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Mixtures of Anionic and Cationic Surfactants with Single and Twin Head Groups: Solubilization and Adsolubilization of Styrene and Ethylcyclohexane A. Fuangswasdia, A. Charoensaenga, D.A. Sabatinib,d,*, J.F. Scamehornc,d, E.J. Acostab–d, K. Osathaphane, and S. Khaodhiare a

National Research Center, Environmental Hazardous Waste Management, Chulalongkorn University, Bangkok, Thailand; b Civil Engineering and Environmental Science Department, Carson Engineering Center, cChemical, Biological, and Materials Engineering Department, Sarkeys Energy Center, and dInstitute for Applied Surfactant Research (IASR), Sarkeys Energy Center, The University of Oklahoma, Norman, Oklahoma; and e Environmental Engineering Department, Chulalongkorn University, Bangkok, Thailand

ABSTRACT: Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilize styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1:3; however, the SHDPDS/DPCl mixture at a ratio of 3:1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCl adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHDPDS/DPCl systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle. Paper no. S1509 in JSD 9, 29–37 (Qrt. 1, 2006).

Micelles are formed when the surfactant concentration exceeds the critical micelle concentration (CMC). Above the CMC, surfactant monomers associate with one another to form micelles, which have a hydrophobic interior. When an oil phase is in contact with an aqueous micellar solution, oil molecules partition into the hydrophobic core of these micelles, a process known as solubilization (1–3). Solubilization is sometimes expressed as the amount of oil solubilized per mass, volume, or moles of surfactant present in micelles at saturation (1) (Fig. 1). Surfactant systems with higher solubilization capacities are desirable, as they reduce surfactant requirements and formulation costs in applications such as surfactant-based separation processes, enhanced oil recovery, and environmental remediation technologies (4–6). When supra-CMC ionic surfactant solutions are contacted with solid surfaces of opposite charge, the surfactant will adsorb on the solid surface and form “adsorbed micelles” or admicelles. Similar to micelles, admicelles have a hydrophobic interior that can solubilize oil, a process known as adsolubi-

KEY WORDS: Adsolubilization, anionic surfactant, cationic surfactant, ethylcyclohexane, mixed surfactant, silica, solubilization, styrene.

*To whom correspondence should be addressed at School of Civil Engineering and Environmental Science, The University of Oklahoma, 202 West Boyd, Rm. 334, Norman, OK 73019-1024. E-mail: [email protected] Abbreviations: CMC, critical micelle concentration; DPCl, dodecylpyridinium chloride; GC, gas chromatography; MSR, molar solubilization ratio; PODD, pentamethyl-octadecyl-1,3-propane diammonium dichloride; SDS, sodium dodecyl sulfate; SHDPDS, sodium hexadecyldiphenyloxide disulfonate.

FIG. 1. Solubilization of organic solutes in mixed micelles.

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JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 9, QTR. 1, 2006

30 A. FUANGSWASDI ET AL.

FIG. 2. Adsolubilization of organic solutes in the core zone and palisade layer of the admicelle.

lization (7,8). Adsolubilization can occur in the hydrophobic inner core of the admicelle or in a region of medium polarity located near the surfactant head groups, known as the palisade layer (7) (see Fig. 2). Previous studies (9,10) have indicated that organic solutes tend to partition into the region of the admicelle that has polarity similar to the solute. Thus, while a nonpolar solute is expected to partition primarily within the core region, a polar solute is expected to preferentially adsolubilize in the palisade layer (Fig. 2). Since adsolubilization may be used to remove organic pollutants from contaminated groundwater or wastewater, it is important to understand the partitioning of contaminants in the different regions of the admicelle (11–16). For economic reasons, it is important to find ways to improve the solubilization and adsolubilization capacity of surfactant systems. Recent work on microemulsions has found that anionic and cationic surfactant mixtures help increase the solubilization capacity of these systems (17). This finding agrees with previous observations suggesting that mixed anionic and cationic (18) and mixed ionic and nonionic (19–21) surfactants exhibit synergistic behavior (i.e., lower CMC, higher solubilization enhancement) when mixed at an appropriate ratio. The hypothesis of this work was that by introducing mixtures of anionic and cationic surfactants, it would be possible to improve the solubilization capacity of micelles and admicelles, and that this effect would depend on the mole ratios of mixed admicelles and the polarity of the solute. The objectives of this study were as follows: to evaluate the solubilization capacity of micelles and admicelles formulated with mixtures of anionic and cationic surfactants as a function of the anionic/cationic molar ratio, and to probe the internal environment of micelles and admicelles produced with these mixtures by using solutes of differing polarity. Mixtures of single- and double-head ionic surfactant systems were used in this research to minimize the tendency of JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 9, QTR. 1, 2006

these systems to precipitate (17). In particular, two pairs of surfactants were evaluated: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), in mixtures with a twinhead cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Surfactant ratios were selected to avoid precipitation; this required surfactant ratios different from 1:1, as precipitation was prevalent at this ratio. To study adsolubilization, SDS/PODD and SHDPDS/DPCl systems were adsorbed onto silica and alumina, respectively. To evaluate the solubilization and adsolubilization capacity of these systems, styrene and ethylcyclohexane were used as surrogates for polar aromatic and nonpolar hydrocarbon solutes. The partition of the various organic solutes into the micelle can be described by the micellar partition coefficient, Kmic, shown in Equation 1 (1,22): K mic =

X mic X aq

[1]

where Xmic is the mole fraction of the organic solute in the micelle pseudophase and Xaq is the mole fraction of organic solute in the aqueous phase. Xmic and Xaq are calculated using Equations 2 and 3, respectively (1,10,22): X mic =

X aq =

MSR 1 + MSR C eq

C eq + 55.55

[2]

[3]

where the molar solubilization ratio (MSR) refers to the moles of the solute solubilized per mole of surfactant in the micelles and is determined by the slope of the graph of surfactant concentration vs. oil concentration in mol/L. Ceq is the equilibrium concentration of the organic solute in water alone, and 55.55 is the inverse molar volume of water. In solubilization studies, Ceq is the water solubility concentration of the oil. For adsolubilization studies of styrene and ethylcyclohexane in mixed admicelles (SDS/PODD on silica and SHDPDS/DPCl on alumina), the mole fraction of the organic solute in the admicelle, Xadm, can be calculated by Equation 4 (10): X adm =

(S i − S f ) (S i − S f ) + (Ai − A f ) + (C i − C f )

[4]

where Xadm is mole fraction of organic solute in the admicelle, Si and Sf are the initial and equilibrium aqueous concentrations of the organic solute, Ai and Af are the initial and equilibrium aqueous concentrations of anionic surfactant, and Ci and Cf are the initial and equilibrium aqueous concentrations of cationic surfactant. The admicellar partition coefficient, Kadm, is defined similarly to the micellar partition coefficient, as shown in Equation 5 (1,10):

31 SOLUBILIZATION AND ADSOLUBILIZATION OF STYRENE AND ETHYLCYCLOHEXANE IN MIXED SURFACTANTS TABLE 1 Chemical Properties of the Surfactants Chemicals Pentamethyl-octadecyl1,3-propane diammonium dichloridea,b (PODD) Sodium dodecyl sulfate (SDS)

Molecular structure

Molecular weight (g/mol)

CMC (mM)

CH3 CH3   R—N—C3H6—N—CH3(Cl)2   CH3 CH3

463.62

1.3c (0.01 M NaCl)

CH3(CH2)11SO4Na

288.38

6.8d (0.15 M NaCl)

283.88

4.0d (0.15 M NaCl)

642

6.3e (0.15 M NaCl)

Dodecylpyridinium chloride (DPCl)

Sodium hexadecyldiphenyloxide disulfonate (SHDPDS)

a

http://surface.akzonobelusa.com/Product_Data.cfm?PID=100 (accessed January 2006). CMC, critical micelle concentration. http://www.lion.co.jp/laco/e/prod/p/44dqad_e.htm (accessed July 2005). c Fuangswasdi et al. (23). d Stellner et al. (24). e Rouse et al. (4). b

K adm =

X adm X aq

[5]

EXPERIMENTAL PROCEDURES Materials. The cationic surfactant PODD (or Duoquad® T50) was donated by Akzo Nobel Surface Chemistry LLC (McCook, IL) as a 50% solution in isopropanol; while mainly octadecyl (38% C18:1 and 25% C18:0), PODD is also 29% hexadecyl, with the remaining 8% being C14:0, C14:1, C16:1, and C18:2. The isopropanol was evaporated by cyclic heating at 80°C

under vacuum extraction. The purified sample was rediluted and titrated until the remaining alcohol was 0.99). Styrene and ethylcyclohexane concentrations were measured by gas chromatography (GC) using a Varian 3300 GC equipped with a 50-m SPB25 hydrophobic capillary column with auto sampler, model 8100, and flame ionization detector. Additional details on this method can be found elsewhere (26). The experiments for SDS/PODD and SHDPDS/DPCl systems were carried out using 0.01 M or 0.015 M NaCl, respectively, to maintain a constant ionic strength. The experiments were conducted at 25 ± 1°C with at least duplicate and sometimes triplicate samples. Error bars are included for triplicate samples (typical variations were ±8%). Methods. (i) Solubilization study. The solubilization capacities of styrene and ethylcyclohexane were measured for SDS and PODD alone and for differing concentrations of SDS/PODD mixtures in 1:3 and 1:10 ratios; these ratios were selected to be outside the precipitation region (23) (eight to nine concentrations for each ratio). A 10-mL aliquot of each surfactant solution was pipetted into 15-mL vials, followed by addition of an excess amount of solute oil (500 µL styrene or 400 µL ethylcyclohexane), and the vials were subsequently sealed with Teflon caps. The solutions were slowly shaken for 1 d to let the oils solubilize in the aqueous solutions. After centrifuging for 30 min, the aqueous surfactant solutions were carefully sampled (1.5 mL for each sample). The concentrations of styrene and ethylcyclohexane were measured using GC. The solubility studies of mixed SHDPDS/DPCl in JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 9, QTR. 1, 2006

the ratios 3:1, 10:1, SHDPDS alone, and DPCl alone were conducted in the same manner. (ii) Adsolubilization study. Supra-CMC surfactant solutions (20-mL samples in 40-mL vials) of PODD alone and SDS/PODD mixtures were contacted with a given mass of silica in such a way that the final total aqueous surfactant concentration (after adsorption) was just below the CMC (i.e., just below maximum adsorption) for the particular surfactant sample to ensure that no micelles remained in solution after adsorption. Styrene or ethylcyclohexane was added at a concentration less than the solubilization capacity. The volume of oil adsolubilized was calculated by the difference in concentration between the initial and final aqueous concentration of the oil (10). Blank samples (without silica) were used to assess potential volatilization of the solutes, which was shown to be negligible. The samples were shaken for 2 d, which is sufficient to achieve equilibrium (27,28). The solution pH was maintained in the range of 6 to 7 by using NaOH and HCl solutions if pH adjustment was required; in that case, the vials were shaken again for 1 d. The samples were centrifuged and a sample of the aqueous solution was collected to measure surfactant and solute concentrations. The same approach was used for mixed SHDPDS/DPCl and SHDPDS-alone adsorbed onto alumina.

RESULTS AND DISCUSSION Solubilization in micelles. The results of the styrene and ethylcyclohexane solubilization experiments in mixed SDS/PODD, SDS-alone, and PODD-alone micelles are shown in Figures 3 and 4, respectively. Figure 3 plots the solubility of styrene vs. surfactant concentration. At low surfactant concentrations (below the CMC), the styrene solubility is constant (the plot is horizontal) and equals the water solubility, while at higher surfactant concentrations (above the CMC), the styrene solubility increases linearly owing to solubilization in the micelles. The intersection of these two lines is an indicator of the CMC of the system, as noted in the figure. The slope of the plot above the CMC is the MSR, which, along with the solubility of the solute, can be used to calculate the Kmic (see Eqs. 1 to 3). The solubilization of styrene and ethylcyclohexane in mixed SHDPDS/DPCl, SHDPDS-alone, and DPCl-alone micelles is shown in Figures 5 and 6, respectively. The CMC, MSR, and Kmic values for each of these systems are summarized in Table 3. We should note that the CMC is affected by the presence of the solutes, so the values in Table 3 are different than those for surfactant-only systems at those salinities. Mixtures of SDS and PODD produced lower CMC values in comparison with the individual surfactants alone (Figs. 3, 4, respectively; Table 3); this effect is more evident as the ratio of anionic to cationic surfactant approaches equimolar concentrations. The micellar partition coefficient (Kmic) values for styrene and ethylcyclohexane were higher for the SDS/PODD mixtures than for either the SDS-alone or the PODD-alone (see Table 3), which supports the original hy-

33 SOLUBILIZATION AND ADSOLUBILIZATION OF STYRENE AND ETHYLCYCLOHEXANE IN MIXED SURFACTANTS

FIG. 3. Solubilization of styrene in sodium dodecyl sulfate (SDSalone), pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD-alone) and in two SDS/PODD mixtures (0.01 M NaCl, 25°C). [a is the critical micelle concentration (CMC) of SDS/PODD, 1:3; b is the CMC of SDS/PODD, 1:10; c is the CMC of PODD-alone; and d is the CMC of SDS-alone.]

pothesis that mixed anionic–cationic micelles will show higher solubilization capacity compared with single surfactant micelles. In addition, the Kmic of ethylcyclohexane in mixed SDS/PODD and PODD-alone micelles was higher than that of styrene. The different partition behavior between ethylcyclohexane and styrene can be explained by the fact that the nonpolar ethylcyclohexane tends to concentrate in the hydrophobic core of the micelles (8). The higher Kmic values for nonpolar ethylcyclohexane in mixed SDS/PODD systems suggest that the mixed anionic/cationic surfactant micelles had a larger and more hydrophobic nonpolar core region

FIG. 4. Solubilization of ethylcyclohexane in SDS-alone, PODD-alone, and in two SDS/PODD mixtures (0.01 M NaCl, 25°C). (a is the CMC of SDS/PODD, 1:3; b is the CMC of SDS/PODD, 1:10; c is the CMC of PODD-alone; and d is the CMC of SDS-alone.) For abbreviations see Figure 3.

FIG. 5. Solubilization of styrene in sodium hexadecyl-diphenyloxide disulfonate (SHDPDS-alone), dodecylpyridinium chloride (DPClalone), and in three SHDPDS/DPCl mixtures (0.015 M NaCl, 25°C). (a is the CMC of SHDPDS-alone and the three SHDPDS/DPCl mixtures, and b is the CMC of DPCl-alone.) For other abbreviation see Figure 3.

than the individual surfactant micelles. By contrast, the styrene was expected to accumulate in the palisade layer, which would experience less synergism, or even be negatively affected, by the mixed micelles because of the “squeezing out” effect; that is, the second surfactant fills “cavities” in the micelle where the solute might have accumulated (8). Conversely, it has been shown that the π electron/charged group interactions between cationic head groups and aromatic solutes can produce higher solubilization in cationic micelles (ion–dipole interactions) (29). Thus, although is it surprising that ethylcyclohexane (E) solubilized more than styrene (S) in cationic micelles here, it is interesting to note that the ratio of Kmic for E/S was lower in SDS and SHDPDS than for DPCl or PODD, consistent with this discussion. These inter-

FIG. 6. Solubilization of ethylcyclohexane in SHDPDS-alone, DPClalone, and in three SHDPDS/DPCl mixtures (0.015 M NaCl, 25°C). (a is the CMC of SHDPDS-alone and the three SHDPDS/DPCl mixtures, and b is the CMC of DPCl-alone.) For abbreviations see Figures 3 and 5.


34 A. FUANGSWASDI ET AL. TABLE 3 CMC, Molar Solubilization Ratio (MSR), Micellar and Admicellar Partition Coefficient (Kmic, Kadm) Values Medium Silica

Surfactants

SDS/PODD (1:3) SDS/PODD (1:10) PODD-alone SDS-alone Alumina SHDPDS/DPCI (3:1) SHDPDS/DPCI (10:1) SHDPDS/DPCI (30:1) SHDPDS-alone DPCI-alone

CMC (mM) MSR Kmic Kadma Styrene Ethylcyclohexane Styrene Ethylcyclohexane Styrene Ethylcyclohexane Styrene Ethylcyclohexane 0.40 0.80 1.1 6.0 0.80 0.90 1.1 1.0 6.0

0.40 0.80 1.1 6.0 0.80 0.60 0.70 0.80 5.0

1.4 1.3 1.1 0.07 1.2 1.4 1.4 0.9 0.2

1.6 0.55 0.40 0.06 2.2 2.3 2.2 2.2 0.08

0.12 0.12 0.11 0.013 0.11 0.12 0.12 0.10 0.04

0.60 0.35 0.28 0.058 0.68 0.69 0.68 0.68 0.07

0.26 0.21 0.19 NMb 0.35 0.40 NM 0.40 NM

0.16 0.06 0.06 NM 0.75 0.70 NM 0.65 NM

a

Based on high Xaq values (mole fraction of organic solute in the aqueous phase) to more closely reflect the maximum additivity method used to assess Kmic. NM, not measurable—surfactant admicelle does not form because single surfactant and surface are like charged. All SDS/PODD systems had 0.01 M NaCl, and all SHDPDS/DPCI systems had 0.015 M NaCl. For other abbreviations see Table 1.

b

pretations are speculative at this point and should be further evaluated in future research. Table 3 also summarizes the CMC, MSR, and Kmic values for mixed SHDPDS/DPCl, SHDPDS-alone, and DPCl-alone micelles. The CMC values of the surfactant mixtures were nearly the same as that for the SHDPDS-alone. The CMC of the mixtures were virtually the same as for the SHDPDSalone, and the Kmic values were unaffected by the mixtures. These results are consistent with previous research with SHDPDS, which showed that it was not significantly influenced by counterions or cosurfactants, as demonstrated by precipitation and middle-phase microemulsion studies (17). These results raise the question of the kind of micellar structure a double-head, single-tail ionic surfactant forms when combined with a single-head ionic surfactant of opposite charge. One can imagine that a single-head ionic surfactant could complex with the double-head, single-tail surfactant to form a quasi double-head–double-tail structure. A ratio of 1:3 of SDS/PODD could produce a double tail complex that would account for 33% of the total PODD. This more hydrophobic double-head–double-tail structure would approach the hydrophobicity of the SHDPDS system. In fact, the Kmic for ethylcyclohexane in SDS/PODD at a ratio 1:3 approaches the value of this parameter for the SHDPDS-alone system. Adsolubilization in admicelles. Figure 7 shows the admicellar partition coefficient, Kadm, vs. the aqueous concentration of styrene for PODD-alone and for mixtures of PODD and SDS adsorbed onto silica. The first conclusion from this data is that, for any value of Xaq, the Kadm increases with increasing ratios of SDS/PODD. These results are consistent with the micellar solubilization results and help confirm our hypothesis that mixtures of anionic and cationic surfactants produce larger adsolubilization of organic compounds than do singlesurfactant admicelles. A second observation from this data is that for a polar molecule like styrene, the value of Kadm plateaus with an increasing aqueous molar fraction of the polar solute, suggesting a saturation of the palisade layer (28). This trend was observed for the PODD-alone and the SDS/PODD 1:10 admicelles. However, the Kadm values for JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 9, QTR. 1, 2006

the SDS/PODD 1:3 system, although higher, were independent of Xaq. This behavior indicates that the surfactant-modified surface has an equal affinity for the styrene independent of the styrene concentration [i.e., Kmic is independent of styrene loading (Xadm) in the admicelle]. Figure 8 shows the admicellar partition coefficient of ethylcyclohexane, Kadm, as a function of the ethylcyclohexane aqueous mole fraction in mixed SDS/PODD and PODDalone admicelles. All the isotherms in Figure 8 have a positive slope, which indicates that the adsolubilization process increases as additional solute partitions into the admicelles; this is as expected for a core solubilization process. Furthermore, the surfactant ratio of 1:3 was again the most efficient in adsolubilizing ethylcyclohexane (had the largest Kadm), once again supporting our hypothesis of improved adsolubilization as the mixed system approaches equimolar conditions. Figure 9 shows the admicellar partition coefficient of styrene, Kadm, as a function of the styrene aqueous mole frac-

FIG. 7. The styrene admicellar partition coefficient, Kadm, in mixed SDS/PODD and PODD-alone admicelles on silica. For abbreviations see Figure 3.


FIG. 8. The ethylcyclohexane admicellar partition coefficient, Kadm, in mixed SDS/PODD and PODD-alone admicelles on silica. For abbreviations see Figure 3.

tion in mixed SHDPDS/DPCl and SHDPDS-alone admicelles. The values of Kadm were higher at low Xaq values and plateaued toward a minimum value at higher Xaq. This suggests preferential adsolubilization at lower loading (lower Xaq and Xadm) and eventual site saturation with increased loading (i.e., the palisade layer effect described above). It should be noted that the preferential adsolubilization at lower values of Xaq diminished with increasing ratios of SHDPDS/DPCl: This can again be attributed to the “squeezing out” phenomenon mentioned (i.e., the co-surfactant is filling spaces that might have been filled by the solute, thereby diminishing the palisade effect). Figure 10 shows the admicellar partition coefficient of ethylcyclohexane, Kadm, as a function of the ethylcyclohexane aqueous mole fraction in mixed SHDPDS/DPCl and SHD-

FIG. 9. The styrene admicellar partition coefficient, Kadm, in mixed SHDPDS/DPCl and SHDPDS-alone on alumina. For abbreviations see Figure 5.

FIG. 10. The ethylcyclohexane admicellar partition coefficient, Kadm, in mixed SHDPDS/DPCl and SHDPDS-alone. For abbreviations see Figure 5.

PDS-alone admicelles. The curves in Figure 10 resemble the curves in Figure 8 in that the slopes of the curves are positive and the values of the admicelle partition coefficients are larger for the mixed SHDPDS/DPCl system at the highest mole ratio, 3:1. Once again, these results illustrate the more hydrophobic nature of the mixed admicelle and corroborate our hypothesis. In the case of ethylcyclohexane, admicelles formulated with mixtures of anionic and cationic surfactants showed greater solubilization than single SHDPDS systems, even though this was not observed in the case of micelles: This disparity could be due to the two-dimensional nature of admicelles vs. the three-dimensional nature of micelles (i.e., the packing synergy may be better exploited in more planar structures). To compare the values of Kadm reported in this section with the Kmic values reported above, we will focus on the Kadm values at higher levels of Xaq, as this more closely mimics the maximum additivity method used to determine Kmic. When comparing the Kadm and Kmic values in Table 3, it is interesting to note that for the SDS/PODD system, the admicelles were more efficient in solubilizing styrene than the micelles—again, this may be attributed to the more planar structure of admicelles favoring the palisade effect. Conversely, the SDS/PODD admicelles were less efficient for ethylcyclohexane—although from Figure 8 we observed that the isotherm was continuing to rise, and since it was not possible to conduct experiments at higher levels, we may not have realized the maximum value. For the SHDPDS/ DPCl system, we observed that once again the admicelles were more efficient for incorporating styrene, while the micelles and admicelles were equally effective for ethylcyclohexane. Thus, we observed that the relative efficiency of admicelles and micelles was a function of the surfactants and their mole ratio in the mixture and also the nature of the solute of interest.


36 A. FUANGSWASDI ET AL.

ACKNOWLEDGMENTS Financial support for this work was provided by the National Research Center for Environmental and Hazardous Waste Management (NRC-EHWM) Program, Chulalongkorn University, Thailand. In addition, financial support for this research was received from the industrial sponsors of the Institute for Applied Surfactant Research, University of Oklahoma, including Akzo Nobel, Clorox, Conoco/Phillips, Church and Dwight, Ecolab, Halliburton, Huntsman, Oxiteno, Procter & Gamble, Sasol, Shell, and Unilever. Finally, funds from the Sun Oil Company Chair (D.A.S.) and Asahi Glass Chair (J.F.S.) at the University of Oklahoma helped support this research.

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Aranya Fuangswasdi received her B.S. in Geology from Chulalongkorn University, Thailand, her M.S. in groundwater management from the University of New South Wales, Australia, and her Ph.D. in environmental management from Chulalongkorn University. She is currently the director of the Groundwater Restoration Sub-Bureau, Department of Groundwater Resources. Her interest is in the area of groundwater remediation technologies. Ampira Charoensaeng received her B.Sc. in general science and M.Sc. in environmental and hazardous waste management from


Chulalongkorn University, Bangkok, Thailand. She is currently a Ph.D. student at the National Research Center for Environmental and Hazardous Waste Management (NRC-EHWM), Chulalongkorn University, Bangkok, Thailand. Her research interest is in the area of surfactant-based adsorption onto solid interfaces for environmental remediation. Khemarath Osathaphan received his B.E. in civil engineering from Chulalongkorn University, Thailand, and his M.S. and Ph.D. in environmental engineering from Oregon State University in 1998 and 2001, respectively. He is currently an assistant professor at the Department of Environmental Engineering of Chulalongkorn University. His research areas include the transport and fate of chemicals in the subsurface and the chemical treatment of hazardous waste. Sutha Khaodiar received his B.Eng. in environmental engineering from Chulalongkorn University, Thailand, and his M.S. and Ph.D. in environmental engineering from Oregon State University. He is currently an assistant professor, and is head of the Department of Environmental Engineering at Chulalongkorn University. His research interest focuses on the adsorption and complexation reactions of contaminants in aqueous environments. Edgar J. Acosta received his B.S. in chemical engineering from the Universidad del Zulia (Venezuela) in 1996, and his M.S. and Ph.D. in chemical engineering from the University of Oklahoma,

Norman, Oklahoma, in 2000 and 2004, respectively. He is currently an assistant professor in the Department of Chemical Engineering at the University of Toronto. His research encompasses the area of colloids, complex fluids, and formulation engineering. John F. Scamehorn holds the Asahi Glass Chair in Chemical Engineering and is director of the Institute for Applied Surfactant Research at the University of Oklahoma. He received his B.S. and M.S. from the University of Nebraska and his Ph.D. from the University of Texas, all in chemical engineering. Dr. Scamehorn has worked for Shell, Conoco, and DuPont and has been on a number of editorial boards for journals in the areas of surfactants and of separation science. He has edited four books and coauthored over 160 technical papers. His research interests include surfactant properties important in consumer product formulation and surfactantbased separation processes. David A. Sabatini is David Ross Boyd Professor and Sun Oil Company Chair of Civil Engineering and Environmental Science and is associate director of the Institute for Applied Surfactant Research at the University of Oklahoma, Norman, Oklahoma. He received his B.S. from the University of Illinois (1981), his M.S. from Memphis State University (1985), and his Ph.D. from Iowa State University (1989). His research interests include surfactant-based microemulsion systems for consumer products, environmental remediation, and the replacement of organic solvents.
