pubs.acs.org/Langmuir © 2010 American Chemical Society
Micellization of Alkyl-Propoxy-Ethoxylate Surfactants in Water-Polar Organic Solvent Mixtures Biswajit Sarkar, Stephanie Lam, and Paschalis Alexandridis* Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York (SUNY), Buffalo, New York 14260-4200 Received February 5, 2010. Revised Manuscript Received March 4, 2010 The effects of cosolvents (glycerol, ethanol, and isopropanol) on the self-assembly of novel alkyl-propoxy-ethoxylate surfactants in aqueous solutions have been investigated with a focus on the (i) quantification of solvent effects on the critical micelle concentration (cmc), (ii) free-energy contributions to micellization, (iii) local environment in the micellar solution, and (iv) structure of the micelles. The introduction of the polar organic solvents considered in this work into water decreases cohesive forces in the solvent mixture, resulting in an increase in the solubility of the surfactant molecules. As a result, micelle formation becomes less favorable and the cmc increases. The contribution of the cosolvent to the free energy of micellization is positive, and the data for different mixed solvents collapse onto a single straight line when plotted versus a function of the solubility parameters of the surfactant alkyl chains and the mixed solvents. The behavior of the poly(propylene oxide) part of the alkyl-propoxy-ethoxylate surfactants is hydrophilic, albeit less so in the ethanol-water mixed solvent than in plain water. Pyrene fluorescence emission I1/I3 data suggest that the microenvironment in micellar solutions is affected mainly by the cosolvent concentration, not the surfactant degree of ethoxylation. Small-angle X-ray scattering data for both water and ethanol-water surfactant solutions are consistent with oblate ellipsoid micelles and reveal that the introduction of 20% ethanol decreases the micelle long axis by 10-15%.
Introduction Nonionic surfactants of the alkyl-ethoxylate (CiEOj) family exhibit a low critical micelle concentration (cmc), a mild nature, and compatibility with other surfactants and are widely used in detergents, paints, personal care products, controlled drug delivery, and so forth.1-3 Organic solvents are often used along with such nonionic surfactants, especially for pharmaceuticals, personal care products, and paint formulations. The surfactant typically serves as a carrier of active ingredients whereas cosolvents impart beneficial qualities to the formulation (e.g., viscosity and volatility). Thus, the investigation of organic solvent effects on the *To whom correspondence should be addressed. E-mail: palexand@ buffalo.edu. (1) Schick, M. J. Nonionic Surfactants; Marcel Dekker: New York, 1967. (2) Rhein, L. D.; Schlossman, M.; O’Lenick, A.; Somasundaran, P. Surfactants in Personal Care Products and Decorative Cosmetics, 3rd ed.; Surfactant Science Series; CRC Press/Taylor & Francis: Boca Raton, FL, 2006; Vol. 135. (3) Lai, K.-Y., Ed. Liquid Detergents, 2nd ed.; Surfactant Science Series; Taylor & Francis: Boca Raton, FL, 2006; Vol. 129. (4) Zana, R. Aqueous surfactant-alcohol systems - A review. Adv. Colloid Interface Sci. 1995, 57, 1-64. (5) Ameri, M.; Attwood, D.; Collett, J. H.; Booth, C. Self-assembly of alcohol ethoxylate non-ionic surfactants in aqueous solution. J. Chem. Soc., Faraday Trans. 1997, 93, 2545-2551. (6) Aramaki, K.; Olsson, U.; Yamaguchi, Y.; Kunieda, H. Effect of watersoluble alcohols on surfactant aggregation in the C12EO8 system. Langmuir 1999, 15, 6226-6232. (7) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. Phase-behavior of polyoxyethylene surfactants with water - mesophase structure and partial miscibility (cloud points). J. Chem. Soc., Faraday Trans. 1 1983, 79, 975-1000. (8) Barry, B. W.; Eleini, D. I. D. Surface properties and micelle formation of long-chain polyoxyethylene nonionic surfactants. J. Colloid Interface Sci. 1976, 54, 339-347. (9) Rao, I. V.; Ruckenstein, E. Micellization behavior in the presence of alcohols. J. Colloid Interface Sci. 1986, 113, 375-387. (10) Nagarajan, R.; Ruckenstein, E. Theory of surfactant self-assembly: A predictive molecular thermodynamic approach. Langmuir 1991, 7, 2934-2969. (11) Puvvada, S.; Blankschtein, D. Molecular-thermodynamic approach to predict micellization, phase-behavior and phase-separation of micellar solutions. 1. Application to nonionic surfactants. J. Chem. Phys. 1990, 92, 3710-3724.
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self-assembly of nonionic surfactants in aqueous solution is of considerable practical interest.2-4 Several experimental5-8 and theoretical9-11 investigations have addressed the micellization of CiEOj surfactants in aqueous solution. However, the self-assembly of nonionic surfactants in mixtures of water with organic mixed solvents has attracted relatively little attention in the literature, and a proper understanding of solvent effects in selfassembly is lacking. The self-assembly of CiEOj surfactants in aqueous solution can be attributed to the hydrophobic effect that emanates from the less favorable interaction between surfactant alkyl chains and water molecules compared to the more favorable water-water interactions.12 Above the cmc, hydrophobic alkyl chains are localized in the micelle core and hydrophilic headgroups are localized at the micelle core-water interface. The hydrophobichydrophilic interactions of the surfactant molecules with the solvent can be controlled through a modification of solvent quality. For water as the main solvent, this is done by adding suitable additives such as salts, short-chain alcohols, glycerol, dimethyl sulfoxide (DMSO), formamide, and so forth. This introduction of additives provides additional degrees of freedom with which to control the self-assembly of amphiphiles and the properties of the resulting formulation.13,14 Among the organic solvents, glycerol and ethanol are of special interest because of their biocompatibility.2 Ethanol is also relevant in controlling the shape and morphology of nanomaterials synthesized by using micelle templates.15 (12) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (13) Lin, Y. N.; Alexandridis, P. Cosolvent effects on the micellization of an amphiphilic siloxane graft copolymer in aqueous solutions. Langmuir 2002, 18, 4220-4231. (14) Ivanova, R.; Lindman, B.; Alexandridis, P. Effect of pharmaceutically acceptable glycols on the stability of the liquid crystalline gels formed by poloxamer 407 in water. J. Colloid Interface Sci. 2002, 252, 226-235.
Published on Web 03/24/2010
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Article Table 1. Composition of Surfactants Used in This Study
surfactant
molecular structure
molecular weight
EO content (wt %)
alkyl/EO ratio (w/w)
PO/EO ratio (w/w)
S1 S2 S3
C13(PO)12.2(EO)8 C13(PO)12.2(EO)17 C13(PO)12.2(EO)34
1266 1641 2390
30 46 63
0.49 0.24 0.12
1.94 0.91 0.46
Organic additives to aqueous surfactant solutions can be classified into cosolvents and cosurfactants.4,16 Cosurfactants denote weakly amphiphilic polar organic molecules that cannot form micelles alone. Cosurfactants act at low concentrations; they adsorb at the outer portion of the micelle (at the water-micelle core interface) and favor micellization of the surfactant. Cosolvents affect the micellization at much higher concentrations. Cosolvents are not necessarily amphiphilic in nature and may not interact with micelles. Instead, cosolvents affect the cmc via the modification of water-surfactant interactions by changing the properties of the mixed solvent. A given molecule can often act as both a cosurfactant and cosolvent. For example, it has been reported that ethanol acts as a cosurfactant and reduces the cmc at low concentration whereas it increases the cmc at high concentration, indicating its role as cosolvent.17 Our group has a long-standing interest in the role of various organic solvents on the self-assembly of nonionic amphiphiles.13,14,18-20 For example, organic cosolvents greatly modulate the range of stability and structure of lyotropic liquid crystals formed by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EOjPOnEOj) block copolymers.14,19 The introduction of ethanol and glycerol into water caused an increase in the cmc of poly(ethylene oxide)-graft-siloxanes.13 The hydrogen bonding solubility parameter was found to be a solvent property suitable for capturing the cosolvent effect on the cmc in this case. Alkyl-propoxy-ethoxylates (CiPOnEOj) are novel surfactants where the middle poly(propylene oxide) (PPO) part exhibits hydrophobic/hydrophilic character intermediate to that of the alkyl and PEO parts. We have previously investigated the lyotropic phase behavior of these surfactants in water and in aqueous electrolyte solution.21 Recently, we reported on the micellization of CiPOnEOj surfactants in water.22 We examine here the influence of polar organic solvents (glycerol, ethanol, and isopropanol) on the micelle formation, micropolarity, and micelle structure of CiPOnEOj surfactants, with a particular interest in how the role of the middle PPO block might be impacted by cosolvents that have a chemical composition and polarity similar to those of PPO. We first examine possible correlations between (15) Denkova, A. G.; Mendes, E.; Coppens, M. O. Effects of salts and ethanol on the population and morphology of triblock copolymer micelles in solution. J. Phys. Chem. B 2008, 112, 793-801. (16) Chennamsetty, N.; Bock, H.; Scanu, L. F.; Siperstein, F. R.; Gubbins, K. E. Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide. J. Chem. Phys. 2005, 122, 094710. (17) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley-Interscience: Hoboken, NJ, 2004. (18) Holmqvist, P.; Alexandridis, P.; Lindman, B. Phase behavior and structure of ternary amphiphilic block copolymer-alkanol-water systems: Comparison of poly(ethylene oxide) poly(propylene oxide) to poly(ethylene oxide) poly(tetrahydrofuran) copolymers. Langmuir 1997, 13, 2471-2479. (19) Svensson, B.; Olsson, U.; Alexandridis, P. Self-assembly of block copolymers in selective solvents: Influence of relative block size on phase behavior. Langmuir 2000, 16, 6839-6846. (20) Yang, L.; Alexandridis, P. Polyoxyalkylene block copolymers in formamide-water mixed solvents: Micelle formation and structure studied by small-angle neutron scattering. Langmuir 2000, 16, 4819-4829. (21) Shusharina, N. P.; Balijepalli, S.; Gruenbauer, H. J. M.; Alexandridis, P., Mean-field theory prediction of the phase behavior and phase structure of alkylpropoxy-ethoxylate “graded” surfactants in water: Temperature and electrolyte effects. Langmuir 2003, 19, 4483-4492. (22) Sarkar, , B.; Alexandridis, , P. Alkyl-propoxy-ethoxylate “graded” surfactants in aqueous solutions: Micelle formation and structure. J. Phys. Chem. B 2010, doi: 10.1021/jp910939q.
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the cmc and physicochemical properties of the solvent mixtures in order to reveal the origin of the cosolvent effect. The effect of solvent quality on the thermodynamics of micellization is discussed next. We then probe the local environment in the micellar solution and assess the effect of solvent on the micelle structure using small-angle X-ray scattering.
Materials Surfactants. Three CiPOnEOj surfactants with varying poly(ethylene oxide) (PEO) length were obtained from Dow Chemical Company (Midland, MI) and were used as received. Their nominal compositions are shown in Table 1. These surfactants can also be considered to be short-chain ABC triblock terpolymers with hydrophobic poly(ethylene) and hydrophilic poly(ethylene oxide) blocks at the two ends and a poly(propylene oxide) block in the middle. Solvents. Glycerol, ethanol, and 2-propanol (isopropanol) (obtained from Acros Co., Belgium) were used as cosolvents. All solutions were prepared with Milli-Q filtered water (18 MΩ cm). Spectroscopic Probes. Pyrene (Fluka, Switzerland) was used to probe the cmc and the micropolarity of the surfactant solution. 1,6-Diphenyl-1,3,5-hexatriene (DPH) (Sigma, St. Louis, MO) was used to determine the cmc. Sample Preparation. Stock solutions of surfactants were prepared by dissolving the surfactants in water-cosolvent mixtures. Samples were prepared by diluting the stock solutions to the desired surfactant concentration (in the range of 0.0001-2 wt %). The solutions were tested within a few days of preparation. Pyrene Addition. A stock solution of 1 mM pyrene in ethanol was prepared and was further diluted to 1 μM. Two microliters of the 1 μM pyrene/ethanol solution was added to 3 g of surfactant solution. The resulting pyrene and ethanol concentrations were about 0.006 μM and 0.067 vol %, respectively. DPH Addition. A stock solution of 0.7 mM DPH in methanol was prepared; 25 μL of the DPH/methanol solution was added to 3 g of surfactant solution so that the final solution contained about 0.004 mM DPH and 1% v/v methanol.23 The same DPH concentration (0.004 mM) was used for all samples. The solutions were left in the dark to equilibrate for at least 3 h (and no more than 24 h) before the spectroscopic measurement.
Methods Fluorescence Spectroscopy. The fluorescence emission intensity of pyrene-containing surfactant solutions was recorded in the 350-600 nm range using a Hitachi 2500 fluorescence spectrophotometer at 25 °C. The excitation wavelength of pyrene was λ = 335 nm. Five characteristic vibronic peaks were observed. The ratio of the first to the third vibronic peak (I1/I3) of the pyrene emission spectrum indicates the polarity of the pyrene microenvironment.24 This ratio decreases with decreasing micropolarity. As the surfactant concentration increased, pyrene started to partition to hydrophobic sites and, as a result, the I1/I3 ratio started to decrease because the polarity of a hydrophobic medium (23) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Micellization of poly(ethylene oxide)-poly(proplene oxide)-poly(ethlene oxide) triblock copolymers in aqueous-solutions - thermodynamics of copolymer association. Macromolecules 1994, 27, 2414-2425. (24) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Fluorescence probe studies of Pluronic copolymer solutions as a function of temperature. Langmuir 1995, 11, 730-737.
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Figure 1. Pyrene fluorescence emission intensity I1/I3 ratio plotted against CiPOnEOj surfactant concentration for various cosolvent-water compositions: (b) 0% cosolvent, (2) 20% cosolvent, and (9) 40% cosolvent. Top panels: S1. Middle panels; S2. Bottom panels: S3. Left column: glycerol. Middle column: ethanol. Right column: isopropanol. Temperature: 25 °C. was lower than that of an aqueous medium (Figure 1). At concentrations above the cmc, the I1/I3 value stabilized as pyrene accumulated in the micelle hydrophobic moieties. UV-Vis Spectroscopy. Absorption spectra of the DPHcontaining surfactant solutions were recorded in the 300500 nm range using a Hitachi U-1800 UV-vis spectrophotometer. DPH is sparingly soluble in water whereas its solubility in organic media is high. Thus, in an aqueous micellar solution DPH tends to be soluble in the micelle core. The absorption intensity is a function of the concentration of solubilized DPH. The main absorption intensity peak at 356 nm, characteristic of DPH, was plotted versus the logarithm of surfactant concentration (data not shown here). cmc values were determined from the first inflection point of the absorption intensity versus surfactant concentration curve.23 The cmc values obtained from DPH absorption compared well with those obtained from pyrene emission spectroscopy. Small Angle X-ray Scattering (SAXS). The SAXS technique was applied to examine the effect of cosolvent on the micellar structure of S2 and S3 surfactants in 20/80 vol % ethanol-water. The SAXS experiments were performed using a Nano-STAR instrument (Bruker-AXS, Madison, WI) operated at 40 kV and 35 mA. The surfactant solutions were tested at 25 °C in a quartz capillary with an outer diameter of 2 mm. The sample-to-detector distance was 1015 mm. The X-ray wavelength (λ) used was 0.1542 nm (Cu KR). The angular distribution of the scattered electrons was recorded in a 2D detector. The scattering intensity was obtained by averaging the intensity of all points in the 2D detector space for a scattering vector value, q, defined as 4π Θ q¼ sin ð1Þ λ 2
(25) Pedersen, J. S. A flux- and background-optimized version of the NanoSTAR small-angle X-ray scattering camera for solution scattering. J. Appl. Crystallogr. 2004, 37, 369-380.
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Θ is the angle between the incident beam and the scattered radiation. The reduction of data to an absolute scale was carried out following procedures described in ref 25. Scattering data taken from pure solvent was used as the background in the analysis of the data from surfactant solutions. Transmission measurements were made indirectly by measuring the scattering from glassy carbon with and without the sample. Noise correction was made, and data obtained from pixels close to the beam stop were corrected using a first-order correction procedure as discussed in ref 25. The absolute scattering intensity obtained for the micellar solutions was evaluated, taking into consideration the core-shell ellipsoid form factor used to capture the micelle shape and the hard sphere structure factor used to describe the intermicellar interactions. More information on the selection of the form factor as well as the equations that describe the form and structure factors is presented elsewhere.22
Results and Discussion Effect of Cosolvents on the cmc. The pyrene I1/I3 ratios are plotted in Figure 1 versus the logarithm of surfactant concentration for mixed water-cosolvent solutions of S1, S2, and S3. (Refer to Table 1 for the surfactant notation.) The cmc is reflected in the second inflection of the (I1/I3)-concentration curve.13 The cmc values of CiPOnEOj surfactants in the presence of glycerol, ethanol, or isopropanol thus determined are presented in Table 2. The introduction of ethanol and isopropanol into water increased the cmc values for all three surfactants. To compare the relative influence of cosolvents on the micellization of the three CiPOnEOj surfactants having different cmc0 values (i.e., the cmc value in the absence of cosolvent), cmc values have been normalized with respect to cmc0. The log(cmc/cmc0) varies linearly with cosolvent concentration (Figure 2). The effect of isopropanol and ethanol on the cmc is more pronounced than that of glycerol. At 40 vol % cosolvent, a 20-fold increase in the cmc Langmuir 2010, 26(13), 10532–10540
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Table 2. Influence of Cosolvent on the Critical Micelle Concentration of S1, S2, and S3 S1
S2
S3
cosolvent concentration
cmc wt %
cmc/ cmc0
cmc wt %
cmc/ cmc0
cmc wt %
cmc/ cmc0
0% 20% glycerol 40% glycerol 20% ethanol 40% ethanol 20% isopropanol
0.01 0.015 0.025 0.03 0.20 0.06
1.0 1.5 2.5 3.0 20.0 6.0
0.02 0.03 0.04 0.06 0.40 0.08
1.0 1.5 2.0 3.0 20.0 4.0
0.05
1.0
0.20 1.00 0.20
4.0 20.0 4.0
Figure 3. Dependence of cmc/cmc0 on the surfactant degree of ethoxyation (NEO) for various cosolvent types and concentrations: (9) 20% isopropanol; (b) 20% ethanol; and (2) 40% ethanol. The dotted lines are linear fits to the data.
Figure 2. Variation of cmc/cmc0 of (b) S1, (9) S2, and (2) S3 with cosolvent concentration at 25 °C. Filled symbols: ethanol-water. Open symbols: water-isopropanol. (þ) symbols: glycerol-water. The dotted lines are guides to the eye.
values of S1, S2, and S3 is observed in the case of ethanol, whereas a 2-fold increase in the cmc is found in the case of glycerol for S1 and S2. At 40 vol % isopropanol, no micelles form as shown by both pyrene fluorescence and DPH absorption. From Figure 2, it can be seen that the cmc/cmc0 values for all three surfactants tend to collapse onto a single line for each cosolvent type, suggesting that the cmc is primarily influenced by the cosolvent type rather than the surfactant composition. The lack of effect of the surfactant degree of ethoxylation (NEO) on the cmc can be seen in Figure 3, where cmc/ cmc0 at different cosolvent concentrations is plotted against NEO. The micellization of CiPOnEOj surfactants in aqueous solution exhibits similarities to that of CiEOj surfactants.22 Therefore, it is interesting to compare the effect of solvent quality on the micellization of these two classes of nonionic surfactants. Nishikido et al. found an exponential increase in the cmc of C12EOj (j = 6, 11, 20, 31) nonionic surfactants with an increase in ethanol concentration.26 From their reported results, we calculated the cmc/cmc0 values and observed little variation in the cmc/cmc0 ratio with NEO for 20% ethanol. Glycerol was found to increase the cmc of CiEOj surfactants. However, its influence on the cmc was less pronounced than that of ethanol;27,28 this is similar to what we have observed for CiPOnEOj. For example, the cmc of C16EO20 remained unchanged for up to 5 vol % glycerol in water whereas a 6-fold increase in the cmc was reported at 40 vol % glycerol.27 (26) Nishikido, N.; Moroi, Y.; Uehara, H.; Matuura, R. Effect of alcohols on micelle formation of nonionic surfactants in aqueous-solutions. Bull. Chem. Soc. Jpn. 1974, 47, 2634-2638. (27) D’Errico, G.; Ciccarelli, D.; Ortona, O. Effect of glycerol on micelle formation by ionic and nonionic surfactants at 25 degrees C. J. Colloid Interface Sci. 2005, 286, 747-754. (28) Cantu, L.; Corti, M.; Degiorgio, V.; Hoffmann, H.; Ulbricht, W. Nonionic micelles in mixed water-glycerol solvent. J. Colloid Interface Sci. 1987, 116, 384-389.
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The effects of various cosolvents and cosolutes on the selfassembly of EOjPOnEOj (Pluronic) amphiphiles have been studied.29-32 The addition of glycerol as a cosolvent favors micelle formation32 whereas the introduction of ethanol is reported to improve the solvent quality for EOjPOnEOj and disfavor micellization.29 Methanol, ethanol, and 1-propanol disfavor micelle formation and thus increase the cmc of EO19PO69EO19 (Pluronic P123).30 For 20% ethanol in water, a cmc/ cmc0 value of 3.6 was calculated for EO61PO40EO61 (Pluronic F87) from the reported cmc data.29 Ethanol and isopropanol have the same effect on the micellization of the CiPOnEOj surfactant, but the effect is of a higher magnitude than that observed for the EOjPOnEOj amphiphiles. Origin of Solvent Effect. The micellization of surfactants in water is driven by the low solubility of the alkyl chains (hydrophobic effect). The introduction of a cosolvent alters the bulk properties of the mixed solvent according to the properties of individual solvents and their interactions. In general, the addition of an organic solvent improves the alkyl solubility and micellization becomes less favorable compared to that in water. Thus, in an organic solvent an increase in the cmc is observed. The solvation of a solute molecule requires the cohesive forces of the solvent and the solute to be overcome. Properties related to the polarity such as the dipole moment, dielectric constant, surface tension, Hildebrand solubility parameter (δ), Hansen solubility parameters (dispersion (δD), polar (δP), and hydrogen bonding solubility parameter (δH)),33 and octanol-water partition coefficient (log P) reflect the cohesive force of the solvent and are often used to capture the effect of the solvent on the cmc.13,34 Values for these properties are presented in Table 3. We note that the log P value of isopropanol is slightly positive (0.05), indicating a weak affinity for water, and for ethanol, (29) Armstrong, J.; Chowdhry, B.; Mitchell, J.; Beezer, A.; Leharne, S. Effect of cosolvents and cosolutes upon aggregation transitions in aqueous solutions of the Poloxamer F87 (Poloxamer P237): A high sensitivity differential scanning calorimetry study. J. Phys. Chem. 1996, 100, 1738-1745. (30) Bharatiya, B.; Guo, C.; Ma, J. H.; Hassan, P. A.; Bahadur, P. Aggregation and clouding behavior of aqueous solution of EO-PO block copolymer in presence of n-alkanols. Eur. Polym. J. 2007, 43, 1883-1891. (31) Chaibundit, C.; Ricardo, N.; Costa, F.; Wong, M. G. P.; Hermida-Merino, D.; Rodriguez-Perez, J.; Hamley, I. W.; Yeates, S. G.; Booth, C. Effect of ethanol on the micellization and gelation of Pluronic P123. Langmuir 2008, 24, 12260-12266. (32) Alexandridis, P.; Yang, L. SANS investigation of polyether block copolymer micelle structure in mixed solvents of water and formamide, ethanol, or glycerol. Macromolecules 2000, 33, 5574-5587. (33) Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (34) Rodriguez, A.; Graciani, M. D.; Moya, M. L. Effects of addition of polar organic solvents on micellization. Langmuir 2008, 24, 12785-12792.
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Sarkar et al. Table 3. Physical Properties of Solvents Used in This Study
water glycerol ethanol 2-propanol
molar volume (mL/mol)
dielectric constant
dipole moment (D)
surface tension (dyn/cm)
Solubility parameter (δ) (MPa1/2)
δD (MPa1/2)
δP (MPa1/2)
δH (MPa1/2)
log P
18 73.3 58.5 85.0
80.1 25.3 20.2 46.5
3.11 2.68 1.69 1.66
72.8 64.0 22.3 23.0
47.8 36.2 26.5 27.21
15.5 17.4 15.8 20.0
16.0 12.2 8.8 18.0
42.3 29.3 19.4 4.1
-1.38 -2.55 -0.3 0.05
Figure 4. cmc for (b) S1, (2) S2, and (9) S3 plotted vs the physicochemical properties of mixed solvents: solubility parameter (δ) (left column, top); dipole moment (left column, bottom); partial solubility parameter due to hydrogen bonding (δH) (right column, top); partial solubility parameter due to polar forces (δP) (right column, middle); and partial solubility parameter due to dispersion forces (δD) (right column, bottom). The dotted lines are linear fits to the data.
log P = -0.32, indicating slight hydrophilicity. The log P value of glycerol (-2.55) indicates its strong affinity toward water (log P = -1.38). Thus, the behavior of glycerol should be closer to that of water than to that of ethanol and isopropanol. Recall that the impact of the cosolvent on the cmc follows the order isopropanol > ethanol > glycerol. Thus, log P appears to capture qualitatively the effect of cosolvents on the cmc. To evaluate the impact of the mixed solvent on the CiPOnEOj self-assembly, we examine possible correlations between the cmc and solvent properties by plotting in Figure 4 the cmc for each 10536 DOI: 10.1021/la100544w
surfactant against different mixed solvent properties: the dipole moment (D), δ, δP, δH, and δD (we estimated the properties of the solvent mixture as the volumetric average of these properties for pure solvents35). This Figure reveals that log(cmc) increases as the D, δ, δH, and δP values decrease. In fact, for a given surfactant, the log(cmc) values obtained from different binary solvent mixtures fall on a straight line (the correlation coefficient values are in the (35) Antoniou, E.; Alexandridis, P. Polymer conformation in mixed aqueous polar organic solvents. Eur. Polym. J. 2010, 46, 324-335.
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Figure 5. Dependence of the cosolvent contribution to the micellization free energy for (b) S1, (2) S2, and (9) S3 on the cosolvent concentration: (filled symbols) ethanol-water and (open symbols) isopropanol-water.
range of 0.87-0.99). However, no such clear trend is observed for δD. The bulk physicochemical parameters related to the cohesive forces due to hydrogen bonding and the polarity of the solvent decrease with an increase in the concentration of the cosolvent in the mixture, resulting in a reduction in the cohesive energy difference between the surfactant alkyl chains and the solvent. Thus, the solubility of surfactant molecules improves because of a reduction in the hydrophobic effect, and as a consequence, the cmc increases. Such correlation between the cmc and solvent physicochemical properties (D, δ, δH, and δP) should enable the estimation of polar organic solvent effects on the micelle formation of nonionic surfactants using the readily available bulk properties of solvents. Influence of Cosolvents on the Thermodynamics of Micellization. The micellization of nonionic surfactants can be described by the closed association model that assumes an equilibrium between individually dissolved surfactant molecules and micelles.23 According to this model, the standard free energy of micellization (ΔGomic) for the transfer of one mole of surfactant molecules from an aqueous solution to the micellar phase is given by23 ΔGomic ¼ RT lnðX cmc Þ
ð2Þ
where R is the ideal gas constant, T is the absolute temperature, and Xcmc is the surfactant mole fraction at the critical micelle concentration. Positive values of ΔHo and ΔSo were observed for the micellization of CiPOnEOj surfactants in aqueous solution.22 ΔHo was 11.0, 34.1, and 45.1 kJ/mol for S1, S2, and S3, respectively. The positive enthalpies of micellization indicate that the transfer of surfactants from the solution to the micelles is an endothermic process. Micellization is instead driven by an entropy gain. ΔSo was found to be 0.15, 0.22, and 0.26 kJ/(mol K) for S1, S2, and S3, respectively. The effect of cosolvent on the free energy of micellization can be captured using the so-called free energy of transfer (ΔGcosolvent) through the following equation:36-38 ΔGcosolvent ¼ ΔGomicðwater þ cosolventÞ - ΔGomicðwaterÞ
ð3Þ
(36) Ruiz, C. C.; Molina-Bolivar, J. A.; Aguiar, J.; MacIsaac, G.; Moroze, S.; Palepu, R. Thermodynamic and structural studies of Triton X-100 micelles in ethylene glycol-water mixed solvents. Langmuir 2001, 17, 6831-6840. (37) Ruiz, C. C.; Molina-Bolivar, J. A.; Aguiar, J.; MacIsaac, 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. (38) Das, C.; Das, B. Thermodynamic and interfacial adsorption studies on the micellar solutions of alkyltrimethylammonium bromides in ethylene glycol (1) þ water (2) mixed solvent media. J. Chem. Eng. Data 2009, 54, 559-565.
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Figure 6. Dependence of free energies of micellization (top) and cosolvent contribution to the free energy of micellization (bottom) on solute-solvent interaction parameters. (b) S1, (2) S2, and (9) S3. The dotted lines are linear fits.
ΔGcosolvent is plotted as a function of cosolvent concentration in Figure 5. The ΔGcosolvent values are positive, consistent with the finding that the introduction of the cosolvents considered here disfavors micellization. As the cosolvent concentration increases, the ΔGcosolvent values also increase for all three CiPOnEOj surfactants. It can be seen from Figure 5 that the ΔGcosolvent values fall onto a single straight line irrespective of the surfactant degree of ethoxylation, similar to the trend observed in Figure 2, reaffirming that the effect of the surfactant hydrophilic part on micellization is less pronounced than that of the solvent. Because the alkyl chain lengths for the three surfactants of interest here are the same, the ΔGcosolvent values are comparable at a fixed cosolvent concentration for all surfactants. A similar effect on ΔGcosolvent has been observed for the micellization of EOjPOnEOj block copolymers in binary and ternary mixed solvents, where micellization is mainly affected by the hydrophobic part.39 The micellization free-energy contribution due to the transfer of the surfactant alkyl chain from the bulk solvent to the micelle core is typically estimated using the solubility of alkyls in the particular solvent. The solubility of a solute in a solvent depends on the difference between their chemical potentials, which can be expressed in terms of the solubility parameter.40 Recall that we found the cosolvent effect on cmc to correlate to the solubility parameter of the mixed solvent (Figure 4). An empirical solubility parameter function (ΛCS) has been defined by combining various Hansen solubility parameters40 ΛCS 2 ¼ ðδCD - δSD Þ2 þ ðδCP - δSP Þ2 þ 0:5ðδCH - δSH Þ2
ð4Þ
(39) Ravi, V.; Sarkar, B.; Alexandridis, P. Micellization of amphiphilic block copolymers in presence of binary and ternary mixed solvents. Submitted for publication. (40) Nagarajan, R.; Wang, C. C. Estimation of surfactant tail transfer free energies from polar solvents to micelle core. Langmuir 1995, 11, 4673-4677.
DOI: 10.1021/la100544w
10537
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Sarkar et al. Table 4. Free-Energy Contributions for CiPOnEOj Micellization in 20% Ethanol in Water
surfactant
ΔG(C) (kJ/mol)
ΔG(EO-PO) (kJ/mol)
ΔG(EO) (kJ/mol)
ΔG(PO) (kJ/mol)
ΔG(EO)/NEO (kJ/ mol of EO segment)
ΔG(PO)/NPO (kJ/ mol of EO segment)
(ΔG(EO)/ NEO)/(ΔG(PO)/NPO)
S1 S2 S3
-37.4 -37.4 -37.4
7.2 8.3 10.3
6.6 7.4 8.9
0.6 0.9 1.4
0.82 0.43 0.26
0.05 0.07 0.11
15.4 5.9 2.3
where C is the solute (in our case, a surfactant alkyl chain) and S is the solvent. ΔGtransfer for dodecane and hexadecane in polar solvents was found to have a linear relationship with ΛCS2.40 ΔGmic and ΔGcosolvent for the CiPOnEOj surfactants are plotted in Figure 6 against values of ΛCS2 calculated for the mixed solvents of interest here. ΔGmic decreases linearly (correlation coefficient 0.97) as ΛCS2 increases. ΔGcosolvent also decreases linearly with increasing ΛCS2. Interestingly, the ΔGcosolvent values for the three surfactants collapse onto a single line, which reinforces the previous observation that the cosolvent mainly affects the free energy because of the transfer of the surfactant tail (which is common in all three surfactants). Role of PPO in Self-Assembly. The total free energy of micellization can be expressed in terms of two major contributions; one favoring micellization [ΔG(hydrophobic), related to the solubility of the alkyl chain and hence to the cohesive energy density] and the other opposing micellization [ΔG(hydrophilic)]. Such a simplification allows us to ascribe the free energy of micellization to the chemical structure of the surfactant molecule with the aim of establishing the role of the middle PPO block in self-assembly. We accomplished this by extracting the PPO contribution to the micellization free energy of CiPOnEOj in aqueous ethanol (20%) solutions for which reliable data are available for the micellization of CiEOj surfactants.26 For the alkyl-propoxy-ethoxylate nonionic surfactants of interest here, the total free energy of micellization can be broken down into three contributions (eq 5): (i) free energy due to hydrophobic alkyl chains, (ii) free energy due to PEO, and (iii) free energy due to PPO. ΔGomic ¼ ΔGðCÞ þ ΔGðEOÞ þ ΔGðPOÞ
ð5Þ
ΔG(C) (kJ/mol) can be estimated from the solubility of hydrocarbons in the mixed solvent by the following empirical correlation for dodecane (eq 6):41 ΔGðCÞ ΔGtransfer ¼ ð- 0:0191ΛCS 2 ÞRT
ð6Þ
This correlation needs to be corrected (ΔGcorrection(C)) for the energy penalty due to a restriction in the free rotation of the alkyl chain that can be accounted for by using the following empirical correlation (eq 7):9 ΔGcorrection ðCÞ ¼ ð- 0:50 þ 0:24N C ÞRT
ð7Þ
Here, NC is the number of carbon atoms present in the alkyl chain. ΔGcorrection(C) renders ΔG(C) less negative by about 10%. The ΔG(C) value thus calculated for a 20% ethanol in water mixture is reported in Table 4. ΔG(EO) has been obtained from ΔGmico following the subtraction of ΔG(C). We used eq 2 to calculate ΔGmico from cmc values reported in the literature26 for a series of C12EOj (where j = 6, 11, 20, and 31) surfactants in a 20% aqueous ethanol solution. ΔG(EO) was extrapolated for NEO = 34 and interpolated for NEO = 8 and 17 (relevant to the compositions of the (41) Evans, D. F.; Wennerstr€om, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed.; Wiley-VCH: New York, 1999.
10538 DOI: 10.1021/la100544w
CiPOnEOj surfactants of interest here). The ΔG(EO) values are presented in Table 4. ΔG(PO) can now be calculated using eq 5; values are given in Table 4. ΔG(PO) is positive, on the order of 1.0 kJ/mol, for the CiPOnEOj surfactants in 20% ethanol-water mixed solvents at 25 °C, intimating that PPO disfavors micellization just like PEO. Having established the hydrophilic (solvophilic) role of PPO, we now compare its solvophilicity to that of PEO. To this end, ΔG(EO) and ΔG(PO) are normalized with respect to NEO and NPO, respectively. The ΔG(EO)/NEO values for S1, S2, and S3 CiPOnEOj surfactants in a 20% ethanol-water mixed solvent are 0.82, 0.43, and 0.27 kJ/mol of EO segment, respectively. The ΔG(PO)/NPO values for S1, S2, and S3 in 20% ethanol in water are 0.05, 0.07, and 0.12 kJ/mol of PO segment, respectively. An EO unit is thus found to be about 15, 6, and 2 times more solvophilic than a PO unit for S1, S2, and S3, respectively. In comparison, in aqueous solution we found that an EO unit was about 5 and 2 times more hydrophilic than a PO unit for S1 and S2, respectively; for S3 in water, ΔG(EO)/NEO and ΔG(PO)/NPO were comparable.22 The values of both ΔG(EO)/NEO and ΔG(PO)/NPO in ethanol-water are lower than those in water. The higher ratio of ΔG(EO)/NEO to ΔG(PO)/NPO in ethanol-water compared to that in pure water is attributed to the improved solvation of PEO in ethanol-water. Hammouda found that a water-ethanol mixed solvent is more effective at solvating PEO than either water or ethanol alone.42 Influence of Solvent Quality on the Pyrene Microenvironment. The changes in the pyrene fluorescence emission intensity I1/I3 ratio at various surfactant and cosolvent concentrations (Figure 1) reflect changes in the micropolarity of the environment sensed by pyrene.43,44 In this section, we analyze the micelle microenvironment in terms of the local solvent composition. I1/I3 data in various solvent mixtures (in the absence of surfactant) are used in this analysis. For glycerol-water mixtures, a rather linear decrease in the I1/I3 ratio is found with increasing cosolvent content. For ethanol-water and isopropanol-water mixed solvents, a sigmoidal decrease in the I1/I3 ratio is found, which can be attributed to the associating nature of the alcohols.4 The pyrene fluorescence emission intensity ratio (I1/I3) for a binary system (A and B) can be approximated as45 I1 ¼ ½xI 1A þ yI 1B =½xI 3A þ yI 3B ð8Þ I3 Here I1A, I1B, I3A, and I3B are the emission intensities of the vibronic bands of pyrene in solvents A and B and x and y are the mole fractions of A and B, respectively. The above equation can be further approximated by considering I3 to be independent of (42) Hammouda, B. Solvation characteristics of a model water-soluble polymer. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 3195-3199. (43) Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Temperature effects on structural-properties of Pluronic P104 and F108 PEO-PPO-PEO block-copolymer solutions. Langmuir 1995, 11, 1468-1476. (44) Ruiz, C. C.; Diaz-Lopez, L.; Aguiar, J. Micellization of sodium dodecyl sulfate in glycerol aqueous mixtures. J. Dispersion Sci. Technol. 2008, 29, 266-273. (45) Acree, W. E.; Wilkins, D. C.; Tucker, S. A.; Griffin, J. M.; Powell, J. R. Spectrochemical investigations of preferential solvation 0.2. Compatibility of thermodynamic models versus spectrofluorometric probe methods for tautomeric solutes dissolved in binary-mixtures. J. Phys. Chem. 1994, 98, 2537-2544.
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Sarkar et al.
Article Table 5. Influence of Cosolvent on the Pyrene Microenvironmenta
surfactant
solvent composition
(I1/I3)mic
(I1/I3)solvent
x (mol %)
S1 0% cosolvent 1.16 1.80 64.2 S1 20% glycerol 1.17 1.79 62.6 S1 40% glycerol 1.20 1.74 57.5 S1 20% ethanol 1.17 1.69 48.4 S1 40% ethanol 1.28 1.42 22.6 S1 20% isopropanol 1.19 1.61 51.9 S1 40% isopropanol 1.29 1.30 2.0 S2 0% cosolvent 1.16 1.80 64.5 S2 20% glycerol 1.17 1.79 62.6 S2 40% glycerol 1.17 1.74 62.6 S2 20% ethanol 1.16 1.69 59.6 S2 40% ethanol 1.26 1.42 25.8 S2 20% isopropanol 1.18 1.61 53.09 S2 40% isopropanol 1.29 1.30 2.00 S3 0% cosolvent 1.15 1.80 65.1 S3 20% glycerol 1.16 1.79 63.6 S3 40% glycerol 1.16 1.74 63.6 S3 20% ethanol 1.18 1.69 57.3 S3 40% ethanol 1.27 1.42 24.2 S3 20% isopropanol 1.27 1.61 49.4 S3 40% isopropanol 1.28 1.30 4.0 a x and y denote the compositions of the alkyl chain and mixed solvent, respectively.
y (mol %)
X (vol %)
Y (vol %)
35.8 37.4 42.6 41.6 77.4 48.2 98.0 35.5 37.4 37.4 40.5 74.2 46.9 98.0 34.9 36.4 36.4 42.7 75.8 50.6 96.0
95.5 94.3 91.7 92.2 71.2 91.4 14.2 95.5 94.3 94.3 93.7 74.7 91.8 14.2 95.6 94.6 94.6 93.1 73.0 90.6 25.3
4.5 5.7 8.3 7.8 28.3 8.6 85.8 4.5 5.7 5.7 6.3 25.4 8.2 85.8 4.4 5.4 5.4 6.9 27.0 9.4 74.7
the microenvironment:45
I1 I1 I1 x þy I3 I3 A I3 B
ð9Þ
The I1/I3 ratio above the cmc remains almost constant with varying surfactant concentration for a particular solvent (Figure 1). This indicates that the I1/I3 ratio for a micellar solution is not a function of surfactant concentration, and pyrene is preferentially located in the micelle core. Thus, for the micellar solutions, if we assume that the micelle core consists mostly of the surfactant alkyl chains, then eq 9 can be written as I1 I1 I1 ¼x þy ð10Þ I 3 mic I 3 alkyl chain I 3 solvent Here, x and y are the alkyl chain and solvent mole fractions, respectively (x þ y = 1). (I1/I3)alkyl chain is taken to be equal to the I1/I3 value (0.8) in dodecane. Data for (I1/I3)solvent (Table 5) are obtained from binary solvent mixtures (in the absence of surfactant). Compositions x and y, reflecting the environment where pyrene resides, can now be calculated from eq 10; the results are presented in Table 5. At 0% cosolvent, the pyrene local environment is composed of about 95.5 vol % alkyl chains and 4.5 vol % solvent (water) and is independent of the surfactant composition. A similar observation of the pyrene microenvironment was found for C16EOj micellar solutions where the I1/I3 values collapse onto a single straight line for varying degrees of ethoxylation.46 For aqueous solutions of CiEO8 micelles, it has been reported that the pyrene microenvironment becomes more hydrophobic with increasing alkyl chain length.47 The introduction of cosolvent affects the pyrene microenvironment. At 20% cosolvent, pyrene resides in a location that consists of about 5.5, 7.0, and 8.7 vol % solvent in glycerol-water, ethanol-water, and isopropanol-water solvent mixtures, respectively (Table 5). When the cosolvent content was increased to 40 vol %, the solvent sensed by pyrene increased to about 27 vol % (46) Zhang, Z. Q.; Xu, G. Y.; Wang, F.; Du, G. Q. Aggregation behaviors and interfacial properties of oxyethylated nonionic surfactants. J. Dispersion Sci. Technol. 2005, 26, 297-302. (47) Rusdi, M.; Moroi, Y.; Hlaing, T.; Matsuoka, K. Micelle formation and surface adsorption of octaethylene glycol monoalkyl ether (CnE8). Bull. Chem. Soc. Jpn. 2005, 78, 604-610.
Langmuir 2010, 26(13), 10532–10540
Figure 7. SAXS absolute intensity data for 15 wt % S2 (0) and S3 (O) in water (filled symbols) and ethanol-water mixed solvent (open symbols) at 25 °C. The solid lines represent data fits to the core-shell oblate ellipsoid form factor and hard sphere structure factor. The fit parameters are presented in Table 6.
for the ethanol-water mixture whereas no such significant change was observed for glycerol-water. This is consistent with the literature report that the addition of glycerol does not change the pyrene microenvironment in aqueous micellar solutions of anionic surfactant sodium dodecyl sulfate.44 For an isopropanolwater solvent mixture of 40 vol % isopropanol, the above analysis shows pyrene to reside in an environment consisting mostly of solvent, indicating the absence of well-defined micelles. Effect of Solvent on Micelle Structure. The absolute scattering intensity data for the S2 and S3 surfactants in water DOI: 10.1021/la100544w
10539
Article
Sarkar et al.
Table 6. Micelle Structure Parameters Obtained from Fitting the Core-Shell Oblate Ellipsoid Form Factor for 15 wt % S2 and S3 Surfactants in 100% Water and 80/20% Water-Ethanol surfactant S2 S3
ethanol concentration (vol %)
minor core radius (A˚)
major core radius (A˚)
minor shell radius (A˚)
major shell radius (A˚)
hard sphere radius (A˚)
association number
molecular weight of micelle (kD)
0 20 0 20
15.0 15.0 15.0 15.0
40.0 33.0 35.0 34.0
41.0 43.0 42.0 40.0
69.0 60.0 69.0 62.0
75.0 70.0 70.0 68.0
287 207 220 207
471.0 340.0 525.8 495.3
and 80/20 water-ethanol mixed solvent are presented in Figure 7. The surfactant concentration was kept constant at 15 wt %. As we move from water to the ethanol-water mixture, the main features of the scattering intensity remain the same, suggesting that the shape of the micelles does not change. A similar observation was made for the C12EO23 surfactant (Brij 35) where the micelle shape remained unaffected by the addition of ethanol.48 However, the scattering intensity is lower in 20/80 ethanol-water. The decrease in the surfactant scattering intensity in the mixed solvent as compared to that in the aqueous system can be attributed to a decrease in both the micellar volume fraction as well as the scattering contrast (ethanol has a lower scattering-length density (7.39 10-6 A˚-2) than does water (9.46 10-6 A˚-2)). The effect of the volume fraction is stronger than that of the scattering contrast. The introduction of ethanol increases the cmc, and the number of surfactant molecules forming micelles decreases. Among the form factor models that we have considered in order to extract micelle structure information from the scattering data, the core-shell oblate (disk-shaped) ellipsoid provided the most reasonable fit for both aqueous and ethanol-water mixtures (solid lines in Figure 7). An ellipsoid form factor was also found to be better than the spherical core-shell model in describing the C12EO23 (Brij 35) micelle structure in water and butanol-water.49 Structural parameters obtained from the fits for different systems are presented in Table 6. In these fits, we assumed the micelle minor core radius to be equal to the extended alkyl chain length (15 A˚). The major core radius is in the range of 35-40 A˚. Following the introduction of ethanol into the solution, a decrease in the major core and shell radii was observed for both surfactants whereas the minor shell radii remained more or less steady at about 40 A˚. This indicates a shortening of the long axis of the oblate by about 10-15%. Correspondingly, both the micelle association number (estimated as the ratio of the micelle core volume (Vcore) to the volume of an alkyl chain) and its molecular weight decreased because of the addition of ethanol (Table 6). The present findings regarding the effect of ethanol on the CiPOnEOj micelle size are consistent with literature reports of ethanol effects on C12EO23 micelles. A generalized inverse Fourier transform analysis (that does not depend on the preselection of a form factor model) of SAXS data from C12EO23 solutions revealed the formation of inhomogeneous nearly spherical micelles that were smaller in ethanol-water than in pure water.48 Small-angle neutron scattering data for C12EO23 micellar solu(48) Tomsic, M.; Bester-Rogac, M.; Jamnik, A.; Kunz, W.; Touraud, D.; Bergmann, A.; Glatter, O. Ternary systems of nonionic surfactant Brij 35, water and various simple alcohols: Structural investigations by small-angle X-ray scattering and dynamic light scattering. J. Colloid Interface Sci. 2006, 294, 194-211. (49) Preu, H.; Zradba, A.; Rast, S.; Kunz, W.; Hardy, E. H.; Zeidler, M. D. Small angle neutron scattering of D2O-Brij 35 and D2O-alcohol-Brij 35 solutions and their modelling using the Percus-Yevick integral equation. Phys. Chem. Chem. Phys. 1999, 1, 3321-3329. (50) Meziani, A.; Touraud, D.; Zradba, A.; Pulvin, S.; Pezron, I.; Clausse, M.; Kunz, W. Comparison of enzymatic activity and nanostructures in water ethanol Brij 35 and water 1-pentanol Brij 35 systems. J. Phys. Chem. B 1997, 101, 3620-3625.
10540 DOI: 10.1021/la100544w
tions also revealed a significant decrease in micelle size due to the addition of 20% ethanol.50
Conclusions Solvent effects on the micellization of alkyl-propoxy-ethoxylated nonionic surfactants are reported. The cosolvents considered have a chemical composition and polarity similar to those of PPO. The addition of ethanol or isopropanol to water disfavors micelle formation and results in a significant increase in the cmc. The influence of glycerol is less pronounced. The cmc/cmc0 ratios for each cosolvent type at different solvent compositions are little affected by the surfactant degree of ethoxylation. This suggests that the cmc is mainly influenced by the improvement in the solvation of the alkyl chains in the mixed solvents. CiPOnEOj surfactants are affected by the cosolvents considered here in a manner similar to that for CiEOj surfactants. The cmc under different solvent conditions has been correlated with solvent physicochemical properties that reflect cohesive forces. The introduction of a polar organic solvent decreases the cohesive forces due to polar and hydrogen bonding in the solvent mixture and, as a result, increases the cmc. The correlations thus obtained can be extended to estimate solvent effects on the cmc’s of similar surfactants. The cosolvent contribution (ΔGcosolvent) to the free energy of micellization (ΔGomic)is positive (i.e., the cosolvent renders ΔGomic less negative) and does not depend on the surfactant degree of ethoxylation. A linear correlation is observed between the free energy of micellization and a function of the solubility parameters of the surfactant alkyl chain and the mixed solvent. The free-energy contribution to micellization due to the PPO part of the CiPOnEOj surfactant opposes micellization, but to a lesser degree in ethanol-water than in pure water. Each EO unit is about 15, 6, and 2 times more hydrophilic than PO for S1, S2, and S3, respectively, in 80/20 vol % ethanol-water. The micelle microenvironent, obtained from an analysis of the pyrene I1/I3 ratio, is affected by the cosolvent type and not by the surfactant composition. With an increase in cosolvent concentration in the solvent mixture from 20 to 40 vol %, the pyrene microenvironment is enriched with ethanol from about 7 to 27 vol % whereas it remains unaffected by increasing glycerol content. The core-shell oblate ellipsoid form factor has been found to describe the structure of S2 and S3 micelles well in both water and ethanol-water mixtures. The long axis of the ellipsoid micelle, micelle association number, and molecular weight were found to decrease by 10-20% as 20% ethanol was introduced into the aqueous solution. To the best of our knowledge, this is a first report of the influence of solvent quality on the self-assembly of CiPOnEOj surfactants. Solvent quality has been proven to be a readily controlled variable in tuning the amphiphile organization and nanostructure. Acknowledgment. We thank Dr. S. Balijepalli and Dr. H. J. M. Gruenbauer of Dow Chemical Co. for providing the CiPOnEOj surfactants and the NSF (grant CBET-0421154) for funding the SAXS facility used in this work.
Langmuir 2010, 26(13), 10532–10540
