Physicochemical Studies on the Micellization of Cationic, Anionic

Article pubs.acs.org/jced

Physicochemical Studies on the Micellization of Cationic, Anionic, and Nonionic Surfactants in Water−Polar Organic Solvent Mixtures Sibani Das, Satyajit Mondal, and Soumen Ghosh* Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700 032, India S Supporting Information *

ABSTRACT: The effect of cosolvent (ethane-1,2-diol and dimethyl sulfoxide) on the self-assembly of three surfactants, N,N,N-trimethyl-1-dodecanaminium bromide (DTAB), sodium [dodecanoyl(methyl)amino]acetate (SDDS), and polyoxyethylene (20) sorbitan monolaurate (Tween-20) in aqueous solution have been investigated by conductometric, tensiometric, and viscometric techniques at 298 K. The main focus was on the effect of solvent on critical micelle concentration (cmc), free energy contribution to micellization (ΔG0m), tail transfer Gibbs free energy (ΔG0trans), Gibbs adsorption energy (ΔG0ads), and some micellar interfacial parameters, for example, Gibbs surface excess (Γmax), minimum area per surfactant molecule (Amin), surface pressure (Πcmc), and pC20(= −log(C20), where C20 is the surfactant molar concentration required to reduce the surface tension of mixed solvent by 20 mN m−1). With increasing concentration of cosolvent in the binary mixture, the cohesive force decreases, and surfactant molecules are more soluble in mixed solvent. As a result, micellization process becomes less favorable, and an increase in cmc was obtained. Steady state fluorescence spectroscopy was used to determine the aggregation number (Nagg) of the surfactants in organic solvent−water binary mixture and also the micropolarity of the mixed solvent. It was observed that Nagg decreased with the increase of organic solvent concentration. The micropolarity of the mixed solvent and packing parameter (P) were also determined.

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INTRODUCTION Surfactants are amphiphilic in nature. They contain a hydrophilic headgroup and a lipophilic tail. Due to the dual nature of the surfactant, they self-aggregate in aqueous solution, and the simplest aggregated form is called micelle; the corresponding concentration is called the critical micelle concentration (cmc). For this property, they are widely used in detergency, drug delivery, emulsion stabilization, personal care products, and paint formulations1 and so forth. The micellar interfacial and thermodynamic properties of surfactant are widely dependent on environmental factors, namely, pH, temperature, pressure, and additives, such as cosolvent, cosurfactants, and added electrolytes,2−14 and so forth. The surfactants generally serve as a carrier of active ingredients and cosolvents help to improve this property by increasing the viscosity and volatility. They affect the cmc via the modification of water−surfactant interactions by changing the properties of the mixed solvent. Generally, the cmc of a given surfactant increases by the addition of cosolvents in comparison to water. But at low concentration, ethanol acts as a cosurfactant and reduces the cmc; at higher concentrations, ethanol acts as a cosolvent.15 Aggregations of many cationic, anionic, and nonionic surfactants, for example, alkyltrimethylammonium bromides of various chain lengths, sodium dodecyl sulfate (SDS), Triton X-100, Tween-20, alkyl-ethoxylate, and some gemini surfactants, and so forth, in aqueous−organic solvent and nonaqueous polar medium were extensively studied in the past few years.16−21 The scopes of these studies are to © 2013 American Chemical Society

understand the nature of self-assembly in the presence of polar organic solvents and use of the surfactants in the field which requires water-free or water-poor media.22 Water-like polar organic solvents, for example, ethylene glycol, glycerol, formamide, N-methyl formamide, dimethyl sulfoxide, acetonitrile, alcohol, N,N-dimethylformamide, hydrazine, and so forth, have been studied extensively.23−26 These studies provide information about surfactant−solvent interactions. These solvents have a high dielectric constant, a high cohesive energy, and a significant hydrogen bonding ability. The hydrogen bonding ability is important for micellization.27 To understand the interaction involved in the solvophobic effect, many investigations have been carried out where water is partially replaced by the organic solvent. In the solvophobic effect, the tail part of the surfactant has less contact with water and helps to aggregate. This effect can be changed by altering the nature of the amphiphile as well as the nature of the bulk phase after addition of polar organic solvent. In the present work, micellization of three surfactants, N,N,N-trimethyl-1-dodecanaminium bromide (DTAB) (cationic), sodium [dodecanoyl(methyl)amino]acetate (SDDS) (anionic), and polyoxyethylene (20) sorbitan monolaurate (Tween-20) (nonionic) was studied in two different solvent systems, ethane-1,2-diol (EG) + water and dimethyl sulfoxide Received: May 18, 2013 Accepted: July 26, 2013 Published: August 8, 2013 2586

dx.doi.org/10.1021/je4004788 | J. Chem. Eng. Data 2013, 58, 2586−2595

Journal of Chemical & Engineering Data

Article

Scheme 1

Table 1. Specification of Chemical Samples chemical name

molecular weight (density/g·mL−1)

source

purification method

final mass fraction purity

N,N,N-trimethyl-1-dodecanaminium bromide (DTAB) sodium [dodecanoyl(methyl)amino]acetate (SDDS) polyoxyethylene (20) sorbitan monolaurate (Tween-20) 1-hexadecylpyridinium chloride (CPC) ethane-1,2-diol (EG) dimethyl sulfoxide (DMSO) pyrene

308.35 293.38 1227.54 340 62.07 (1.112) 78.13 (1.099)

Fluka Sigma Sigma Sigma Merck (India) Merck (India) Aldrich

none none none none none none recrystallized

0.99 0.99 0.995 0.99 0.99 0.995 0.994

(DMSO) + water combinations. The study of micellization in a DMSO + water mixture is limited. All of the surfactants used have identical tails (constituting 12 carbon atoms in the linear chain), but they differ in the type and size of the headgroup; there are 20 ethylene oxide linked with a sorbitan moiety in Tween-20, there are trimethylmmonium ions present in DTAB, and there is a sarcosinate moiety in SDDS. Recently, we have reported an investigation on the mixed micellar properties of cationic gemini and cationic monomeric surfactants in aqueous ethylene glycol mixture showing synergism.12 Previously, the micellization of DTAB and Tween-20 in EG + water mixed solvent were studied by another research group,15,17 but that was not investigated in DMSO + water system. Actually, micellization in the DMSO + water system is limited. Both EG and DMSO are polar solvents having dielectric constants less than water. Water is gradually replaced by EG and DMSO, and the ratio of mixed solvent used in this study ranged from 0 to 0.5267 (0.5256) mass fraction. We have carried out conductance, surface tension, and viscosity measurements on the systems discussed above to study the cmc, thermodynamics of micellization and adsorption, counterion binding (when amphiphile is ionic in nature), interfacial parameters, and so forth. The aggregation number of the surfactants and micropolarity were also determined from the static fluorescence quenching method. The combined analysis of the experimental results reported by these techniques allows a detailed description of the behavior of surfactants in the presence of EG and DMSO.

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(DMSO) were bought from Merck (India). All of the surfactants, solvents, and probe used in the experiment are AR grade products and used as received. Pyrene was purchased from Aldrich and recrystallized several times and then used. The information of the chemicals used in this work is listed in Table 1. Double-distilled water was used for all sample preparation at 298 K. Methods. Electrical Conductivity Measurements. The conductivity measurements were performed with Eütech (Singapore) (cell constant = 1 cm−1). The temperature of the solution was maintained at 298 K with a temperaturecontrolled circulator water bath of accuracy ± 0.1 K. The cell constant was calibrated by using KCl solution. A sample of 6 mL of solution was placed in a container, and surfactant solution (made in the desired solvent medium) of the desired (∼10 to 15 times the cmc) concentration was progressively added using a microsyringe. The specific conductance (κ) was measured after each addition followed by thorough mixing. The solvent (water or water + organic solvent mixture) was equilibrated at 298 K for at least 20 min before the addition of the desire surfactant solution. At each concentration, the conductivity measurement was repeated three times, and the average value was obtained with an uncertainty within ± 2 μS. The cmc values were estimated from the break points in the specific conductance (κ)−[surfactant] plots. The data above and below the inflection point are fitted to two linear equations, and the cmc is obtained from the intersection. The basic data are given in the Supporting Information. Tensiometry. Tensiometric measurements were taken with a du Noüy tensiometer (Jencon, India) by the platinum ring detachment method. A portion of 5 mL of solution was taken in a double-wall jacketed container placed in a thermostatted water bath (accuracy, ± 0.1 K) at the requisite temperature of 298 K, and surfactant solution (in desire solvent medium, water or water + organic solvent mixture) of known concentration (∼10 to 15 times the cmc) was added progressively. During such measurements, the 15 min time interval for equilibration was allowed after surfactant addition and thorough mixing. Prior to each measurement, the ring was heated briefly by holding it above a Bunsen burner until glowing. The cmc values

EXPERIMENTAL SECTION

Materials. Sodium [dodecanoyl(methyl)amino]acetate (SDDS, mass fraction purity = 0.99) was purchased from Fluka, and N,N,N-trimethyl-1-dodecanaminium bromide (DTAB, mass fraction purity = 0.99), polyoxyethylene (20) sorbitan monolaurate (Tween-20, mass fraction purity = 0.995), and 1-hexadecylpyridinium chloride (CPC, mass fraction purity = 0.99) were purchased from Sigma. Structures of DTAB, SDDS, and Tween-20 have been presented in Scheme 1. Ethane-1,2-diol (EG) and dimethyl sulfoxide 2587



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Figure 1. (a) Plot of specific conductance (κ) vs [DTAB/mol·kg−1] in different mass fractions of EG (1) + water (2): ■, water; □, w1 = 0.0553; ●, w1 = 0.11; ○, w1 = 0.2175; ▲, w1 = 0.3228; △, w1 = 0.4257. (b) Plot of specific conductance (κ) vs [SDDS/mol·kg−1] in different mass fractions of DMSO (1) + water (2): ■, water; □, w1 = 0.0547; ●, w1 = 0.1088; ○, w1 = 0.2155; ▲, w1 = 0.32; △, w1 = 0.4228.

Figure 2. Plots of surface tension (γ) vs (a) log[DTAB/mol·kg−1] in different mass fractions of DMSO (1) + water (2): ■, water; □, w1 = 0.0547; ●, w1 = 0.1088; ○, w1 = 0.2155; ▲, w1 = 0.32; △, w1 = 0.4228. Plots of surface tension (γ) vs (b) log[SDDS/mol·kg−1] and vs (c) log[Tween-20/ mol·kg−1] in different mass fractions of EG (1) + water (2): ■, water; □, w1 = 0.0553; ●, w1 = 0.11; ○, w1 = 0.2175; ▲, w1 = 0.3228; △, w1 = 0.4257.

Fluorescence Measurements. Fluorescence measurements were made by using Perkin-Elmer LS55 fluorescence spectrophotometer. The temperature was kept at 298 K by using a water-flow thermostat connected to the cell compartment. In all cases, the concentration of surfactant was used above the cmc. Pyrene was excited at 332 nm, and its emission was recorded at 373 nm and 384 nm, which corresponded to the first and third vibrational peaks, respectively. The excitation and emission slits were 9 nm and 4 nm, respectively. A scan speed of 250 nm/min was used. The aggregation number (Nagg) was determined by the fluorescence quenching study of pyrene by 1-hexadecylpyridinium chloride (CPC). Pyrene solution was added to the water + EG or water + DMSO micellar solution of surfactants. The probe concentration was kept low enough to avoid eximer formation. The quencher was added progressively, and the intensity was recorded for data analysis. The basic data are given in the Supporting Information.

were estimated from the break points in the surface tension-log [surfactant] plots. Determined surface tension (γ) values were accurate within ± 0.1 mN·m−1. The basic data are provided in the Supporting Information. Viscometry. The relative viscosity measurements were taken in a calibrated two-limbed Ostwald viscometer (placed in a thermostatted water bath at 298 K of accuracy ± 0.1 K) with a clearance time of 98.5 s for 15 mL of water. The mixed solvent without surfactant was taken in the viscometer, and surfactant solution of known concentration was progressively added with a microsyringe. Then the flow time of the solutions were measured after thorough mixing. Each measurement was repeated thrice, and the average time of flow was used for calculation. The errors of the measurements were within ± 0.5 %. The cmc values were determined from the inflection points in the plots of relative viscosity (η/η0) against concentration of surfactants (where η and η0 denote the viscosities of the solvent with and without surfactant, respectively). The data above and below the inflection point are fitted to two linear equations, and the cmc is obtained from the intersection. The basic data are given in the Supporting Information. 2588



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Figure 3. Plots of relative viscosity (η/η0) vs (a) [DTAB/mol·kg−1] and vs (b) [SDDS/mol·kg−1] in DMSO (1) + water (2): ■, w1 = 0.1088; □, w1 = 0.2155; ●, w1 = 0.32; ○, w1 = 0.4228. Plot of relative viscosity (η/η0) vs (c) [Tween-20/mol·kg−1] in EG (1) + water (2): ■, w1 = 0.11; □, w1 = 0.2175; ●, w1 = 0.3228; ○, w1 = 0.4257.

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RESULTS AND DISCUSSION Critical Micelle Concentration (cmc). The cmc values for three surfactants, DTAB (cationic), SDDS (anionic), and Tween-20 (nonionic), in different mass fractions (w1) of EG (1) + water (2) and DMSO (1) + water (2) at 298 K were determined from conductivity, tensiometry, and viscosity methods, and representative plots are shown in Figures 1 to 3. The average cmc values were used for all calculations and are listed in Table 2. The parenthetic values in this table represent the cmc of the three surfactants in DMSO + water medium. The values of cmc of micellar solutions of DTAB in pure water as well as in EG + water are in agreement with literature data.9 But the cmc value of DTAB in 0.2175 mass fraction of EG + water and Tween-20 in water deviate from the literature value.9,17 It was found that the cmc values obtained from different methods are quite close and increased with increasing mass fraction of EG + water and DMSO + water. The inhibitory effect of solvent for each surfactant depends upon the nature of solvent. EG is a polar protic structured solvent, but DMSO is a polar aprotic nonstructured solvent. The dielectric constant values of EG and DMSO (37.7 and 46.7) are less than water, and with the increasing mass fraction of solvent in the mixture, the dielectric constant decreases. This decrease in dielectric constant is expected to cause an increase in electrostatic repulsions between the ionic head groups at the micellar surface and decrease the hydrophobic interaction between the hydrocarbon tails. The oxygen atom of DMSO is partially negatively charged, whereas EG is neutral. Both EG and DMSO form H-bonds with water and break the threedimensional structure of water. DMSO is known to form stoichiometric hydrates with water of the type DMSO·2H2O. The structure-breaking ability and solubilization power of DMSO are higher than that of EG. The formation of hydrate restricts the motion of amphiphilic molecules and reduces hydrophobic interaction with a concomitant increase in cmc. The head group of DTAB is more solubilized in DMSO +

water by the attractive electrostatic interaction, whereas the anionic headgroup of SDDS is less solubilized due to electrostatic repulsion. As a result the delay of micellization is more prominent for DTAB in DMSO + water. Adsorption at the Air/Solvent Interface. Amphiphiles orient at the air−water interface and decrease surface tension (γ). The decrease in γ value continues until the adsorption of surfactant at the air−water interface reaches a maximum. At a certain concentration, the γ value does not change. This concentration of surfactant is called cmc, and corresponding surface tension is known as γcmc. Actually, γcmc is the measure of efficiency of the surfactant to populate at the air−water interface. The surface excess concentration Γmax have been calculated using the Gibbs equation Γmax = −

⎡ dγ ⎤ 1 ⎢ ⎥ 2.303nRT ⎣ d log C ⎦

(1)

where R and T are the universal gas constant and absolute temperature, respectively, C is the concentration of surfactant taken, and n is the number of species in the solution and n is taken as 2 for the ionic surfactants (DTAB and SDDS) and 1 for the nonionic surfactant (Tween-20). Equation 1 is valid for ideal solutions. It can be extended for binary solvents also.27,28 Assuming complete monolayer formation at cmc, the minimum area per surfactant molecule at air−solvent interface (Amin) was calculated from the equation: A min =

1018 NA Γmax

(2)

where NA is Avogadro’s number. The values of surface pressure, Πcmc, was calculated from

Πcmc = γsol − γcmc 2589

(3)



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Table 2. Critical Micellar Concentration (cmc) of DTAB, SDDS, and Tween-20 and Counter Ion Binding (β) of DTAB and SDDS in Different Mass Fractions (w1) of EG (1) + Water (2) [DMSO (1) + Water (2)] at 298 Ka 103 cmc (mol·kg−1) 100w1

conductance

surface tension

viscosity

β

average

DTAB 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

14.7 15.5 (15.9) 16.0 (16.9) 16.7 (18.2) 16.9 (20.2) 18.4(28.8) 21.9 (37.2) 25.9 (44.7)

14.7 15.4 15.5 16.2 17.1 19.1 21.6 26.4 34.2

0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

13.4 14.1 14.6 15.5 16.0 16.9 19.0 21.3

14.6 14.8 15.1 16.1 17.4 17.9 19.8 22.0 28.5

(15.2) (16.5) (17.7) (20.6) (29.7) (38.6) (46.1) (53.9)

15.3 15.6 16.0 16.7 17.3 18.9 21.9 25.7 33.7

(15.9) (16.5) (17.6) (21.1) (28.9) (37.0) (45.7) (53.8)

14.9 15.5 15.9 16.5 17.1 18.8 21.8 25.9 33.9

(15.7) (16.6) (17.8) (20.6) (29.1) (37.6) (45.5) (53.8)

14.3 14.9 15.0 16.0 17.0 18.3 20.0 21.6 28.4

(14.8) (15.4) (15.7) (16.1) (17.1) (18.8) (21.5) (23.3)

14.1 14.6 14.9 15.9 16.8 17.7 19.6 21.6 28.5

(14.8) (15.3) (15.8) (16.3) (17.5) (19.8) (20.7) (23.1)

0.78 0.78 0.77 0.75 0.73 0.69 0.68 0.51

(0.76) (0.75) (0.71) (0.69) (0.64) (0.63) (0.60)

0.52 0.46 0.45 0.44 0.42 0.41 0.40 0.27

(0.47) (0.45) (0.44) (0.43) (0.42) (0.41) (0.30)

SDDS (14.0) (14.7) (15.3) (15.7) (17.1) (20.0) (21.2)

(15.8) (15.9) (16.4) (17.0) (18.4) (20.1) (20.6) (23.9) Tween-20 103 cmc (mol·kg−1)

100w1 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

surface tension 0.047 0.050 0.054 0.060 0.070 0.081 0.087 0.100 0.132

viscosity 0.051 0.058 (0.053) 0.060 (0.061) 0.066 (0.069) 0.075(0.076) 0.086 (0.088) 0.091 (0.094) 0.105 (0.107) 0.129 (0.156)

(0.051) (0.056) (0.063) (0.072) (0.084) (0.092) (0.117) (0.153)

average 0.049 0.054 0.057 0.063 0.073 0.084 0.090 0.103 0.130

(0.052) (0.056) (0.066) (0.074) (0.086) (0.093) (0.112) (0.154)

The standard uncertainty u(w1) = 0.0001 and combined expanded uncertainties (Uc) are Uc(cmc) = 7.10−4 mol·kg−1 and Uc(β) = 0.005 (0.95 level of confidence). The combined expanded uncertainty (Uc) is Uc(cmc) = 0.0025 mol·kg−1 (0.95 level of confidence). a

where γsol and γcmc are the values of surface tension of the mixed solvents without surfactant and surface tension of the mixture including surfactant at the cmc, respectively. The increase in Γmax with the decrease in mass fraction of organic solvent (Table 3) denotes the enhanced efficiency of the surfactant monomers to populate at the interface at lower concentration of organic solvent. From Table 3, it was obtained that Amin increased with a decrease in Γmax and Πcmc decreased with an increase in the amount of organic solvent in the solution. Thermodynamics of Micellization and Interfacial Adsorption. To quantify how the addition of organic solvents affects the micellization process, the standard Gibbs free energy of micellization, ΔG0m, was calculated for ionic surfactants by using the following equations: ΔGm0 = (1 + β)RT ln Xcmc

and premicellar (S1) slopes of the plot of the specific conductance vs concentration of the surfactant solution from unity, i.e., β=1−

(6)

This method of evaluation of β is frequently used for its simplicity and easy adoptability. Data in Table 2 as well as those in the literature show that, with an increase in concentration of organic solvent in the mixture, counterion binding for the ionic surfactants were decreasing gradually with increasing cmc. Initially, the counterion binding of DTAB in water is 78 %, which decreases to 51 % and 60 % in 0.4257 mass fraction of EG−water and 0.4228 mass fraction of DMSO + water (in the parentheses of Table 2), respectively. The counterion binding of SDDS in water is 52 %, which decreases to 27 % and 30 % in 0.4257 mass fraction of EG + water and 0.4228 mass fraction of DMSO + water, respectively. The fraction of counterion located very close to the micellar surface is affected by the surface solubilization of cosolvent. The solubilization at the micellar surface reduces the surface charge density. As a result,

(4)

and for nonionic surfactants by

ΔGm0 = RT ln Xcmc

S2 S1

(5)

where β is the counterion binding, determined conductometrically by the subtraction of the ratio of the post-micellar (S2) 2590



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Table 3. Gibbs Free Energy of Micellization (ΔG0m), Gibbs Free Energy of Transfer (ΔG0trans), and Interfacial Parameters such as the Gibbs Free Energy of Adsorption (ΔG0ads), Gibbs Surface Excess (Γmax), Minimum Area per Surface Molecule (Amin), Surface Pressure (Πcmc), and Efficiency of Adsorption (pC20) for (a) DTAB, (b) SDDS, and (c) Tween-20 in Different Mass Fractions (w1) of EG (1) + Water (2) (DMSO (1) + Water (2)) at 298 K −ΔG0m 100 w1

−1

kJ·mol

ΔG0trans −1

kJ· mol

−ΔG0ads

106 Γmax

−1

−2

kJ·mol

mol·m

Πcmc

Amin

mN·m−1

pC20

1.06 1.12 (1.10) 1.18 (1.12) 1.2 (1.16) 1.24 (1.28) 1.36 (1.40) 1.42 (1.56) 1.50 (1.94) 1.72 (2.32)

40.5 36.7 (37.1) 33.8 (33.5) 32.8 (31.8) 31.34 (29.9) 26.6 (28.2) 22.9 (25.7) 20.2 (23.2) 16.5 (19.2)

2.97 2.79 (2.81) 2.64 (2.57) 2.60 (2.48) 2.49 (2.32) 2.18 (2.12) 1.86 (1.86) − (1.62)

1.34 1.54 1.55 1.61 1.69 1.73 1.78 2.13 2.34

(1.38) (1.43) (1.45) (1.49) (1.60) (1.77) (1.88) (2.05)

33.8 31.1 29.6 28.5 27.8 25.6 21.4 15.7 12.0

(31.1) (27.9) (26.4) (23.6) (21.3) (18.7) (16.1) (11.6)

0.67 0.74 0.79 0.80 0.90 1.02 1.23 1.40 1.58

(0.78) (0.83) (0.89) (0.99) (1.18) (1.23) (1.35) (1.38)

31.0 29.1 28.2 27.4 26.8 24.6 22.2 19.5 17.1

(30.3) (28.8) (25.4) (24.0) (23.4) (21.4) (19.6) (16.3)

nm ·molecule 2

−1

a

0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

36.3 36.1 35.7 35.0 34.3 32.7 31.5 27.3

(35.6) (35.1) (33.9) (32.7) (29.9) (28.3) (26.6)

0.20 0.53 1.25 1.97 3.53 4.72 8.93

(0.65) (1.14) (2.39) (3.59) (6.34) (7.96) (9.68)

62.2 61.0 59.7 59.0 57.8 54.5 51.2 45.6

0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

31.2 30.4 30.1 29.6 29.0 28.3 27.2 23.4

(30.1) (30.0) (28.4) (27.3) (26.3) (25.2) (24.3)

0.75 1.05 1.59 2.17 2.86 3.93 7.79

(1.01) (1.15) (2.79) (3.90) (4.82) (5.91) (6.87)

58.4 59.2 57.7 57.2 57.3 55.0 50.3 43.4

0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

34.5 34.3 34.1 33.8 33.3 32.8 32.4 31.8 30.9

(34.4) (34.1) (33.7) (33.3) (32.7) (32.3) (31.5) (30.5)

0.20 0.35 0.67 1.16 1.70 2.09 2.68 3.56

(0.11) (0.41) (0.78) (1.21) (1.79) (2.24) (2.98) (4.04)

47.1 47.3 47.6 47.1 47.8 48.0 48.8 48.3 47.2

(a) DTAB 1.56 (60.0) 1.47 (1.52) (57.9) 1.41 (1.47) (56.2) 1.37 (1.42) (55.7) 1.33 (1.30) (54.0) 1.22 (1.17) (52.5) 1.16 (1.06) (53.9) 1.10 (0.85) 0.96 (0.72) (b) SDDSb 1.24 (56.0) 1.08 (1.20) (54.0) 1.07 (1.16) (51.5) 1.03 (1.14) (48.5) 0.98 (1.11) (46.8) 0.96 (1.04) (45.1) 0.93 (0.94) (42.5) 0.78 (0.88) 0.71 (0.81) (c) Tween-20a 2.46 (48.6) 2.24 (2.13) (48.5) 2.10 (2.0) (47.3) 2.06 (1.87) (47.6) 1.85 (1.67) (49.3) 1.62 (1.41) (48.1) 1.35 (1.35) (47.5) 1.19 (1.23) (44.0) 1.05 (1.20)

2.80 2.73 2.59 2.54 2.46 2.23 1.82

(2.31) (2.39) (2.26) (2.03) (1.83) (1.57)

2.10 2.02 1.94 1.84 1.80 1.57 1.32 0.92

(2.09) (1.98) (1.68) (1.55) (1.52) (1.22) (0.86)

The combined expanded uncertainties (Uc) are Uc(ΔG0m) = 0.5 kJ·mol−1, Uc(ΔG0trans) = 0.4 kJ·mol−1, Uc(ΔG0ads) = 0.5 kJ·mol−1, Uc(Γmax) = 0.08 mol·m−2, Uc(Amin) = 0.06 nm2·molecule−1, Uc(Πcmc) = 0.5 mN·m−1, and Uc(pC20) = 0.04 (0.95 level of confidence). bSame as a, except Uc(pC20) = 0.05 (0.95 level of confidence). a

considers that the surfactant tail inside the micelle has a conformation different from that in a pure hydrocarbon liquid because of the molecular packing requirements inside the micelle and finally, (e) the steric Gibbs free energy, ΔG0steric, considers the steric repulsions between the head groups of amphiphiles at the micellar surface. Among all of these Gibbs free energies, the dependence of cmc on bulk phase composition is mainly controlled by ΔG0trans whereas such dependence of cmc on other terms are negligibly small.8,28 The tail transfer Gibbs free energy of surfactants in water and in a mixed solvent system (ΔG0trans) was calculated with the help of the following equation:

counterion binding to the micellar surface decreases in mixed solvent. The values of ΔG0m calculated by using eqs 4 and 5 are listed in Table 3 (the parenthetic values in this table represent ΔG0m of the three surfactants in DMSO + water medium). The cmc values were taken in the mole fraction scale. The free energy of micellization (ΔG0m) is negative and becomes less negative as the EG and DMSO content in the mixed solvent system increases, indicating that the formation of micelles become less favorable at higher mass fractions of EG + water and DMSO + water. According to the theory of surfactant self-assembly suggested by Nagarajan et al.,29 the Gibbs energy contributions to ΔG0m are as follows: (a) the surfactant tail transfer Gibbs free energy, ΔG0trans, accounts for the change in Gibbs free energy due to the transfer of the surfactant tail from the bulk phase into the micellar core; (b) the aggregate−core solvent interfacial Gibbs free energy, ΔG0intf, denotes that the formation of a micelle creates an interface as a result, contact between the hydrophobic core and the bulk phase; (c) the headgroup interaction Gibbs free energy, ΔG0elect, represents the electrostatic repulsion between the surfactant head groups at the micellar surface; (d) the deformation Gibbs free energy, ΔG0def,

ΔGm0 = (ΔGm0)sol − (ΔGm0)H2O

(7)

where (ΔG0m)sol and (ΔG0m)H2O are Gibbs free energies of EG + water or DMSO + water mixture and pure water, respectively. 0 The increase of ΔGtrans is observed with an increasing percentage of organic solvent in the mixture. The values observed are higher in DMSO + water media than that in EG + water mixed solvents. The positive values of ΔG0trans can be attributed to the reduction of the solvophobic interaction 2591



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Table 4. Surface Tension of the Solutions (γsol), Molar Volume (Vm), Tail Transfer Contribution of the Gibbs Free Energy to the Total Gibbs Free Energy of Micellization (Δμ0g/kT)tr,S−W, and Gordon Parameter (G) for Different Mass Fractions (w1) in EG (1) + Water (2) (DMSO (1) + Water (2)) at 298 Ka γsol 100 w1 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 52.67 (52.56)

mN·m 70.9 68.5 66.8 66.2 65.8 63.8 61.4 59.6 57.2

(Δμ0g/kT)tr,S−W

Vm −1

(69.0) (66.7) (65.7) (64.2) (62.8) (61.1) (59.6) (56.6)

3

−1

18.1 18.3 18.5 20.0 21.8 25.6 29.4 33.2 36.9

(18.3) (18.6) (20.7) (23.4) (28.7) (34.0) (39.3) (44.6)

dm ·mol

−1

kJ·mol −19.8 −19.9 −19.9 −20.1 −20.5 −21.1 −21.6 −22.2 −22.7

(−19.8) (−19.8) (−19.9) (−20.0) (−20.2) (−20.4) (−20.5) (−20.9)

G J·m−3 2.70 2.60 2.53 2.44 2.35 2.16 1.99 1.85 1.72

(2.62) (2.60) (2.39) (2.24) (2.05) (1.88) (1.75) (1.60)

The combined expanded uncertainties (Uc) are Uc(γsol) = 0.43 mN·m−1, Uc(Vm) = 0.02 dm3·mol−1, Uc(Δμ0g/kT)tr,S−W = 0.22 kJ·mol−1, and Uc(G) = 0.06 J·m−3 (0.95 level of confidence). a

content of EG or DMSO in the mixed solvent, the transfer free energy increases which indicate that the solubilization of the chain of the surfactant becomes more favorable. The increase of transfer free energy for EG + water is higher than in DMSO + water. The standard free energy of interfacial adsorption at the air/ saturated monolayer interface can be evaluated from the relation.11,25−27

caused by the improved solvation of the hydrocarbon tail and the preferential interactions of the hydrocarbon part of an amphiphile with the hydrophobic part of a cosolvent, leading to an increase in the cmc.14−16 The contribution of the tail transfer free energy toward the total Gibbs free energy of micellization (Δμg0/kT)tr,S−W can be calculated from the equation developed by Nagarajan and Wang29 ⎛ Δμ0 ⎞ ⎛ Δμ0 ⎞ ⎛ Δμ0 ⎞ g ⎟ g ⎟ ⎜ ⎜ g⎟ = φW ⎜ + φS⎜⎜ ⎟ ⎟ ⎜ kT ⎟ ⎝ kT ⎠tr ,S ⎝ kT ⎠tr ,W ⎠tr ,S−W ⎝ − φW ln

V VW − φS ln S + χW−S φW φS Vm Vm

0 ΔGads = ΔGm0 −

(10)

The higher negative value of ΔG0ads, the higher is the efficiency of the surfactant to be adsorbed at the surface. An increase in ΔG0ads is observed for DTAB and SDDS where the values are slightly higher in EG + water media compared to DMSO + water. The values of ΔG0ads are more or less constant for Tween-20. The same prediction can also be drawn for DTAB and SDDS from another physical quantity, pC20 (the values decrease in case of Tween-20). It is defined as pC20 = −log(C20); C20 is the surfactant molar concentration required to reduce the surface tension of mixed solvent by 20 mN·m−1 and is also an indication of the presence of surfactant prior to the air−water interface compared to pC20 = −log(C20) bulk phase. The pC20 value can measure the efficiency of the surfactant to adsorb at the interface. A high value of pC20 denotes the adsorption of the surfactant more efficiently at the surface and reducing the surface tension of the solution. With the increasing concentration of EG + water and DMSO + water, pC20 value decreases; that is, the adsorption at the interface decreases with an increasing population of surfactants in the bulk phase. Gordon Parameter (G). The ability of a solvent to promote amphiphile self-assembly is controlled by the solvophobic effect and can be related to solvent cohesiveness which can be characterized by its Gordon parameter,32 G. It can be determined for the binary mixtures from the experimental surface tension values of the different homologous mixtures. Gordon parameter was determined from the equation:

(8)

where W and S refer to water and solvents (EG and DMSO), φW and φS are their volume fractions in the mixture, and Vm is the molar volume of the mixed solvent and determined from Vm = VWx1 + VS(1 − x1). The factor Δμ0g represents the difference in the standard chemical potential between a surfactant molecule in the aggregate of size g and a singly dispersed surfactant molecule in the solvent. x1 is the mole fraction of water and VS and VW are molar volumes of EG or DMSO and water, respectively. The term χW−S is the Flory interaction parameter between water and solvent (EG and DMSO). These values for EG + water and DMSO + water mixture are obtained by fitting the activity data to the Flory− Huggins equation ⎛ 1 ⎞ ln aW = ln(1 − φEG) + ⎜1 − ⎟φ + χW−S φS2 NS ⎠ S ⎝

Πcmc Γmax

(9)

Here, NS is the ratio of the molar volume of solvent to that of water. The values of χW−S and NS for EG + water are −2.3 and 3.1 and for DMSO + water are −0.49 and 3.96, respectively, at 298 K.29−31 The negative value of χW−S is an indication of stronger water−solvent interaction than water−water and solvent−solvent interactions. The total transfer free energy depends on the temperature and the tail length of the surfactants, but in our study all three surfactants have equal tail lengths. We calculate the total transfer energy for dodecyl chain length. The details of the calculations are summarized in the Supporting Information. The tail transfer contributions of the Gibbs free energy to the total Gibbs free energy of micellization for different mass fraction of EG + water and DMSO + water were shown in Table 4. With increasing

G = γsol /Vm1/3

(11)

where γsol is the surface tension of the mixed solvents and Vm is the molar volume of the mixed solvents. Table 4 shows the Gordon parameter values for different mixtures used as the bulk phase in the micellar solution studied. The Gordon parameter 2592



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Figure 4. Plot of Gibbs free energy of micellization (ΔG0m) vs Gordon parameter (G) for (a) DTAB, (b) SDDS, and (c) Tween-20 (■, EG and □, DMSO).

Figure 5. (a) Plot of ln[F0/F] vs [quencher/mol·kg−1] for SDDS in EG + water (w1 = 0.3228). In the inset, the corresponding spectrum is given. (b) Co-plot of ln [F0/F] vs [quencher/mol·kg−1] for Tween-20 in DMSO (1) + water (2) medium: ■, w1 = 0.1088; ●, w1 = 0.320; ▲, w1 = 0.4228.

points out that an increase in percentage of EG and DMSO (in the parentheses) in the mixed solvents results in a decrease in solvent cohesiveness and as a result increasing the solvation of hydrophobic tail in the bulk phase with decreasing the solvophobic effect. Moyá et al.8 showed that G is linearly dependent on ΔG0m; in our work, we also observe the same type of behavior in the used systems (Figure 4). Packing Parameter (P). The structure of the micelle can be predicted from packing parameter (P). The packing properties of the aggregates depend on three parameters: (i) the surface area A of the polar headgroup; (ii) the volume v of the hydrophobic chain which can be considered to be fluid and incompressible and determined by using Tanford’s equation as: v ≈ (0.0274 + 0.0269nc)

P=

(14)

The structure of the micelle will be spherical when P < 1/3, nonspherical when 1/3 < P < 1/2, vesicles or bilayers when 1/2 < P < 1, or inverted structures when P > 1. From Table 4, it is obtained that the shape of the aggregates for all the three surfactants in water and also in mixed solvent is spherical in nature. Aggregation Number (Nagg) and Micropolarity. To evaluate the effects of EG and DMSO addition on the micellar aggregation number of DTAB, SDDS, and Tween-20, we have employed the static quenching method which is based on the quenching of a fluorescence probe by a known concentration of a quencher. The aggregation number of the surfactants in the mixed solvent system can be determined by using the following equation:

(12)

(iii) the maximum effective length, called the critical chain length, lc. For saturated hydrophobic chain with nc number of carbon atoms: lc ≤ lmax ≈ (0.154 + 0.1265nc)

v Alc

Nagg[Q] ⎛F ⎞ ln⎜ 0 ⎟ = ⎝ F ⎠ [S] − cmc

(13)

(15)

where F0 and F are the fluorescence intensities of the surfactant in the absence and presence of quencher, respectively. [S] and [Q] are the concentrations of surfactant and quencher, respectively. The aggregation number (Nagg) is obtained from

According to Israelachvili,33 the geometry of the micellar aggregate can be predicted from the packing parameter, P, which can be expressed as 2593



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the fitting of the fluorescence intensity data at a various concentrations of a quencher (Figure 5). The Nagg value decreases with increasing concentration of EG and DMSO in the mixed solvents that is in the higher mass fraction of mixed solvent less the number of monomers were required to form the micelle. The micropolarity of the pyrene environment in the micelles is determined from the ratio of the fluorescence intensities of the first and third vibronic peaks (I1/I3) in the pyrene spectra. From Table 5, it was observed that micropolarities of DTAB

DMSO + water due to attractive interaction, whereas the anionic headgroup of SDDS is less solubilized due to electrostatic repulsion. As a result, the polarity index of pyrene in DTAB micelle solubilized in DMSO + water is higher than that of SDDS micelle in the same medium.

■

CONCLUSIONS In this investigation, the experimental evidence indicates that the surfactant micellization in aqueous medium is affected by the presence of ethylene glycol and DMSO in the mixture. With increasing concentration of the solvent in the mixture, the cmc of the surfactant increases for all methods used. In case of DTAB, the effect of DMSO is higher than that of EG. Counterion binding of DTAB and SDDS decreases with increasing concentration of solvent in aquo-organic media. The Gibbs free energy of micellization, ΔG0m, and Gibbs free energy of interfacial adsorption, ΔG0ads, are negative and increase with increasing concentration of DMSO and EG in the mixture indicating the spontaneity of formation of the micelle. In case of Tween-20, the influence of solvent is small. The values of Γmax decrease with the increasing concentration of cosolvent in the mixture. Amin follows the reverse trend. The Gordon parameter decreases with the increasing concentration of the cosolvent in the mixture, denoting a decrease in cosolvent cohesiveness. Nagg decreases with the increase of organic solvent concentration.

Table 5. Aggregation Number (Nagg), Micropolarity, and Packing Parameter (P) of DTAB, SDDS, and Tween-20a 100 w1 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28) 0 0.56 (0.55) 1.11 (1.10) 5.53 (5.47) 11.0 (10.88) 21.75 (21.55) 32.28 (32.0) 42.57 (42.28)

Nagg 95

84 83 74 68 55

(84) (72) (65) (57) (52) 64

62 56 51 45

41 37 33 14

1.034 1.043 1.030 1.035 1.036 SDDS 0.843

(63) (57) (47) (42)

0.857 0.860 0.835 0.863 Tween-20 47 0.986

(42) (31) (19) (14)

P

micropolarity DTAB 1.036

0.983 1.006 1.028 1.058

(1.046) (1.045) (1.046) (1.050) (1.064)

0.20 0.19 0.18 0.18 0.17 0.16 0.15 0.14

(0.19) (0.19) (0.18) (0.17) (0.15) (0.14) (0.11)

(0.864) (0.883) (0.898) (0.912)

0.16 0.14 0.14 0.13 0.13 0.12 0.12 0.10

(0.15) (0.15) (0.15) (0.14) (0.14) (0.12) (0.11)

(1.008) (1.023) (1.193) (1.266)

0.31 0.28 0.27 0.26 0.23 0.21 0.17 0.15

(0.27) (0.25) (0.24) (0.21) (0.18) (0.17) (0.16)

■

ASSOCIATED CONTENT

S Supporting Information *

Determination of transfer coefficients EG/DMSO to alkane core and supporting figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +91 33 24146266. E-mail: [email protected]. Funding

S.D. and S.M. thank CSIR and UGC, Govt. of India, for senior research fellowships. Notes

The authors declare no competing financial interest.

a

The combined expanded uncertainties (Uc) are Uc(Nagg) = 3.5, Uc(micropolarity) = 2·10−3, and Uc(P) = 4·10−3 (0.95 level of confidence).

■

REFERENCES

(1) Lee, Y. S.; Woo, K. W. Micellization of Aqueous Cationic Surfactant Solution at the Micellar Structure Transition Concentration-based upon the Concept of the Pseudophase Separation. J. Colloid Interface Sci. 1995, 169, 34−38. (2) Chakraborty, T.; Chakraborty, I.; Ghosh, S. The Methods of Determination of Critical Micellar Concentrations of the Amphiphilic Systems in Aqueous Medium. Arabian J. Chem. 2011, 4, 265−270. (3) Ghosh, S.; Das Burman, A.; De, G. C.; Das, A. R. Interfacial and Self-Aggregation of Binary Mixtures of Anionic and Nonionic Amphiphiles in Aqueous Medium. J. Phys. Chem. B 2011, 115, 11098−11112. (4) Khatua, P. K.; Ghosh, S.; Ghosh, S. K.; Bhattacharya, S. C. Characterization of Binary Surfactant Mixtures (Cetylpyridinium Chloride and Tween 60) in an Aqueous Medium. J. Dispersion Sci. Technol. 2004, 25, 741−748. (5) Das, C.; Chakraborty, T.; Ghosh, S.; Das, B. Physicochemistry of Mixed Micellization: Binary and Ternary Mixtures of Cationic Surfactants in Aqueous Medium. Colloid J. 2010, 72, 788−798. (6) Das, C.; Chakraborty, T.; Ghosh, S.; Das, B. Mixed Micellization of Anionic−Nonionic Surfactants in Aqueous Media: A Physicochem-

micelles as a function of EG addition are more or less constant, but in DMSO + water (in the parentheses) these values gradually increase. On the other hand, micropolarities of SDDS and Tween-20 micelles were found to increase as a function of EG and DMSO addition. This can be explained from the surface solubilization of DMSO which is higher than that of EG. Pyrene is solubilized in the micellar palisade layer, close to micellar surface. The increase of (I1/I3) indicates that pyrene molecules sense a more polar environment with increasing cosolvent concentration. The observed increase in the pyrene (I1/I3) ratio is not necessarily due to the increase of micropolarity. It is possibly due to the increase of the surface area per headgroup which forces the probe to locate further outward of the micelle, for sensing of pyrene toward a more polar environment. The nonionic surfactant, Tween-20, contains a polyoxyethylene headgroup, which is more solvated. The cationic headgroup of DTAB was more solubilized in 2594



Article

ical Study with Theoretical Consideration. Colloid Polym. Sci. 2008, 286, 1143−1155. (7) Chakraborty, T.; Ghosh, S. A Unified Survey of Applicability of Theories of Mixed Adsorbed Film and Mixed Micellization. J. Surfactants Deterg. 2008, 11, 323−334. (8) Rodríguez, A.; Graciani, M. M.; Angulo, M.; Moyá, M. L. Effects of Organic Solvent Addition on the Aggregation and Micellar Growth of Cationic Dimeric Surfactant 12-3-12,2Br−. Langmuir 2007, 23, 11496−11505. (9) Rodríguez, A.; Graciani, M. M.; Moyá, M. L. Effects of Addition of Polar Organic Solvents on Micellization. Langmuir 2008, 24, 12785−12792. (10) Zhang, L.; Somasundaran, P.; Maltesh, C. Electrolyte Effects on the Surface Tension and Micellization of n-Dodecyl β-D-Maltoside Solutions. Langmuir 1996, 12, 2371−2373. (11) Tikariha, D.; Ghosh, K. K.; Barbero, N.; Quaglioltto, P.; Ghosh, S. Micellization Properties of Mixed Cationic Gemini and Cationic Monomeric Surfactants in Aqueous-Ethylene Glycol Mixture. Colloids Surf., A 2011, 381, 61−69. (12) Majhi, P. R.; Moulik, S. P.; Burke, S. E.; Rodgers, M.; Palepu, R. Physicochemical Investigations on the Interaction of Surfactants and Salts with Polyvinylpyrrolidone in Aqueous Medium. J. Colloid Interface Sci. 2001, 235, 227−234. (13) Chakraborty, T.; Ghosh, S. Mixed Micellization of an Anionic Gemini Surfactant (GA) with Conventional Polyethoxylated Nonionic Surfactants in Brine Solution at pH 5 and 298 K. Colloid Polym. Sci. 2007, 285, 1665−1673. (14) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley-Interscience: Hoboken, NJ, 2004. (15) Ruiz, C. C. Thermodynamics of Micellization of Tetradecyltrimethylammonium Bromide in Ethylene Glycol-Water Binary Mixtures. Colloid Polym. Sci. 1999, 277, 701−707. (16) Ruiz, C. C.; Molina-Bolívar, J. A.; Aguiar, J. Thermodynamic and Structural Studies of Triton X-100 Micelles in Ethylene Glycol-Water Mixed Solvents. Langmuir 2001, 17, 6831−6840. (17) Ruiz, C. C.; Molina-Bolívar, J. A.; Aguiar, J.; Maclsaac, G.; Moroze, S.; Palepu, R. Effect of Ethylene Glycol on the Thermodynamic and Micellar Properties of Tween 20. Colloid Polym. Sci. 2003, 281, 531−541. (18) Sarkar, B.; Lam, S.; Alexandridis, P. Micellization of AlkylPropoxy-Ethoxylate Surfactants in Water-Polar Organic Solvent Mixtures. Langmuir 2010, 26, 10532−10540. (19) Rodríguez, A.; Graciani, M. M.; Muñoz, M.; Robina, I.; Moyá, M. L. Effects of Ethylene Glycol Addition on the Aggregation and Micellar Growth of Gemini Surfactants. Langmuir 2006, 22, 9519− 9525. (20) Sohrabi, B.; Bazyari, A.; Hashemianzadeh, M. Effect of Ethylene Glycol on Micellization and Surface Properties of Gemini Surfactant Solutions. Colloids Surf., A 2010, 364, 87−93. (21) Wärnheim, T. Aggregation of Surfactants in Nonaqueous, Polar Solvents. Curr. Opin. Colloid Interface Sci. 1997, 2, 472−476. (22) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. Thermodynamics of Micellization of Aerosol OT in Binary Mixtures of Water, Formamide, Ethylene Glycol, and Dioxane. J. Phys. Chem. 1994, 98, 4713−4718. (23) Misra, P. K.; Mishra, B. K.; Behera, G. B. Micellization of Ionic Surfactants in Tetrahydrofuran-Water and Acetonitrile-Water Mixed Solvent Systems. Colloids Surf. 1991, 57, 1−10. (24) Lee, D. J.; Huang, W. H. Enthalpy-Entropy Compensation in Micellization of Sodium Dodecylsulfate in Water/Methanol, WaterEthylene Glycol and Water/Glycerol Binary Mixtures. Colloid Polym. Sci. 1996, 274, 160. (25) Chauhan, M. S.; Sharma, K.; Kumar, G.; Chauhan, S. A Conductometric Study of Dimethylsulfoxide Effect on Micellization of Sodium Dodecyl Sulfate in Dilute Aqueous Electrolyte Solutions. Colloids Surf., A 2003, 221, 135−140. (26) Sjöberg, M.; Henriksson, U.; Wärnheim, T. 2H Nuclear Magnetic Relaxation of [1,l-2H]Hexadecyltrimethylammonium Bro-

mide in Micellar Solutions of Nonaqueous Polar Solvents and Their Mixtures with Water. Langmuir 1990, 6, 1205−1211. (27) Das, C.; Das, B. Thermodynamic and Interfacial Adsorption Studies on the Micellar Solutions of Alkyltrimethylammonium Bromides in Ethylene Glycol (1) + Water (2) Mixed Solvent Media. J. Chem. Eng. Data 2009, 54, 559−565. (28) Rodríguez, A.; Graciani, M. M.; Fernández, G.; Moyá, M. L. Effects of Glycols on the Thermodynamic and Micellar Properties of TTAB in Water. J. Colloid Interface Sci. 2009, 338, 207−215. (29) Nagarajan, R.; Wang, C. Theory of Surfactant Aggregation in Water/Ethylene Glycol Mixed Solvents. Langmuir 2000, 16, 5242− 5251. (30) Abraham, M. H.; Grellier, P. L.; McGill, R. A. A Quantitative Measure of Solvent: Solvophobic Effect. J. Chem. Soc., Perkin Trans 2 1988, 3, 339−345. (31) Peyre, V.; Bouguerr, S.; Testard, F. Micellization of Dodecyltrimethylammonium Bromide in Water−Dimethylsulfoxide Mixtures: A Multi-Length Scale Approach in a Model System. J. Colloid Interface Sci. 2013, 389, 164−174. (32) Gordon, J. E. The organic chemistry of electrolyte solutions; Wiley Interscience: New York, 1975. (33) Israelachvili, J. N. Intermolecular and surface force, 2nd ed.; Academic Press: London, 1991; Chapter 17, p 370.

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