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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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
3
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)
8
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-
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).
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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
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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
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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).
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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
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
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
31
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:
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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.
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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).
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
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
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
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.
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.
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.
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
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
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
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 .
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
Figure 4. 14 Ethylcyclohexane admicellar partition coefficient (Kadm) for SHDPDS and three mixture mole fractions for SHDPDS and DPC
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
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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
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
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.
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.
50
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6 7 ( 3
6ROXWH�UHP RYHG�IURPSURFHVV�VWUHDP
E\�DGVROXELOL] DWLRQ
6 7 ( 3
6XUIDFWDQW�OD\HUZLWK�DGVROXELOL] DWH
GHVRUEHG
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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)
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.
51
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