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Environ. Sci. Technol. 2008, 42, 1492–1498

Effects of Linear Alkylbenzene Sulfonate on the Sorption of Brij 30 and Brij 35 onto Aquifer Sand SHWETA TRIPATHI AND DERICK G. BROWN* Department of Civil & Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015

Received August 21, 2007. Revised manuscript received November 21, 2007. Accepted November 23, 2007.

Surfactant sorption is of considerable importance to environmental applications, including surfactant flushing to mobilize hydrophobic contaminants; effects of surfactants on the transport of dissolved contaminants, microorganisms, and colloids through porous media; and bioremediation of hydrophobic organic compounds, as well as understanding the fate and transport of surfactants as environmental contaminants themselves. Although most sorption studies consider pure surfactants, commercial detergent formulations typically consist of mixtures of nonionic and anionic surfactants. In this study, the effects of varying concentrations of the anionic surfactant linear alkylbenzene sulfonate (LAS) on micelle formation and sorption behavior of the two commonly used nonionic surfactants Brij 30 and Brij 35 onto aquifer sand were examined. A strong linear relationship was observed between the critical micelle concentration (CMC) of the Brij surfactants and the concentration of LAS in the mixture, with the CMC decreasing with increasing concentration of LAS. The relative change in CMC as a function of the LAS concentration was identical for the two Brij surfactants, indicating that LAS interacted with their common alkyl chains. Sorption isotherms were developed for Brij 30 and Brij 35 present as single surfactants in an aqueous solution as well as when present with LAS. Although LAS had minor effects on the maximum sorption plateaus of the Brij surfactants, Brij sorption at was significantly enhanced as a function of the LAS concentration for Brij aqueous concentrations below the CMC. Application of a multi-interaction isotherm model indicated that the formation of surface aggregates (e.g., hemimicelles) decreased with increasing LAS concentration. Overall, these results provide insight into the complex sorption behavior of surfactant mixtures.

Introduction Surfactants are molecules that have both hydrophobic and hydrophilic moieties. This amphiphilic structure bestows surfactants with properties that are used in both industrial applications (e.g., agricultural crop applications, industrial cleaning, and textiles) and consumer goods (e.g., laundry and dishwashing detergents, bath soaps, shampoos, and bathroom and kitchen cleaners) (1). A key result of surfactant’s amphiphilic structure is that surfactant molecules gather at interfaces in an aqueous system, including air/water and solid/water interfaces. This * Corresponding author: phone: 610-758-3543; fax: 610-758-6405; e-mail: [email protected]. 1492

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sorption can have beneficial uses, such as with the remediation of contaminated soil and water. For example, surfactant sorption can be used to reduce interfacial surface tensions and to mobilize nonaqueous phase liquids trapped in the subsurface (2), and surfactant sorption on the bacterial cell surface can result in the formation of hemimicelles, which can enhance the bioavailability of hydrophobic compounds (3–5). Surfactant sorption can also affect the movement of microorganisms through groundwater via alteration of the interfacial properties (6–9). This process can be detrimental where surfactant release into the subsurface, such as through septic systems, can enhance the transport of pathogens to drinking water wells. It can also be beneficial, such as the use of surfactants with bioaugmentation schemes to facilitate transport of bacteria into contaminant zones (8, 10). Sorption of pure surfactants is fairly well understood, and it is known that surfactants often exhibit an S-shaped isotherm. This isotherm occurs as surfactant monomers sorb onto the surface at low aqueous surfactant concentrations, followed by the formation of surface aggregates (e.g., hemimicelles) as the aqueous surfactant concentration increases, with a sorption plateau ultimately occurring as the aqueous surfactant concentration increases beyond the critical micelle concentration (CMC) (3, 11–16). A few isotherm models have been developed that capture this complex shape (3, 17), and one has been successfully applied to elucidate the impact of surfactant sorption on the biodegradation of hydrophobic compounds (3, 18). Although surfactants are typically studied as pure compounds, in application they are predominantly used in mixtures because this often provides better performance over a single surfactant (1, 19–22). For example, in household detergents, which are typically mixtures of anionic and nonionic surfactants, the anionic surfactants are used to increase solubility, and nonionic surfactants are used to increase tolerance to hardness in water (23). Despite the wide use of surfactant mixtures, the sorption behavior of surfactant mixtures remains relatively unexplored. Kibbey and Hayes (24, 25) explored the sorption of polydisperse ethoxylated nonionic surfactants to aquifer materials, and they developed a thermodynamic model that accurately represented this system. Wieslaw et al. (26) investigated the effects of propanol on the sorption of linear alkylbenzene sulfonate (LAS) at the air–water interface. Yang et al. (27) examined the impact of sodium dodecyl-benzene sulfate on the sorption of Triton X-100 onto calcium-enriched montmorillonite, and they observed a reduced sorption of individual components within the surfactant mixtures as compared to individual components. In contrast, Zhang et al. (28) observed cooperative sorption of the two nonionic surfactants NP-10 and n-dodecyl-β-D-maltoside on silica and alumina surfaces. To elucidate potential synergistic effects with surfactant mixtures, the current study focused on binary systems consisting of an anionic surfactant and a nonionic surfactant. Three surfactants commonly used in commercial products were examined, including the nonionic surfactants Brij 30 and Brij 35 (11, 29–31) and the anionic surfactant LAS (1, 26, 32–34). The goal was to identify the effects of LAS on the sorption of the nonionic surfactants onto an aquifer sand. To achieve this goal, there were two main objectives. The first objective was to experimentally develop sorption isotherms for the single surfactants and surfactant mixtures on aquifer sand. The second objective was to apply the isotherm model developed by Brown and Al Nuaimi (3) to 10.1021/es0720964 CCC: $40.75

 2008 American Chemical Society

Published on Web 01/25/2008

the experimental data and to use this model to interpret the impact of LAS on the sorption behavior of the Brij surfactants.

Materials and Methods Sand and Surfactants. The experiments were conducted using Ottawa sand (AFS 50/70, US Silica), consisting of sand grains with diameters between 250 and 300 µm. The sand was prepared for each experiment by washing with deionized water and drying at 25 °C for 24 h prior to use. The two nonionic surfactants used in this study are Brij 30 (C12E4) and Brij 35 (C12E23). These linear polyoxyethylene (POE) alcohol surfactants are often represented as CxEy, where x is the number of carbons in the alkyl chain, and y is the number of ethylene oxide units in the POE chain. The anionic surfactant used was LAS with a linear alkyl chain length of 12 carbon atoms. These surfactants were procured from Sigma-Aldrich (St. Louis, USA) and used without any further processing. The aqueous nonionic surfactant concentrations were determined using an iodine-iodide (I-I) assay developed by Baleux (35) and modified by Brown and Jaffé (36). With this assay, a colored complex of I-I and nonionic surfactant is quantified spectrophotometrically. The I-I reagent was prepared by mixing 1 g of iodine with 2 g of potassium iodide in 100 mL of deionized water. For the I-I assay, 0.25 mL of I-I reagent is added to 10 mL of the sample surfactant solution and allowed to equilibrate for 30 min. The difference in absorbance at a wavelength of 500 nm between surfactantfree solution with I-I reagent and the surfactant solution with I-I reagent is used to determine the surfactant concentration from predetermined calibration curves. The aqueous LAS concentrations were measured using the crystal violet assay for anionic detergents developed by Hach Company (37). In this method, the surfactant is associated with crystal violet dye, followed by extraction of the ion-pair complex into benzene using liquid phase separation. The absorbance of the benzene solution at a wavelength of 605 nm is then used to determine the LAS concentration from a predetermined calibration curve. Surfactant mixtures with known concentrations of nonionic Brij 30 or Brij 35 and anionic LAS were subjected to the I-I and crystal violet assays to verify the applicability of these methods to surfactant mixtures. No interference from the other surfactants present in the mixture was observed with either the I-I or crystal violet assays. The CMC of each the three surfactants was determined from equilibrium surface tension measurements using a DuNouy Surface tensiometer (Fisher Scientific). Apparent CMC values were also determined for Brij 30 and Brij 35 with LAS present at various concentrations. Sorption Isotherm Experiment. Sorption isotherm experiments were performed by placing 30 g of washed sand and 25 mL of surfactant solution at a specified concentration in 50 mL glass centrifuge tubes. The samples were gently rotated end-over-end on a rotary shaker at 25 rpm for 24 h. The samples were then centrifuged at 1500g for 45 min, and the aqueous concentrations of the surfactants were measured in the supernatant. The sorbed surfactant concentration was determined as the difference between the total surfactant added to each tube and the aqueous concentration. The adsorption experiments were performed for single surfactant solutions of Brij 30, Brij 35, and LAS, as well as for surfactant mixtures of either Brij 30 or Brij 35 and LAS. The isotherms were developed for surfactant concentrations up to a minimum of 4× CMC, as prior studies have shown that sorption plateaus occur at surfactant concentrations of 1-2× CMC (3). For the surfactant mixtures, the impacts of fixed concentrations of LAS (10, 50, 100, and 300 mg/L) on the Brij 30 and Brij 35 isotherms were determined. These LAS concentrations were selected based on results from the LAS

sorption experiments, where the sorption increased strongly between 10 and 100 mg/L LAS and leveled off at the CMC value (∼195 mg/L; results from isotherm experiments are discussed below). Isotherm Model. Both nonionic and anionic surfactants exhibit S-shaped sorption isotherms. It is believed that the S-shaped isotherm results from monolayer surfactant sorption followed by lateral interactions between the sorbed surfactant molecules (16). These lateral interactions lead to the formation of surface aggregates such as hemimicelles, resulting in sorption beyond the levels that occur with monolayer coverage (3). Brown and Al Nuaimi (3) developed a multi-interaction isotherm to model this S-shaped isotherm, which accounts for both monolayer sorption and lateral interactions. This multi-interaction isotherm model can be written as shown in eq 1, S ) Γmax,1

x xp + Γmax,2 K1 + x K2 + xp

(1)

where S is the sorbed concentration (mg/g), Γmax,1 and Γmax,2 are the maximum sorption concentrations for the monolayer and lateral interactions, respectively (mg/g); K1 and K2 are half-saturation constants for each interaction (unitless); p is an exponent (unitless); and x is defined as the ratio between the aqueous surfactant concentration, C (mg/L), and the surfactants critical micelle concentration, CCMC (mg/L). The first term of eq 1 describes the monolayer sorption of the surfactants via a Langmuir model, and the second term describes the lateral interactions between the adsorbed surfactant molecules via a modified Langmuir model. The maximum sorption plateau of the isotherm (Γmax) is then given by eq 2. Γmax ) Γmax,1 + Γmax,2

(2)

Parameter Estimation. The multi-interaction isotherm (eq 1) was fit to the experimental data using the nonlinear parameter estimate code PEST (Version 6.05, Watermark Numerical Computing). The optimization method used by PEST focuses on reducing the weighted residual, which is described by eq 3. Φ ) Σ(wiri)2

(3)

i

where Φ is the weighted residual, wi is the weight attached to the ith data point, and ri is the difference between the model output and the experimental data for the ith data point. For the current study, the value for each individual weight was inversely proportional to the data point to which it pertained, and in this manner a log least-squares estimation process was obtained (38). This allows observations with small values to have an equivalent impact on the parameter estimation process as observations with larger values. The outputs from this optimization process were the isotherm parameters that best-fit the experimental data.

Results and Discussion CMC of Pure Surfactants and Surfactant Mixtures. Plotted in Figure 1 is the aqueous surface tension as a function of surfactant concentration for the three surfactants examined in this study. The CMC for each surfactant was determined from the break point in the appropriate curve, and the resulting CMC values are presented in Table 1. A comparison of the data in Figure 1 shows that each surfactant impacts the aqueous surface tension differently as a function of surfactant concentration. First, the decrease in surface tension occurred at different rates for the three surfactants, with Brij 30 showing the most rapid drop in surface tension with increasing concentration and with LAS showing a minimum drop. Second, Brij 30 was observed to cause the VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Surface tension as a function of surfactant concentration for Brij 30, Brij 35, and LAS. maximum decrease in surface tension, followed by LAS and then Brij 35. This CMC analysis was repeated for Brij 30 and Brij 35 in the presence of LAS. For each experiment, the LAS concentration was fixed at a specific concentration, and the aqueous surface tension was determined over a range of Brij 30 and Brij 35 concentrations. Four different concentrations of LAS (10, 50, 100, and 300 mg/L) were used for these experiments. The apparent CMCs of Brij 30 and Brij 35 in the presence of LAS were determined, and the results are presented in Figure 2a. As seen in Figure 2a, the Brij 30 and Brij 35 apparent CMC values decreased linearly as the LAS concentration increased. The aqueous surface tension at these Brij 30 and Brij 35 apparent CMC values were the same as that for the pure surfactant, for example, with LAS at a fixed concentration, Brij 30 reached CMC at the same aqueous surface tension obtained at the CMC of Brij 30 alone (Figure 1), irrespective of the LAS concentration present (data not shown). However, as LAS was able to achieve a lower aqueous surface tension than Brij 35 (see Figure 1), the highest LAS concentration of 300 mg/L resulted in a surface tension below that which was achievable by Brij 35 alone. This resulted in Brij 35 having no effect on the aqueous surface tension with LAS present at 300 mg/L. Ultimately, because the surface tension of LAS falls below that of Brij 35 at an LAS concentration of ∼150 mg/L (Figure 1), an apparent CMC for Brij 35 cannot be determined for LAS concentrations above this value. It is well-known that mixed surfactant micelles can result when surfactants are present in mixtures (19, 20, 39), and studies of anionic/nonionic mixtures (40, 41) and cationic/ anionic mixtures (19) have showed appreciable synergistic effects including reduction in the CMC due to this mixed micelle formation. The results in Figure 2a indicate that for Brij 30 and Brij 35 this CMC reduction is a strong linear function of the concentration of LAS present in the mixture. To further investigate this relationship, the ratio of apparent CMC to the CMC of pure surfactant was calculated as a function of LAS concentration, and the results are presented in Figure 2b. Here it is seen that when the results are plotted as the relative change in CMC, LAS had an identical effect on Brij 30 and Brij 35. The occurrence of the same relative change in apparent CMC of both Brij 30 and Brij 35 as a function of LAS concentration is indicative of a similar interaction between these two nonionic surfactants and LAS. Brij 30 and Brij 35 have the same alkyl chain length and differ 1494

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only in their POE chain length (y ) 4 for Brij 30 and y ) 23 for Brij 35). This suggests that, in the mixed micellization process, LAS interacts with the Brij surfactants through their alkyl chains and not through their POE chains. Sorption of Pure Surfactants. The isotherms for Brij 30, Brij 35, and LAS sorption onto the sand are shown in Figure 3. As shown in this figure, the sorbed concentration reaches a plateau when the aqueous surfactant concentration is near the CMC. The adsorption plateau is generally attributed to saturation of the adsorption sites and formation of surface aggregates, such as hemimicelles (3, 12–16, 42, 43). The proximity of the plateau to the CMC is due to the sorption process being driven by the concentration of surfactant monomers in solution, which becomes constant as the aqueous surfactant concentration increases beyond the CMC (44). The observations of the sorption plateau occurring at or above the CMC for various surfaces have been previously reported in literature. For example, this has been observed for nonionic surfactant sorption onto silica surfaces (13, 45, 46), activated carbon (14) and the bacterial cell surface (3), and for LAS sorption onto anionic clays (44). These studies indicate that the attainment of the sorption plateau around or above CMC values is a general sorption behavior for both nonionic and anionic surfactants. Examination of the sorption plateaus in Figure 3 shows that although Brij 30 had a higher sorbed concentration than Brij 35, LAS had a significantly higher level of sorption than either of the nonionic surfactants, with the plateaus of the nonionic surfactants being 2 orders of magnitude lower than that of LAS. The sorption of nonionic surfactants in aqueous systems has been reported to be governed by hydrophobic interactions on the interfaces, with the alkyl group adhering to the surface (11, 13, 14, 33). Some researchers have also proposed sorption of the POE chain via hydrogen bonding; however, for this to occur the bond formed between the POE chain and the mineral surface must be greater than the bond between the mineral surface and the adjacent water molecules (11). The increasing sorption with decreasing POE chain length observed here for the Brij surfactants has been reported in other studies (3, 13, 14, 45, 46) and is most likely due to a combination of the surfactant’s hydrophobicity increasing with decreasing POE chain length and the surfactant physically requiring more surface area per molecule as the POE chain length increases (i.e., packing density increases with decreasing POE chain length) (11). For anionic surfactants, both hydrophobic and electrostatic interactions govern sorption to surfaces (11). Here, the significantly greater sorption of LAS over the Brij surfactants may be due to a combination of increased packing density of the smaller LAS molecules; stronger sorption occurring as the hydrophilic headgroup branches off from within the alkyl chain as compared to being attached to one end of the chain as with the Brij surfactants; and sorption of the anionic headgroup to any local positively charged sites on the sand surface (11). To characterize and compare these isotherms, the multiinteraction isotherm model (eq 1) was fit to the experimental data. For comparison, the Langmuir isotherm was also fit to the experimental data (eq 4), S ) Γmax,L

x KL + x

(4)

where Γmax,L is the maximum sorption level (mg/g), and KL is the Langmuir half-saturation constant (unitless). A comparative plot of the best-fit Langmuir isotherm (eq 4) versus the best-fit multi-interaction isotherm (eq 1) for the three surfactants is shown in Figure 3. Analysis of the weighted residuals (eq 3) indicated that the multi-interaction isotherm model, which accounts for the lateral interactions that occur during surfactant sorption, was capable of replicating the

TABLE 1. Best-fit Parameters for the Multi-interaction Isotherm (eq 1) for Single-surfactant Sorption mg surfactant/g sand surfactant Brij 30 Brij 35 LAS

MW (g/mol) 362 1198 348

CMC (mg/L) 14.9 49.9 194.9

Γmax,1 10-3

4.87 × 7.00 × 10-3 3.13

Γmax,2

Γmax

10-2

3.70 × 1.59 × 10-2 2.26

experimental data whereas the Langmuir isotherm, which is a monolayer sorption model, did not yield as good of a fit (3, 11). Similar results were observed in a previous study where the multi-interaction isotherm was used to model the sorption of five different nonionic surfactants, including Brij 30 and Brij 35, onto the bacterial cell surface (3). The best-fit multi-interaction isotherm parameters for the three surfactants are provided in Table 1. Sorption of Surfactants in Mixture. The sorption isotherms for Brij 30 and Brij 35 with LAS present at concentrations of 10, 50, 100, and 300 mg/L are presented in Figure 4. The symbols in Figure 4 are the experimental data points, and the solid lines are the best-fit multi-interaction isotherms (eq 1) calculated using the apparent CMC values of Brij 30 and 35 in the presence of LAS (Figure 2a). The apparent

10-2

4.19 × 2.29 × 10-2 5.39

K1 (×CMC)

K2 (×CMC)P

p

2.03 6.00 × 10-3 0.231

70.8 0.441 4.11 × 10-2

5.86 2.51 4.77

CMC values and resulting multi-interaction isotherm parameters are presented in Table 2. As noted earlier, the apparent CMC value for Brij 35 with LAS present at 300 mg/L could not be determined, and as such, the multi-interaction isotherm could not be applied to this data set. Two observations can be made from Figure 4. The first observation is related to the maximum sorption plateaus that occur above the CMC, where it was found that the presence of LAS did not have a significant effect on the plateaus for either Brij 30 or Brij 35. Similarly, there was no observed change in the level of LAS sorption in the presence of the two Brij surfactants (data not shown). This observation is supported by the model fits (Tables 1 and 2). For Brij 30, the isotherms converge on a similar plateau value (Γmax) over the LAS concentrations examined in this study, and for Brij

FIGURE 2. (a) Apparent CMC for Brij 30 and Brij 35 as a function of LAS concentration. (b) The ratio of apparent CMC due to the presence of LAS to the CMC of the pure surfactant (Table 1) as a function of LAS concentration shows a highly linear relationship for the LAS concentration range examined.

FIGURE 3. Sorption isotherms for Brij 30, Brij 35, and LAS sorption onto the sand. Symbols are data points and solid lines are the best-fit multi-interaction (eq 1) and Langmuir (eq 4) isotherm model results. Values in parentheses are the weighted residuals (eq 3). VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Sorption of Brij 30 and Brij 35 onto the aquifer sand in the presence of LAS at concentrations of 10, 50, 100, and 300 mg/L. Symbols are experimental data points, and the solid lines are the best-fit multi-interaction isotherm model results (eq 1).

TABLE 2. Effects of LAS on the Best-fit Brij 30 and Brij 35 Multi-interaction Isotherm Parameters (eq 1) mg surfactant/g sand K2 (×CMC)P

LAS concentration (mg/L)

CMC (mg/L)

Γmax,1

Γmax,2

Γmax

Brij 30

10 50 100 300

14.7 14.4 13.7 11.6

5.77 × 10-3 9.67 × 10-3 1.15 × 10-2 1.25 × 10-2

3.66 × 10-2 3.17 × 10-2 3.07 × 10-2 3.00 × 10-2

4.24 × 10-2 4.14 × 10-2 4.22 × 10-2 4.25 × 10-2

1.12 0.739 0.527 0.446

8.90 5.20 2.00 2.64

3.70 3.56 2.52 2.78

Brij 35

10 50 100

49.4 48.5 45.9

1.01 × 10-2 1.15 × 10-2 1.72 × 10-2

1.34 × 10-2 1.28 × 10-2 7.96 × 10-3

2.35 × 10-2 2.43 × 10-2 2.52 × 10-2

6.61 × 10-2 7.43 × 10-2 0.118

0.260 9.90 × 10-2 1.29 × 10-3

4.09 4.54 8.85

35, there was a very minor enhancement in the sorption plateau as a function of LAS concentration. The second observation is related to sorption at Brij concentrations below the CMC. As shown in Figure 4, the sorption plateaus were reached at lower Brij concentrations as the concentration of LAS increased. For sub-CMC Brij concentrations, this resulted in Brij sorption onto the aquifer sand, substantially increasing in the presence of LAS. With regards to both of these observations, it has been shown that a surfactant that strongly sorbs onto a surface can enhance the sorption of a weakly sorbing surfactant via interactions with the hydrophobic hydrocarbon chains (11, 47–49). Following this, the experimental results here suggest that at Brij concentrations below the CMC, the strongly sorbing LAS enhanced the sorption of both Brij surfactants and that this enhanced sorption was a function of the sorbed LAS concentration. Above the CMC of the Brij surfactants, LAS had a very minor impact on the plateau value, most likely due to the packing density restrictions of the Brij POE chains. Examination of the model results in Tables 1 and 2 shows that the maximum sorption concentrations for monolayer sorption (Γmax,1) increased with increasing LAS concentration for both Brij 30 and Brij 35. Concurrently, the maximum sorption concentrations for surfactant aggregate formation (Γmax,2) decreased with increasing LAS concentration. The ratio Γmax,2/Γmax,1, which gives an indication of surface aggregate formation to monolayer coverage (3, 18), is presented in Figure 5 for Brij 30 and Brij 35 as a function of LAS. The ratio decreased rapidly with increasing LAS concentration, suggesting that the sorbed LAS is affecting the formation of surface aggregates (e.g., hemimicelles) on the aquifer sand. This may ultimately have implications for 1496

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K1 (×CMC)

p

partitioning of hydrophobic contaminants within the sorbed hemimicelles at the sand surface, similar to that at the bacterial cell surface proposed in the surfactant-enhanced bioavailability model by Brown (4). Overall, the results of this study indicate that LAS did have an effect on the micellization of Brij 30 and Brij 35, where the apparent CMC of the Brij surfactants decreased as the LAS concentration was increased, presumably through interactions of their hydrophobic tails. The sorption behavior of Brij 30 and Brij 35 was also strongly affected by the presence of LAS, where the sub-CMC sorption of the Brij surfactants

FIGURE 5. Ratio of Γmax,2 to Γmax,1 for Brij 30 and Brij 35 in the presence of LAS. Dashed line shows the ratio for LAS as a pure surfactant.

significantly increased and the formation of surface aggregates (e.g., hemimicelles) decreased, both as a function of the LAS concentration.

Acknowledgments This project was funded by the National Science Foundation through Grant No. 0134362. The authors gratefully acknowledge their valuable support for this work.

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