into both the core

and palisade region, the partition coefficient remains relatively constant with the mole

fraction of solute solubilization (Dickson and O’Haver, 2002).

Kitiyanan et al. (1996) investigated adsolubilization of styrene and isoprene by

cationic surfactants onto silica. They calculated the partition coefficient of the organic

solutes into admicelles. The partition coefficient for styrene remained constant with the

increasing mole fraction of styrene, while the partition coefficient for isoprene decreased

with increasing mole fraction of isoprene. They concluded that styrene was partitioned

primarily both into the core and palisade layer of admicelles, and isoprene was

partitioned primarily to the palisade layer.

Additional research investigated the fundamental aspects of adsolubilization for

organic solutes into admicelles (Wu et al., 1987; Esumi, 2001) and adsolubilization of

organic solutes by mixed surfactant systems (Esumi et al. 2000 and 2001). Moreover,

many researchers are interested in the effect of various parameters to maximum

adsolubilization of organic solutes. Factors investigated included the effects of surfactant

concentration, solution pH (Esumi et al., 1996), electrolyte concentration (Pradubmook

et al., 2003), and structure of organic solute (Dickson and O'Haver, 2002). The results of

these research efforts indicate that the amount of adsorbed surfactants can be changed

by controlling both the amount of surfactants present at the solid-liquid interface and the

structure of the adsorbed layer or adsorbent.

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CHAPTER 3

METHODOLOGY

3.1 Materials

3.1.1 Mixed anionic and cationic surfactants

Figure 3.1 The structures of (a) anionic surfactant - sodium hexadecyl diphenyloxide disulfonate (SHDPDS) and (b) cationic surfactant- dodecyl pyridinium chloride (DPC)

3.1.1.1 Anionic surfactant. Sodium hexadecyl diphenyloxide

disulfonate (SHDPDS), a mixture of single-tailed and double-tailed

diphenyloxide disulfonate (about 3 to 1 mixture), was obtained as Dowfax

8390 (36 % active) from Dow Chemical Company (Midland, MI).

Chemically, this surfactant has two-negatively charged sulfonate groups.

3.1.1.2 Cationic surfactant. Dodecyl pyridinium chloride (DPC) 98%

purity was purchased from Aldrich chemical Co., Inc. (Milwaukee, WI). The

chemical structures of anionic and cationic surfactants are shown in Figure

3.1, and properties of these surfactants are shown in Table 3.1.

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Table 3.1 Properties of the surfactants

Surfactant Molecular formula M.W. a CMCa(mM)

SHDPDS - 642 6.3

DPC C17H30NCl 283 4.0

a data from Doan et al., 2002

3.1.2 Organic solutes

Styrene and ethylcyclohexane were selected as the organic solutes for

solubilization and adsolubilization studies. Styrene and ethylcyclohexane (99%

purity) were purchased from Fisher Scientific. Properties of the organic solutes

are shown in Table 3.2.

Table 3.2 Properties of the organic solute

Organic solute Molecular formula M.W.c Water solubility Kow

Styreneb C8H8 104.15 310 mg/L 2.95

Ethylcyclohexanec C8H16 112.21 - 4.40d b data from http://www.risk.lsd/ornl.gov. cdata from http://www.chemfinder.com. d data from (Gustafson et al., 1997)

3.1.3 Adsorbent

Aluminum oxide or alumina (Al2O3), mesh size 150, was purchased from

Aldrich Chemical Co., Inc. (Milwaukee, WI) and used as received. The surface

area was determined to be 133 m2/g (N2 BET adsorption method). The specific

surface area reported by the manufacturer product was 155 m2/g. The pH of the

point of zero charge (PZC) of alumina is 9.1 (Sun and Jaffe, 1996). Water

suspensions of alumina were weakly acidic (pH of 6.75).

Page 24: Ampira Thesis Draft

24

3.1.4 Chemicals

All chemicals used were ACS analytical reagent grade and used as

received. All solutions were made with double-distilled water. Plastic and

glassware were rinsed well with double-distilled water three times prior to use.

3.2 Experimental method

This study was divided into three experimental parts: adsorption, solubilization

and adsolubilization. All experiments were conducted as batch experiments at electrolyte

concentration of 0.015M NaCl, equilibrium pH of 6.5-7.5 and temperature of 20 ± 2 °C.

The mixtures of anionic and cationic surfactants were varied in mole fraction to

investigate the synergistic effects of mixed anionic and cationic surfactants. Mole

fractions of SHDPDS and DPC were prepared by adding 3:1, 10:1 and 30:1 SHDPDS

and DPC mole fractions with a constant SHDPDS concentration and varying DPC

concentration. The mixed surfactant ratios were selected based on the precipitation

phase diagram for the SHDPDS and DPC system (Doan et at., 2000; see Appendix A.1).

The ranges of parameters studied are shown in Table 3.3.

Table 3.3 Range of parameters evaluated in this study.

Parameter Range studied

Anionic surfactant (SHDPDS) 10-5 – 10-1 M

Cationic surfactant (DPC) 10-6 – 10-1 M

AIS:CIS 3:1, 10:1 and 30:1

pH 6.5-7.5

Electrolyte concentration 0.015 M (NaCl)

Temperature 20 ± 2 oC

Page 25: Ampira Thesis Draft

25

3.2.1 Adsorption study

Adsorption experiments were conducted in 40 ml vials using a constant

volume of 10 ml of mixed anionic and cationic surfactant solution and different

amounts of alumina (based on the estimated adsorption amount of the surfactant

on the alumina). The solution was equilibrated by shaking for at least 48 hours.

After shaking for 12 hours, the pH of the solution was measured and adjusted.

This process was repeated, but with a minimum waiting time of 3 hours, until the

pH of the solution remained constant at the desired level. After equilibration, the

solution was centrifuged to remove the solids. The concentration of anionic and

cationic surfactants in aqueous phase were then measured by Liquid

Chromatography (LC 20, Dionex). The amount of anionic and cationic

surfactant adsorption was calculated by equation 2.1 (also see Appendix 2).

3.2.2 Solubilization study

The extent of organic solute solubilization into surfactant micelles (no

alumina present) was studied for the single surfactants and the three mixtures of

mole fractions for SHDPDS and DPC. The solubilization study was conducted

in 40 ml glass vials with 20 ml of surfactant solution. The excess volume of

organic solute was added to the vials. The vials were equilibrated for 24 hours,

and then the organic solute concentration in aqueous solution was analyzed by

Gas Chromatography (GC3000, Varian).

3.2.3 Adsolubilization study

For adsolubilization studies, the adsorption isotherms were used to

determine an appropriate initial concentration of the mixed surfactant. The

appropriate concentration from adsorption isotherms was one that equilibrated

Page 26: Ampira Thesis Draft

26

just below the CMC of the surfactant (transition point) to ensure maximum

surfactant coverage without the presence of micelles in the bulk solution.

Adsolubilization experiments were conducted in 40 ml glass vials by

varying organic solute concentration with the appropriate surfactant

concentration and the amount of alumina from the adsorption study. The

solution was shaken for 48 hours and centrifuged to remove alumina. The

surfactant concentration and the organic solute concentration in aqueous

solution were analyzed by LC and GC, respectively.

3.3 Analytical method

Liquid Chromatography (LC20, Dionex) was used to quantify the individual

surfactant components of anionic and cationic mixtures. Analytical methods for

detecting anionic and cationic surfactants followed that use in previous research (Doan et

al., 2002). The anionic surfactant (SHDPDS) was analyzed using the coupling agent

tetrabutyle ammonium hydroxide (25 mN). The natural complex was separated with a

reverse phase column (Dionex-NS1) with an acetronitrile-water mobile phase. The

complex was then eluted from the column and de-coupled by ionic suppression and

finally detected by an electrical conductivity detector (Dionex-CD25).

Page 27: Ampira Thesis Draft

27

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Adsorption study

The adsorption of single surfactants and three mixture mole fractions of the

mixed SHDPDS and DPC surfactants were studied onto positively charged alumina

using an electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6.5-7.5 and

temperature of 20±2oC. The surfactant adsorption isotherm was plotted on logarithm

scale. The maximum adsorption was calculated as the mean value at the plateau region

of the surfactant adsorption isotherm. Figure 4.1 is an example of such a surfactant

adsorption isotherm.

Since, the maximum adsorption (plateau region) occurs at the CMC of the mixed

surfactant system, the CMC values of the surfactant systems were also determined

through the surfactant adsorption isotherm. The surfactant molecule per area was

calculated assuming that the surfactant molecule had access to the entire alumina surface,

with the specific area of alumina being 133 m2/g (measured by N2/BET adsorption

method). Table 4.1 summaries the experimentally determined CMCs, maximum total

surfactant adsorption (average at the plateau region), and surfactant molecule per surface

area for SHDPDS, DPC, and their mixtures.

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4.1.1 Adsorption of single surfactant system onto alumina for SHDPDS

and DPC

The adsorption isotherms of single surfactant systems of SHDPDS and

DPC onto positively charged alumina are shown in Figure 4.1. The results show

that the adsorptions of SHDPDS and DPC increase with increasing equilibrium

surfactant concentration. The maximum adsorption of SHDPDS, 2.4 x 10 -4

mole/g (1.09 molecule/nm2), was higher than the maximum adsorption of DPC

3.5 x 10-5 mole/g (0.15 molecule/nm2). The maximum amount of SHDPDS

adsorbed is consistent with the results reported by Sun and Jaffe (1996). Sun and

Jaffe studied the adsorption of Dowfax 8390 (or SHDPDS) onto aluminum

oxide without buffering the pH. They reported that the maximum adsorption of

Dowfax 8390 was 61,000 mg/kg (1.82 x10 -4 mole/g or 0.71 molecule/nm2 ,

calculated with 155 g/m2 for alumina as used in their study).

From Figure 4.1, it can be seen that SHDPDS was highly adsorbed onto

alumina due to the electrostatic attraction between negatively charged SHDPDS

(twin-head anionic surfactant that contains two-negatively charged head group)

and positively charged alumina. On the other hand, the adsorption of DPC was

very low due to the electrostatic repulsion between like charged cationic

surfactant head group and positively charged alumina. Nonetheless, there are

small amounts of DPC adsorbed onto alumina, possibly due to counterions

effect on the adsorption of cationic surfactant onto alumina. The counter ions

(0.015 M NaCl as used in this study) reduced the electrostatic repulsion between

positively charged cationic surfactants and positively charged alumina. Addition

of neutral electrolyte causes a decrease in the adsorption of ionic surfactants onto

oppositely charged adsorbent and an increase in their adsorption onto a similarly

Page 29: Ampira Thesis Draft

29

charged adsorbent (Rosen, 1989). Alumina surfaces at solution pH of 6.5 -.7.5,

which is two pH units below the PZC of alumina, probably has negative sites for

the adsorbed cationic surfactants.

4.1.2 Adsorption of mixed anionic and cationic surfactant system onto

alumina for SHDPDS and DPC

Adsorption of the three SHDPDS and DPC mole fractions of 3:1, 10:1

and 30:1 were conducted to investigate the synergistic effect of mixed anionic

and cationic surfactant adsorption onto alumina as shown in Figure 4.2 at

electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6-5-7.5, and

temperature of 20± 2oC. The total surfactant adsorption was plotted versus the

total equilibrium concentration for the three mole fractions for SHDPDS and

DPC. The total adsorption of the mixed surfactants increased as the total

surfactant increased in concentration, reaching a maximum adsorption for all

surfactant mixtures. The amount of adsorbed surfactant in the plateau maximum

adsorption region increased with increasing cationic surfactant molar ratio as

shown in Figure 4.3. While the mixed surfactant system of 3:1 SHDPDS:DPC

molar ratio provided the highest amount of adsorbed surfactants onto alumina,

this maximum total surfactant adsorption (3.10 x 10 -4 mole/g for 3:1 SHDPDS:

DPC molar ratio) was only 25% percent higher than the SHDPDS alone (2.4 x

10-4 mole/g). The maximum surfactant adsorption versus anionic/cationic

surfactant molar ratio in the mixed surfactant system are shown in Figure 4.3

Figure 4.4 shows the adsorption of SHDPDS alone and for the three

mixture mole fractions of SHDPDS and DPC onto alumina. The results show

that the adsorption of SHDPDS increased with increasing equilibrium surfactant

concentration. However, the maximum SHDPDS adsorption in the three mixed

Page 30: Ampira Thesis Draft

30

surfactant systems was virtually the same. Thus, it can be seen that there is no

significant increase in the SHDPDS adsorbed with additional of DPC.

The adsorption of DPC alone and for the three mixture mole fractions of

SHDPDS and DPC are also shown in Figure 4.5. It is interesting to note that at

the same SHDPDS concentration, the DPC adsorption increased as the DPC

mole fraction in the surfactant mixture was increased, and that this increase was

most dramatic for low surfactant concentrations.

From the adsorption of individual anionic and cationic surfactants in the

mixed surfactant system, it can be inferred that the anionic surfactants readily

adsorbed onto alumina due to the electrostatic attraction between two negatively

charged head anionic surfactant. At the same time, it appears that the positively

charged alumina does in fact contain a number of negatively charged sites that

account for the adsorption of cationic surfactants onto the alumina, even at a pH

of 7 which is two pH units below the PZC. Capovila et al. (2000) studied the

formation of mixed anionic and cationic surfactant adsorbed onto laponite clay at

solution pH of 8.5. They found that cationic surfactant head groups adsorbed

onto negatively charged clay and provided hydrophobic layers for adsorbed

hydrophobic tails of anionic surfactants.

4.2 Solubilization study

Before looking at the adsolubilization into adsorbed surfactant, the solubilization

of organic solute into surfactant micelles was investigated to allow a comparison between

the two types of aggregates. The study of the solubilization potentials (ability to enhance

organic solute in solution) of single surfactants and the three mixture mole fractions for

Page 31: Ampira Thesis Draft

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SHDPDS and DPC were conducted with styrene and ethylcyclohexane at electrolyte

concentration of 0.015 M NaCl and temperature of 20 ± 2 oC. Solubilization isotherms

were plotted with organic solute solubilization versus aqueous surfactant concentration

in the solution in logarithm scale. The transition point was determined as the CMC of

each surfactant mixtures. Table 4.2 shows the CMC values of single surfactants and the

three surfactant mixture mole fractions (which were determined experimentally from

Figure 4.6 and Figure 4.7). It can be seen that the CMC values of each surfactant varied

with presence of the organic solutes. Rosen (1989) reports that surfactant CMC values

are impacted by solubilization of organic solutes could change because the activity of the

surfactant is changed by the introduction of organic solutes.

The micellar partitioning coefficient (Kmic) describes the partitioning of the

various organic solutes into the micelle (Edwards et al., 1991). Kmic is defined as

aq

micmic X

XK = (4.1)

where;

Xmic is the mole fraction of the organic solute in the micelle pseudophase.

Xaq is the mole fraction of the organic solute in the aqueous phase.

The mole fraction are calculated as

)S(S)C(CCC

Xeq0eq0

eq0mic −+−

−=

(4.2)

55.55CC

Xeq

eqaq +

= (4.3)

where:

Page 32: Ampira Thesis Draft

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C0 = the concentration of organic solute at initial

Ceq = the concentration of organic solute at equilibrium

S0 = the concentration of surfactant at initial

Seq = the concentration of surfactant at equilibrium

55.55 = represents 1/molar volume for water

The partitioning of organic solutes is described by the molar solubilization ratio

(MSR), which is the slope of the solubilization isotherm beyond the CMC value (see

Figures 4.8 and 4.9). MSR indicates the moles of organic solute in the micelle per moles

of micellar surfactant. Table 4.2 summarizes the molar solubilization ratio and Kmic of

this study. MSR determinations were determined based on straight-line function with r2

regression greater than 0.97. The mole fraction of the organic solute in micelles is related

to MSR by the simple relationship (Rouse et al., 1995).

MSR)(1MSRXmic +

= (4.4)

55.55)/(CCMSR)MSR/(1K

eqeqmic +

+= (4.5)

From MSR values in Figure 4.8 and Figure 4.9, the result shows that the MSR for

both styrene and ethylcyclohexane increased with increasing surfactant concentration.

The order of solubilization potential (MSR) is DPC <SHDPDS< 30:1< 10:1 <3:1

SHDPDS:DPC molar ratio. This trend is observed for the solubilization of both styrene

and ethylcyclohexane by SHDPDS and DPC. The solubilization potential of styrene and

ethylcyclohexane by SHDPDS is greater than DPC. This is likely because the cationic

head group is larger and more electrostatic, producing a less desirable structure for

solubilization.

Page 33: Ampira Thesis Draft

33

The second trend was observed in the mixed surfactant system. The

solubilization potential increased with increasing a mole ratio of cationic surfactant in the

surfactant mixtures for both styrene and ethylcyclohexane system. As the cationic

surfactant concentration increased, the electrostatic repulsion between anionic surfactant

head group was reduced. Micelles form more easily and the packing density (aggregation

number) increase due to the reduction in electrostatic repulsion between the head groups

reducing the electrical potential in micelles, and providing a more favorable

environmental for the organic solute.

It can be seen that the solubilization potential of ethylcyclohexane is higher than

styrene for the SHDPDS and the three mixtures mole fractions. The MSR value of

ethylcyclohexane is about 3 times that of styrene. The ethylcyclohexane showed much

greater partitioning because the hydrophobicity of ethylcyclohexane (log Kow= 4.40) is

higher than styrene (log Kow = 2.95). Thus, the partitioning of ethylcyclohexane into

hydrophobic site of micelles became more favorable.

4.3 Adsolubilization Studies

The adsolubilization isotherm of styrene and ethylcyclohexane by SHDPDS and

the three mixture feed mole fractions for SHDPDS and DPC onto alumina are shown in

Figure 4.10 and 4.11, respectively at electrolyte concentration of 0.015 M NaCl,

equilibrium pH of 6.5-7.5, and temperature of 20±2 oC. Styrene is a polar organic solute

and ethylcyclohexane is a non-polar organic solute, which is expected to partition into

the palisade region and the core of the admicelles, respectively.

The admicellar partition coefficient (Kadm) is analogous to the micellar partition

coefficient (Nayyar et al., 1994).

Page 34: Ampira Thesis Draft

34

aq

admadm X

XK = (4.6)

where

Xadm is the mole fraction of organic solute in the admicelle phase

Xaq is the mole fraction of organic solute in the aqueous phase

For this study, Xadm values are calculated as:

( )( ) )S(S)(SCC

CCX

CISf,CISi,AISf,AISi,Sf,Si,

Sf,Si,adm ++++−

−=

S (4.7)

where:

Xadm = Mole fraction of organic solute in admicelle

Ci,S = Initial concentration of organic solute (M)

Cf,S = Final concentration of organic solute (M)

Si,AIS = Initial concentration of anionic surfactant, (M)

Sf,AIS = Final concentration of anionic surfactant, (M)

Si,CIS = Initial concentration of cationic surfactant, (M)

Sf,CIS = Final concentration of cationic surfactant, (M)

4.3.1 Styrene Adsolubilization

The adsolubilization isotherms of styrene by SHDPDS and SHDPDS-

DPC mixtures are shown in Figure 4.10. The results show that the amount of

adsolubilized styrene increased with increasing equilibrium styrene concentration

for all adsolubilization isotherms. As expected, the styrene adsolubilization

reached a maximum as the styrene concentration reaches its water solubility, with

the concentration of surfactant below its CMC. Figure 4.11 also shows the

adsolubilized styrene by the mixed anionic and cationic surfactant system for

different mole fractions of cationic surfactants in the surfactant mixtures. It can

be seen that the styrene admicellar mole fraction becomes less favorable with

increasing the mole fraction of cationic surfactants in the mixtures. As the

Page 35: Ampira Thesis Draft

35

concentration of cationic surfactant increased, the net electrostatic charge of the

anionic surfactant head group is reduced, which should facilitate the

incorporation of polar organic solute into the palisade region of admicelles. In

the palisade region, the cationic surfactants attach to the adsorbed anionic

surfactants. As a result, the polar organic solute, styrene, which is expected to

adsolubilize into the palisade layer of admicelles, is then squeezed out. The

admicellar partition coefficient (Kadm) values for styrene adsolubilization in

SHDPDS and the three mixture mole fractions as shown in Table 4.2 are

determined by the slope of adsolubilization isotherm (Figure 4.11). In order to

gain insight into locus of adsolubilization of styrene in the mixed surfactant

admicelle, The Xadm/Xaq of styrene (calculated from Equations 4.6 and 4.7) versus

equilibrium styrene concentrations are shown in Figure 4.12. The The Xadm/Xaq

slowly decreases with increasing equilibrium styrene concentration, supporting

that the polar styrene is adsolubilized into the palisade region of the surfactant

admicelles (Nayyar et al., 1994; Dickson and O’Haver, 2002). However, Kitiyanan

et al. (1996) studied the adsolubilization of styrene by a cetyltrimethylammonium

bromide (CTAB) bilayer onto precipitated silica. The results showed that the

styrene adsolubilization constant is unchanged with increasing equilibrium

styrene concentration in the aqueous phase, suggesting that the styrene was

adsolubilized in the both the core and the palisade region of the admicelles.

4.3.2 Ethylcyclohexane Adsolubilization

The adsolubilization isotherms of ethylcyclohexane by SHDPDS and

SHDPDS-DPC mixtures are shown in Figure 4.13. The results show that the

amount of adsolubilized ethylcyclohexane increased with increasing equilibrium

ethylcyclohexane concentration for all adsolubilization isotherms. Figure 4.13

Page 36: Ampira Thesis Draft

36

also shows the adsolubilized ethylcyclohexane by the mixed anionic and cationic

surfactant system with different mole fractions of cationic surfactant in the

mixtures. It can be seen that the ethylcyclohexane admicellar mole fraction

became more favorable with increases in the mole fraction of cationic surfactants

in the mixtures. However, the adsolubilization of ethylcyclohexane appears to be

independent of the cationic surfactant mole fraction in 30:1 SHDPDS:DPC

molar ratio. The admicellar partition coefficient (Kadm) values for

ethylcyclohexane, as shown in Table 4.2, were determined from slopes in the

adsolubilization isotherm (Figure 4.14). For locus of adsolubilization of

ethylcyclohexane, Figure 4.15 shows Xadm/Xaq of ethylcyclohexane versus

ethylcyclohexane concentration at equilibrium. The Xadm/Xaq of ethylcyclohexane

at low ethylcyclohexane concentration loading increased with increasing

equilibrium ethylcyclohexane concentration and then slowly decreased at the high

concentration. At low ethylcyclohexane concentration suggested that the non-

polar ethylcyclohexane adsolubilized into the core of the admicelles. However, it

is not clear why the Xadm/Xaq ratio decreased at the high ethylcyclohexane

concentration.

From the mixed anionic and cationic surfactant adsorption study, it is

suggested that the adsorbed anionic surfactant molecules lie flat on alumina

surface at a low surfactant concentration, and provided hydrophobic sites, as the

tail group of anionic surfactants facing to aqueous solution. The interactions of

hydrophobic tail groups increase with increasing surface coverage. The

schematic illustration, with mixed anionic and cationic surfactant adsorption and

adsolubilization, helps to explain our results as shown in Figure 4.16. In this

structure, two-negatively charged head groups of anionic surfactant are adsorbed

Page 37: Ampira Thesis Draft

37

onto the positively charged alumina surface with hydrophobic tail groups facing

into the aqueous phase and leading to anionic and cationic surfactant bilayers. As

cationic surfactant molecules increase, the hydrophobic site in the core region

increases which in turn promotes the adsolubilization of non polar

ethylcyclohexane in the admicelle. When the cationic surfactant present in the

system, it decreases the net charge of the surface, thereby making the core more

hydrophobic and promoting the formation of denser aggregates, both of which

would increase hydrophobic adsolubilization. Thus, increasing cationic

surfactant mole fraction in the mixed anionic and cationic surfactant systems

promotes adsolubilization into the core region and resists the adsolubilization

into the palisade region by changing the nature of the adsorbed surfactant

aggregates.

From the partition coefficient values in the solubilization and

adsolubilization studies, it is observed that the Kmic values for styrene and

ethylcyclohexane in solubilization study are the same order as the corresponding

Kadm values in adsolubilization study, but consistently lower. This could be due

to the higher packing density of the admicelles which are fixed on the solid

surface, than that of the micelles. Recent research study found nearly identical

micellar and admicellar partitioning for organic solutes (Park, and Jaffe, 1993;

Rouse et al., 1993). Park and Jaffe (1993) also found that organic solute

partitioning into micelles/admicelles was proportional to the mass of surfactant

molecules. However, the point to be addressed here is that admicellar

partitioning in adsolubilization can be as attractive as micellar uptake in

solubilization for organic solutes. This phenomenon can be useful in

environmental applications.

Page 38: Ampira Thesis Draft

38

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

Surfactant equilibrium concentration (M)

Surf

acta

nt a

dsor

ptio

n ( m

ole/

g)

SHDPDS

DPC

Figure 4. 1 The adsorption isotherm of SHDPDS and DPC onto alumina at

electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6.5-7.5 and temperature of

20±2oC.

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

Total surfactant equilibrium concentration (M)

Tot

al s

urfa

ctan

t ads

orpt

ion

(mol

e/g)

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

DPC

Figure 4. 2 The single surfactants and three mixture mole fractions for SHDPDS and

DPC adsorption onto alumina at electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6.5-7.5, and temperature of 20±2 oC.

Page 39: Ampira Thesis Draft

39

DPC

3:1 SHDPDS:DPC

30:1 SHDPDS:DPC

10:1 SHDPDS:DPC

SHDPDS

0.0E+0

5.0E-5

1.0E-4

1.5E-4

2.0E-4

2.5E-4

3.0E-4

3.5E-4

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Cationic/Anionic surfactant mole ratio

Max

imum

sur

fact

ant a

dsor

ptio

n (m

ole/

g)

Anionic rich Cationic richPrec

ipita

tion

boud

ary

ofSH

DPD

S/D

PC

Figure 4. 3 Maximum surfactant adsorption and cationic/anionic surfactant molar ratio

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

SHDPDS equilibrium concentration (M)

SHD

PDS

adso

rptio

n (m

ole/

g)

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 4 SHDPDS adsorption for SHDPDS alone and for three mixture mole fractions of SHDPDS and DPC onto alumina at electrolyte concentration of 0.015 NaCl, equilibrium of pH 6-5-7.5, and temperature of 20±2 oC.

Page 40: Ampira Thesis Draft

40

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

DPC equilibrium concentration (M)

DPC

ads

orpt

ion

(mol

e/g)

DPC

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 5 DPC adsorption for DPC alone and for three mixture mole fractions of SHDPDS and DPC onto alumina at electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6.5-7.5, and temperature of 20±2 oC.

Table 4. 1 Experimentally determined CMCs, the maximum total adsorption, and molecule per area of single surfactants and three mixture mole fractions for SHDPDS and DPC system.

Surfactants Experimentally

determined CMC (mM)

Max. total adsorption

(mmole/g)

Area per molecule (molecule/nm2)

SHDPDS 0.49 0.239 1.09

30:1 DPDS:DPC 0.46 0.252 1.15

10:1 DPDS:DPC 0.43 0.278 1.26

3:1 DPDS:DPC 0.55 0.301 1.36 DPC - 0.031 0.10

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41

1.0E-3

1.0E-2

1.0E-1

1.0E+0

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

Surfactant concentration (M)

Styr

ene

solu

biliz

atio

n (M

)

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

DPC

Figure 4.6 Solubilization isotherm of styrene by single surfactant and three mixture feed mole fractions for SHDPDS and DPC at electrolyte concentration of 0.015 M NaCl

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0

Surfactant concentration (M)

Eth

yl. s

olub

iliza

tion

(M)

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

DPC

Figure 4. 7 Solubilization isotherm of ethylcyclohexane by single surfactant and three mixture feed mole fractions for SHDPDS and DPC at electrolyte concentration of 0.015 M NaCl

CMC Slope = MSR

CMC Slope=MSR

Page 42: Ampira Thesis Draft

42

MSR = 1.4822

MSR= 0.9167x

MSR = 1.3723

MSR = 1.5747

MSR = 0.2192x

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.02 0.04 0.06 0.08

Surfactant concentration (M)

Styr

ene

solu

biliz

atio

n (M

)

SHDPDS3:1 SHDPDS:DPC10:1 SHDPDS:DPC30:1 SHDPDS:DPCDPC

Figure 4. 8 Molar surfactant ratio (MSR) of styrene for single surfactant and three mixture mole fractions for SHDPDS and DPC

MSR = 2.5191

MSR = 2.1742

MSR = 2.3822

MSR = 2.8753

MSR = 0.0729

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.02 0.04 0.06 0.08

Surfactant concentration (M)

Eth

yl. s

olub

iliza

tion

(M)

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

DPC

Figure 4. 9 Molar surfactant ratio (MSR) of ethylcyclohexane for single surfactant and mixture feed mole fractions for SHDPDS and DPC

Page 43: Ampira Thesis Draft

43

Table 4. 2 CMC of surfactant, MSR value from solubilization study, and partitioning values obtained in this study

Surfactant Log Kow CMC Scmc MSR Log Kmic Log Kadm mM mM

Styrene 2.95

SHDPDS 1.00 3.46 0.9154 3.88 4.52

DPDS:DPC 30:1 1.10 3.48 1.3710 3.96 -

DPDS:DPC 10:1 0.90 3.44 1.4822 3.98 4.36

DPDS:DPC 3:1 0.80 3.52 1.5734 3.99 4.37

DPC 4.00 3.17 0.2216 3.50 -

Ethylcyclohexane 4.40

SHDPDS 0.80 1.59 2.1716 4.38 4.92

DPDS:DPC 30:1 0.70 1.56 2.3848 4.51 4.90

DPDS:DPC 10:1 0.60 1.41 2.5191 4.45 4.91

DPDS:DPC 3:1 0.80 1.21 2.8720 4.42 4.91

DPC 2.00 1.06 0.0729 3.55 - Scmc – organic solute concentration at CMC of surfactant

0.00

0.20

0.40

0.60

0.80

1.00

0 1 2 3 4 5Styrene aqueous mole fraction, Xaq (10-5)

Styr

ene

adm

icel

lar

mol

e fr

actio

n, X

adm

SHDPDS

3:1 SHDPDS

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 10 Adsolubilization of styrene by SHDPDS and three mixture mole fractions for SHDPDS and DPC at electrolyte concentration of 0.015 M NaCl, equilibrium pH 6.5-7.5 and temperature of 20±2 oC

Page 44: Ampira Thesis Draft

44

K = 0.3326 K = 0.2294

K = 0.2341

0.00

0.20

0.40

0.60

0.80

0.00 0.50 1.00 1.50 2.00Styrene aqueous mole fraction, X aq (10-5)

Styr

ene

adm

icel

lar m

ole

frac

tion

,Xad

m

S H D P D S

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 11 Styrene admicellar partition coefficient (Kadm) by SHDPDS and three mixture mole fractions for SHDPDS and DPC.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025

Styrene equilibrium concentration (M)

Styr

ene

adm

icel

lar

part

ition

coe

ffici

ent,

Kad

m

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 12 Ploted Xadm/Xaq of styrene versus styrene concentration at equilibrium for SHDPDS and three mixture mole fractions for SHDPDS and DPC .

Page 45: Ampira Thesis Draft

45

0.00

0.20

0.40

0.60

0.80

1.00

0 1 2 3 4 5Ethylcyclohexene aqueous mole fraction, Xaq (10-5)

Eth

yl. a

dmic

ella

r mol

e fr

actio

n Xad

m

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 13 Adsolubilization of ethylcyclohexane by SHDPDS and three mixture mole fractions for SHDPDS and DPC at electrolyte concentration of 0.015 M NaCl, equilibrium pH of 6.5-7.5 and temperature of 20±2 oC

K = 0.8386

K = 0.8538

K = 0.8144

K= 0.8091

0.00

0.20

0.40

0.60

0.80

1.00

0.00 0.50 1.00 1.50 2.00

Ethylcyclohexene aqueous mole fraction, Xaq (10 -5)

Eth

ycyc

lohe

xene

adm

icel

lar

mol

e fr

actio

n, X

adm

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 14 Ethylcyclohexane admicellar partition coefficient (Kadm) for SHDPDS and three mixture mole fractions for SHDPDS and DPC

Page 46: Ampira Thesis Draft

46

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010

Ethycyclohexene equilibrium concentration (M)

Xad

m/X

aq o

f Eth

ylcy

cloh

exan

e

SHDPDS

3:1 SHDPDS:DPC

10:1 SHDPDS:DPC

30:1 SHDPDS:DPC

Figure 4. 15 Plotted Xadm/Xaq of ethylcyclohexane versus ethylcyclohexane concentration at equilibrium by SHDPDS and for three mixture mole fractions for SHDPDS and DPC

�FRU�UHJLRQ �SOLVDGH�UHJ LRQ

OLTXLGVROLG

��

�FRU�UHJLRQ �SOLVDGH�UHJ LRQ

OLTXLGVROLG

��

Figure 4. 16 Schematic representation of adsolubilization in (a) twin-head anionic surfactant (SHDPDS) and (b) mixed anionic and cationic surfactant (SHDPDS and DPC) aggregates onto positively charge alumina

Page 47: Ampira Thesis Draft

47

CHAPTER 5

SUMMARY CONCLUSIONS

AND ENGINNERING SIGNIFICANCE

5.1 Summary

The synergism of the anionic and cationic surfactant system for SHDPDS-DPC

through the adsorption and adsolubilization of styrene and ethylcyclohexane onto

positively charged alumina was studied at different cationic surfactant mole fractions in

the mixed surfactant system at electrolyte concentration of 0.0015 M NaCl, equilibrium

pH of 6.5-7.5 and temperature of 20±2oC. The results showed that the adsorption,

solubilization, and adsolubilization were promoted by the cationic surfactants. The

adsorption of SHDPDS and DPC system showed low synergism due to only the

adsorbed cationic surfactants were enhanced, but there are no significantly different on

the adsorbed anionic surfactants with the additional cationic surfactants. This may be

due to the twin-head structure of the SHDPDS surfactant, which decreases the

synergism in other precipitation tendency but also appears to reduce synergism.

However, the different mole fractions of cationic surfactants in the mixed anionic and

cationic surfactants system had greater impact on solubilization and adsolubilization than

on adsorption. The solubilization capacity of both styrene and ethylcyclohexane

increased with increasing the cationic surfactant mole fraction in the mixed surfactant

system due to reduced electrostatic repulsion between anionic head groups reducing

increased micelle formation. Through the adsorption study there is expected to exist of a

Page 48: Ampira Thesis Draft

48

maximum form of admicellar aggregates. For the adsolubilization studies, the increasing

cationic surfactant mole fraction in the mixed system increased the adsolubilized

ethylcyclohexane and decreased adsolubilized styrene. From these results, it can be

inferred that the tight packing arrangement of the mixed anionic and cationic surfactant

systems promoted adsolubilized ethylcyclohexane in the core region and resisted the

adsolubilized styrene in the palisade region of the admicelle. The admicellar partition

coefficient data further supported that styrene partitions into the palisade region and

ethylcyclohexane partitions into the core region of admicelles. The admicellar partition

coefficient (Kadm) of the organic solute was of the same order as the corresponding to

micellar partition coefficient (Kmic). As a result, admicellar partitioning can be attractive

as the micellar partitioning and this phenomenon can be use in environmental

applications. Although this mixed anionic and cationic surfactant system demonstrated

low adsorption synergism, it has great potential for enhancing solubilization and

adsolubilization of organic contaminants. Thus, this research provides useful

information for designing surface modification by surfactants to enhanced contaminant

remediation.

5.2 Conclusions

Based on the results of this research, the following conclusions are made.

1. The adsorption of a mixed anionic and cationic surfactant system, SHDPDS

and DPC, onto positively charged alumina showed a slight synergism with

only the adsorption of cationic surfactants increased in the mixed system.

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49

2. Increasing the cationic surfactant mole fraction in the mixed system

promoted slight increases in the solubilization capacity of styrene and

ethylcyclohexane

3. Increasing the cationic surfactant mole fraction in the mixed surfactant

system promoted the adsolubilization of non-polar organic solutes in the core

region and resisted the adsolubilization of polar organic solutes in the

palisade region of the admicelles.

4. The admicellar partition coefficient (Kadm) for the adsolubilization process is

comparable to study can be attractive as the micellar partition coefficient

(Kmic) for the solubilization process.

5.3 Engineering significance

Surfactant-modified surfaces can be used in many industrial and commercial

processes and environmental engineering applications. Metal oxide coated with

surfactants appears particularly promising for treatment of groundwater and wastewater

for removal of organic compounds by the adsolubilization process.

In field application for subsurface remediation, surfactant modified surfaces can

be used in landfill liners or subsurface barriers which effectively prevent organic

contaminants from mitigating in groundwater. For wastewater treatment, surfactant

modified surfaces could be used as a strong adsorbent for organic compound removal in

wastewater streams which is known as admicellar-enhanced chromatography.

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50

6 7 ( 3

6XUIDFWDQW�OD\HUIRUP HG�RQ�SDFNLQJ

6 7 ( 3

6ROXWH�UHP RYHG�IURPSURFHVV�VWUHDP

E\�DGVROXELOL] DWLRQ

6 7 ( 3

6XUIDFWDQW�OD\HUZLWK�DGVROXELOL] DWH

GHVRUEHG

6XUIDFHDJJUHJDWH

0 RQRPHU $JJUHJDWH�ZLWKDGVROXELOL]DWH

$GVROXELOL]DWH

&RQFHQWUDWHG�SURGXFW�VWUHDP

6XUIDFWDQW�DGGHGWR�VWUHDP

(IIOXHQW�ZDWHU3 URFHVV�VWUHDP

(IIOXHQW�ZDWHU (IIOXHQW�ZDWHU

Figure 5.1 The admicellar-enhanced chromatography processes (adapted from Harwell and O’Rear, 1992)

Admicellar-enhanced chromatography (AEC) utilizes adsorbed surfactant

aggregates on solid surfaces and the phenomenon of adsolubilization. If the aqueous

solution, which contains dissolved organic solute, is contacted with solid containing

adsorbed surfactant aggregates, the solute will tend to adsolubilize into these aggregates,

and a purified water stream then results. The adsorption bed can then be contacted with

a solution of different pH, causing the surfactant to be desorbed along with the organic

solutes, producing a concentrated solution. The bed can be retreated with surfactant and

the process repeated indefinitely. The admicellar-enhanced chromatography process is

shown in Figure 5.1.

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51

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