Langmuir 2000, 16, 5579-5583
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Poly(ethylene oxide)13-Poly(propylene oxide)30-Poly(ethylene oxide)13 Electrolyte Interactions in Aqueous Solutions at Some Temperatures R. De Lisi and S. Milioto* Dipartimento di Chimica Fisica, Universita` di Palermo, Viale delle Scienze, Parco D’Orleans II, 90128 Palermo, Italy Received December 6, 1999. In Final Form: January 19, 2000 The apparent molar volumes (VΦ,C) of poly(ethylene oxide)13-poly(propylene oxide)30-poly(ethylene oxide)13 (Pluronic L64) in aqueous electrolyte solutions were determined as functions of L64 concentrations at some temperatures. The electrolytes studied are sodium perchlorate, sodium decyl sulfate (NaDeS), sodium decanoate (NaDec), and decyltrimethylammonium bromide (DeTAB). Sodium perchlorate plays the same role as temperature on VΦ,C data because of its destructuring solvent effect. At a given temperature, sodium decyl sulfate, in both the dispersed and micellized states, strongly affects not only the VΦ,C vs mC profile but also their values. In fact, VΦ,C decreases with mC to the critical micelle concentration (cmc), beyond which it increases, tending to level off. The experimental data are consistent with the idea that the addition of dispersed L64 to the micellar NaDeS solutions leads to the formation of NaDeS micelles with aggregation numbers lower than those in pure water; this likely results from the solubilization of copolymer hydrophobic segments in the micellar core and hydrophilic loops at the micellar surface. When the L64 cmc is reached, VΦ,C increases with mC because of the cooperative effect between L64 and NaDeS in forming highly charged mixed micelles richer in L64 content. The effect of the nature of the polar head of the dispersed surfactants also was studied at 301 K. The VΦ,C vs mC trends in the presence of NaDeS, NaDec, and DeTAB 0.015 mol kg-1 superimpose on that in water at high mC. In the premicellar region, the VΦ,C vs mC slope is positive for DeTAB and NaDec, being larger for NaDec and negative for NaDeS. The difference in the behavior of NaDec and NaDeS is due to the reduction of hydrophilic hydration of the polar head caused by the interaction with L64, which is more important for NaDec, as expected from the solute-solvent interaction contribution to the volume for the -COONa and -SO4Na groups.
Introduction Recently, attention has been paid to the aggregation process of poly(ethylene oxide)-block-poly(propylene oxide)block-poly(ethylene oxide) copolymers (PEO)a-(PPO)b(PEO)a, a and b being the repetitive units, in water. The change in temperature, to which the copolymer aggregation equilibrium is sensitive, is particularly investigated.1-3 At a given temperature, the nature of the solvent can be modulated by the addition of cosolvents. Thus, attention has been focused on the effect of inorganic4,5 and organic5,6 salts and urea4-6 on the cloud point and on the critical micellization temperature of some (PEO)a-(PPO)b-(PEO)a copolymers (commercially available as Pluronics). Methanol and ethanol increase the critical micelle concentration of (PEO)61(PPO)40(PEO)61, whereas butanol and hydrazine have an opposite effect.7 Moreover, perfluoro alcohols favor the (PEO)13(PPO)30(PEO)13 (L64) micellization.8 Millimoles of halothane and isofluorane enhance the dehydration of copolymers inducing the aggregation process.9 In the case of thermodynamic properties, some differential * Corresponding author. E-mail:
[email protected]. (1) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (2) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730. (3) Wen, X. G.; Verrall, R. E. J. Colloid Interface Sci. 1997, 196, 215. (4) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074. (5) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci., Pure Appl. Chem. 1993, A30, 1. (6) Cardoso da Silva, R.; Loh, W. J. Colloid Interface Sci. 1998, 202, 385. (7) Armstrong, J. K.; Chowdhry, B.; Mitchell, J.; Beezer, A. S.; Leharne, S. J. Phys. Chem. 1996, 100, 1738. (8) De Lisi, R.; Milioto, S. Langmuir 1999, 15, 6277. (9) Wen, X. G.; Verrall, R. E.; Liu, G. J. J. Phys. Chem. B 1999, 103, 2620.
scanning calorimetry data aimed at studying the influence of various solutes (inorganic salts,4 nonionic polar compounds,7 and organic cosolvents10) on the behavior of Pluronics are known. The effect of classical surfactants on the micellization of block copolymers is scarcely studied. Hecht and Hoffmann11 observed that sodium dodecyl sulfate (NaDS), hexadecyltrimethylammonium bromide (CTAB), and tetradecyldimethylamine oxide (TDAO) favor the suppression of (PEO)108(PPO)69(PEO)108 (F127) micelles. This finding was ascribed to the F127-surfactant complex formation, in the case of CTAB and NaDS, and to the interactions between dispersed copolymers and nonionic micelles, in the case of TDAO. Almgren et al.12 determined the 13C chemical shifts of carbon atoms in NaDS solution 0.07 mol dm-3 as functions of L64 concentration. It was inferred that the addition of L64 to NaDS micellar solution leads to the formation of NaDS-L64 mixed micelles, each containing one L64 molecule. To obtain new insights about the thermodynamics of copolymers in aqueous mixed solvents, we measured the density of water-electrolyte-L64 ternary systems as a function of the electrolyte and the copolymer concentrations. The inorganic salt sodium perchlorate was chosen because it is one of a very few inorganic salts that increase the cloud point of nonionic surfactants.13 Hydrophobic electrolytes selected were sodium decyl sulfate, sodium decanoate, and decyltrimethylammonium bromide. Be(10) Cheng, Y.; Jolicoeur, C. Macromolecules 1995, 28, 2665. (11) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86. (12) Almgren, M.; van Stam, J.; Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991, 195, 5677. (13) Schott, H. J. Colloid Interface Sci. 1997, 189, 117, and references therein.
10.1021/la991586+ CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000
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cause the latter electrolytes form micelles, the influence of these aggregates on L64 behavior was also analyzed. The effect of temperature was also investigated. Experimental Section Materials. Poly(ethylene oxide)13-poly(propylene oxide)30poly(ethylene oxide)13 triblock copolymer, (PEO)13(PPO)30(PEO)13, was obtained from Fluka (named Pluronic L64). The product was purified according to the literature.14 Its water content was determined by thermogravimetry (Mettler TA 3000). Sodium perchlorate (Fluka) and sodium decanoate (NaDec, Fluka) were used as received. Sodium decyl sulfate (NaDeS, Kodak) and decyltrimethylammonium bromide (DeTAB, Kodak) were crystallized from absolute ethanol and ethanol-ethyl acetate mixture (1:7, v/v), respectively. They were dried in a vacuum oven at 313 K for at least 4 d. For the surface tension measurements, NaDeS was crystallized four times. Urea (Fluka) was crystallized twice by water-ethanol mixture (60%, v/v) and dried in a vacuum oven at 313 K for 1 week. All solutions were prepared by mass using degassed conductivity water, and their concentrations were expressed as molalities. Equipment. Density. The solution densities were measured at 293, 298, and 301 K by using a vibrating tube flow densimeter (Model 03D, Sodev Inc.) sensitive to 3 ppm. The temperature was maintained constant within 0.001 K by using a closed-loop temperature controller (Model CT-L, Sodev Inc.). The calibration of the densimeter was made with water and urea aqueous solutions whose densities as functions of concentration and temperature are reported in the literature.15 The density values of water are 0.998206 g cm-3 at 293,16 0.997047 g cm-3 at 298,17 and 0.996237 g cm-3 at 301 K.16 Calculations. The apparent molar volumes (VΦ,C) of the copolymer in the given solvent were calculated by means of the following equation:
VΦ,C )
3 M 10 (d-do) d mC ddo
(1)
Here, mC and M are the molality and the molecular weight (2900 g mol-1) of the copolymer, d is the density of the solutions, and do is the corresponding property for the solvent. Copolymer solutions of concentrations larger than those reported in this paper were not studied because of experimental difficulties. Surface Tension. Surface tension was measured at 300.8 ( 0.1 K with a programmable tensiometer (KSV Sigma 70) using the Wilhemy plate. The tensiometer is an electrobalance with a lifting system controlled by a computer. The solutions were prepared in a glass beaker by successive addition of the copolymer concentrated solution to the corresponding surfactant-water solvent. This was accomplished using a computer-controlled Methrom dosimat titration unit. For each solution the surface tension was measured twice. The precision is 0.05 dyne cm-1. Conductivity. The specific conductivity measurements were performed at 300.8 ( 0.1 K (digital conductimeter Analytical Control 120) to evaluate the critical micelle concentration (cmc) of NaDeS and the degree of ionization of the micelles (β). The former corresponds to the intersection point of the straight lines (below and above the cmc) of the plot of specific conductivity vs surfactant concentration, whereas β is given by the ratio of the slopes of these straight lines.18
copolymer concentration (mC) at 298 K is typical of micellization behavior and gives the cmc at 0.065 mol kg-1. The standard partial molar volume (VoC) was calculated from data in the premicellar region as
VΦ,C ) VoC + BVmC
(2)
Here, BV is the solute-solute pair interaction parameter. The values of 2561.9 ( 0.3 cm3 mol-1 for VoC and 60 ( 6 cm3 mol-2 kg for BV were obtained. The VoC value is consistent with those8 at 293 and 301 K. The partial molar volume of the L64 monomer in the micellar phase (VM) was derived by applying the following equation19 to data in the postmicellar region:
VΦ,C ) VM +
EV 1 + FV(mC - cmc)
(3)
The values of the fitting parameters EV and FV are -147.7 ( 0.1 cm3 mol-1 and 4.910 ( 0.008 kg mol-1, respectively. We reported elsewhere8 that at 293 and 301 K, the VM values are equal to those calculated on the basis of the additivity rule. We assumed that the micelles are formed by a core of pure PPO and a hydrated hydrophilic shell of PEO units.3,8 This hypothesis holds also at 298 K where the VM value (2712.9 ( 0.1 cm3 mol-1) is equal to that calculated (2712.3 cm3 mol-1). By assuming the pseudophase transition model,20 we obtained the volume of micellization (∆Vm) as
∆Vm ) VM - Vm
(4)
Here, Vm is the partial molar volume of L64 in the aqueous phase calculated by means of eq 5 and the VoC and BV values reported above:
Vm ) VoC + 2BV cmc
(5)
Apparent Molar Volumes of L64 in Water. The dependence of the apparent molar volume of L64 on
From the present ∆Vm value (143.2 cm3 mol-1) and that8 at 293 and 301 K, the expansibility of micellization was evaluated. As is observed for classical surfactants,21 it has a negative value (-1.74 ( 0.08 cm3 mol-1 K-1) because the L64 expansibility in the aqueous phase is larger than that in the micellar phase. Apparent Molar Volumes of L64 in Sodium Perchlorate Aqueous Solutions. At 293 K, the presence of NaClO4 0.015 and 0.05m does not affect, within experimental error, the VΦ,C values in the whole range of copolymer concentration studied. At mNaClO4 ) 0.1 mol kg-1, the VΦ,C vs mC trend is parallel shifted toward values larger by about 5 cm3 mol-1 than those in pure water (Figure 1). Similar results were obtained at 301 K but, at this temperature, the presence of NaClO4 0.05m also affects VΦ,C (Figure 2). Moreover, VΦ,C data in the presence of NaClO4 0.015 mol kg-1 at 298 K are equal to those in pure water. On the basis of the Friedman and Krishnan approach,22 the volume of transfer of L64 from water to the
(14) Michels, B.; Waton, G.; Zana, R. Langmuir 1997, 13, 3111. (15) Desrosiers, N.; Perron, G.; Mathieson, J. G.; Conway, B. E.; Desnoyers, J. E. J. Solution Chem. 1974, 3, 789. (16) Lide, D. R., Ed. Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995-96. (17) Kell, G. S. J. Chem. Eng. Data 1967, 12, 66. (18) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.
(19) De Lisi, R.; Marongiu, B.; Milioto, S.; Pittau, B.; Porceddu, S. J. Solution Chem. 1997, 26, 891. (20) Mukerjiee, P. Adv. Colloid Interface Sci. 1967, 1, 241. (21) De Lisi, R.; Milioto, S.; Verrall, R. E. J. Solution Chem. 1990, 19, 665. (22) Friedman, H. L.; Krishnan, C. V. J. Solution Chem. 1973, 2, 115.
Results and Discussion
Electrolyte Interactions in Aqueous Solutions
Figure 1. Apparent molar volumes as functions of concentration for L64 in water (-) and in aqueous solutions of NaClO4 0.015 (∆), 0.05 (2), and 0.10 (o) mol kg-1 at 293 K.
Figure 2. Apparent molar volumes as functions of concentration for L64 in water (-) and in aqueous solutions of NaClO4 0.015 (∆), 0.05 (2), and 0.10 (o) mol kg-1 at 301 K.
electrolyte aqueous solution, VΦ,C(W+E) - VΦ,C(W), can be expressed as
VΦ,C(W+E) - VΦ,C(W) ) 2mEVCE + 3m2E VCEE + 3mEmCVCCE +... (6) Here, VCE, VCCE, and VCEE are the copolymer-electrolyte pair and triplet interaction parameters. Eq 6 shows that for mC f 0 VoC (W+E) - VoC (W) is a second-order polynomial in mE while, for a given mE value, VΦ,C(W+E) - VΦ,C(W) vs mC is a straight line. Our experimental data agree with these correlations. It was suggested23 that for poly(ethylene oxides)water-salt systems around the polymer chains, there are zones with a salt concentration lower than that in the bulk, because the interactions between ions and polymer are weaker than those between ions and water. The positive VoC(W+E) - VoC(W) values and their dependence on mNaClO4 confirm this finding and agree with the idea that NaClO4 reduces the amount of ordered water around the copolymer chains because of its destructuring solvent effect. This latter effect was also invoked to explain the increase of the cloud point of nonionic surfactants by perchlorate ion13 and of L64 by amides5 due to the favored hydration of poly(ethylene oxide) units. Therefore, (23) Florin, E.; Kjellander, R.; Eriksson, J. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2889.
Langmuir, Vol. 16, No. 13, 2000 5581
Figure 3. Apparent molar volumes as functions of concentration for L64 in water (-) and in aqueous solutions of sodium decyl sulfate 0.015 (b), 0.10 (2), 0.30 (∆), and 0.37 (o) mol kg-1 at 301 K.
the electrolyte plays the same role as temperature. Similar results were obtained for hydrophobic electrolytes24-26 in the presence of urea, a compound that acts as a structurebreaker cosolvent. As far as VM is concerned, from experimental data it is expected that its value is larger than that in water as a consequence of the dehydration of the PEO units at the micellar surface, because the micellar core (PPO units) is water-free. At a given electrolyte concentration, increasing temperature causes the VΦ,C vs mC trends to shift toward larger values; therefore, the expansibility is positive. The standard partial molar expansibility of L64 increases in a nonlinear manner with mNaClO4 as predicted by eq 6 applied to the expansibility. Apparent Molar Volumes of L64 in Sodium Decyl Sulfate Aqueous Solutions. The presence of NaDeS affects the volumetric properties differently than NaClO4. As Figure 3 shows, at 301 K the VΦ,C vs mC profiles differ from those in water in the dilute region; VΦ,C decreases with mC up to ≈0.02 mol kg-1, beyond which it increases, tending to level off. Moreover, the VΦ,C vs mC trends at mNaDeS ) 0.30 and 0.37 mol kg-1 superimpose on each other. At 293 K, VΦ,C was determined in the presence of NaDeS 0.015 mol kg-1. The profile of VΦ,C vs mC is equal to the corresponding one at 301 K, but in this case, the minimum occurs at ≈0.08 mol kg-1 (Figure 4). The minima in the VΦ,C vs mC trends can be ascribed to the cmc of L64 and are expected to be lower than those in water (0.095 and 0.035 mol kg-1 at 293 and 301 K, respectively.8) Accordingly, from the plots of surface tension as a function of mC, the cmc value of ≈0.015 mol kg-1 for L64 in some water-NaDeS mixtures can be evaluated at 301 K (Figure 5). As for classical surfactants in the presence of hydrophobic additives,27 the depression of cmc can be interpreted in terms of copolymer-surfactant interactions in both the aqueous and the micellar phases. At a given temperature, by increasing NaDeS concentration, the VΦ,C vs mC curves are shifted toward values larger than those in water; the same effect is observed for a given NaDeS concentration by increasing temperature. (24) Perron, G.; Desrosiers, N.; Desnoyers, J. E. Can. J. Chem. 1976, 54, 2163. (25) Musbally, G. M.; Perron, G.; Desnoyers, J. E. J. Colloid Interface Sci. 1976, 54, 80. (26) Causi, S.; De Lisi, R.; Milioto, S. J. Phys. Chem. 1991, 95, 5664. (27) De Lisi, R.; Milioto, S. In Solubilization in Surfactant Aggregates; Christian, S. D., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 1995.
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Figure 4. Dependence of the apparent molar volume on concentration for L64 in water (-) and in aqueous solution of sodium decyl sulfate (o) and decyltrimethylammonium bromide (b) 0.015 mol kg-1 at 293 K.
Figure 5. Surface tension as a function of copolymer concentration in the presence of sodium decyl sulfate 0.015 (o), 0.19 (0) and 0.29 (∆) mol kg-1 at 301 K.
Let us consider the standard partial molar volume of L64 in the NaDeS solutions, VoC(W+NaDeS), which reflects the copolymer-solvent interactions. In these conditions, the copolymer is a thermodynamic probe that permits us to detect structural changes in the solvent and does not affect them. Thus, it can be assumed that L64 does not influence the NaDeS micellization. VoC(W+NaDeS) increases with mNaDeS in the dilute region and is constant in the concentrated one. According to eq 6, the dependence of VoC(W+NaDeS) on mNaDeS in the premicellar region can be correlated to the L64-NaDeS pair and triplet interaction parameters. Indeed, according to the previous findings for L64 in NaClO4, and because the NaDeS concentration is low, one can state that the VoC(W+NaDeS) vs mNaDeS slope is given by the pair interaction parameter. If the hydrophobic hydration spheres of both solutes are destroyed because of their overlap, the interaction parameter value is expected to be negative, which is not the case here. Negative values for this parameter were obtained for L64 and alcohols.8 On the contrary, a positive contribution to the volume is expected from the destruction of the hydrophilic shell of the two solutes. Therefore, the predominance of the latter contribution could justify the present results. Indeed, conformational effects of L64 should be also considered. From the comparison between VoC values in water and those calculated on the basis of the additivity rule and volume data of PEO and PPO units,3 it turns out that the
De Lisi and Milioto
Figure 6. Critical micelle concentration (b) and degree of ionization of micelles (o) for sodium decyl sulfate as functions of the copolymer concentration at 301 K.
copolymer folding process involves a negative volume. Thus, a positive contribution to VoC(W+NaDeS) due to conformational effects is expected as a consequence of L64-NaDeS interactions, because it is reported that NaDS causes the unfolding of F127.11 In conclusion, it is likely that the positive slope of VoC(W+NaDeS) vs mNaDeS reflects the contributions due to the loss of the hydrophobic (negative volume) and hydrophilic (positive volume) hydration and the L64 conformational changes (positive volume). The VoC(W+NaDeS) values in the postmicellar region are independent of mNaDeS. According to the positive standard partial molar volume of transfer of amphiphilic additives from water to the micellar phases,27 the L64 solubilization in the NaDeS micelles can be invoked to explain this finding. Almgren et al.12 evidenced NaDSL64 mixed micelles (formed by one L64 and 60 NaDS molecules) with the coiled PPO units solubilized in the NaDS micellar core and PEO loops at the micellar interface. Therefore, the L64 solubilization process in the NaDeS micelles reflects not only the interaction changes for the L64 transfer from the hydrophilic solvent (water) to the hydrophobic moiety (NaDeS micelles), but also the copolymer conformational changes. The sign of the latter contribution cannot be evaluated, because the degrees of L64 folding in water and in NaDeS micelles are different. Returning to Figure 3, the decrease of VΦ,C upon the addition of L64 to the NaDeS aqueous solution 0.015 mol kg-1 can be explained on the basis of eq 6 in terms of the L64-NaDeS triplet and quadruplet interaction parameters, because both L64 and NaDeS are in the dispersed form. Although the profiles of VΦ,C vs mC at high NaDeS concentrations are equal to that at 0.015 mol kg-1, these considerations are no more valid at higher NaDeS concentrations, because micelles are also present. To explain the decrease of VΦ,C by increasing mC, we consider that the presence of L64 in the dispersed form shifts the NaDeS cmc toward values slightly larger than that in pure water (Figure 6). Moreover, it was reported28 that the decrease in the aggregation number of micelles leads to the increase in the cmc and to the decrease in the volume. Thus, the decrease of VΦ,C with mC can be explained by invoking the formation of NaDeS micelles with aggregation numbers lower than those in pure water. It is likely that the copolymer hydrophobic segments interact with the micellar core while the hydrophilic loops screen the (28) Inglese, A.; De Lisi, R.; Milioto, S. J. Phys. Chem. 1993, 100, 2260.
Electrolyte Interactions in Aqueous Solutions
charged polar heads. This hypothesis is supported by the present micelle’s degree of ionization values that strongly increase with mC, tending to 1 at high mC values (Figure 6). Our hypothesis agrees with evidence that the solubilization of polymers into micelles decreases both the aggregation number and the degree of counterion binding of micelles.29 When the L64 cmc is reached, VΦ,C increases with mC because of the cooperation between L64 and NaDeS in forming highly charged mixed micelles richer in L64 content. This process favors the NaDeS aggregation and, therefore, the cmc decreases with mC (Figure 6). Apparent Molar Volumes of L64 in Dispersed Surfactant Aqueous Solutions. It is reported30 that cationic surfactants, because of their bulky polar head and unfavorable polar charge, do not show affinity toward nonionic polymers. However, strong interactions between hydrophobic polymers and cationic surfactants have recently been demonstrated.31 Thus, the subject is not yet well understood. As far as the thermodynamic aspect is concerned, poly(ethylene oxides) slightly influence the apparent molar volumes and heat capacities of CTAB.31,32 On the contrary, poly (propylene glycols) strongly affect these properties mainly in the region near the cmc.31 Because the present copolymer can be considered as a 1:1 mixture of PEO and PPO segments, on the basis of these findings, we determined VΦ,C values in the water-DeTAB mixture 0.015 mol kg-1 at 301 and 293 K. At the latter temperature, as can be seen in Figure 4, VΦ,C data are not influenced by the presence of DeTAB while they are at 301 K. In fact, VΦ,C changes with mC, showing an inflection point at ≈0.03 mol kg-1, beyond which it increases monotonically, reaching the value in water at high concentration (Figure 7). Because temperature strongly affects the L64 dehydration process, its increase reduces the L64 polarity; therefore, hydrophobic interactions between L64 and DeTAB occur at 301 K. Hecht and Hoffmann11 reported that CTAB affects the enthalpy of micellization of F127 1 wt % solution at a concentration that is about 5 times lower than its cmc. A comparison with our data is not straightforward, because the literature data were obtained from differential scanning calorimetry and, therefore, they do not strictly refer to a fixed temperature. To investigate the effect of the polar head of the surfactant, VΦ,C in the presence of sodium decanoate (NaDec) 0.015 mol kg-1 was also determined at 301 K. The VΦ,C vs mC profile is shifted toward values larger than those in water and in DeTAB (Figure 7). The VΦ,C vs mC trends in the presence of NaDeS, NaDec, and DeTAB superimpose on that in water at high
Langmuir, Vol. 16, No. 13, 2000 5583
Figure 7. Dependence of the apparent molar volume on concentration for L64 in water (-) and in aqueous solutions of decyltrimethylammonium bromide (o), sodium decanoate (b), and sodium decyl sulfate (∆) 0.015 mol kg-1 at 301 K.
mC, indicating that VM is not affected by the presence of the surfactant. In the premicellar region, the VΦ,C vs mC slope is positive for DeTAB and NaDec and negative for NaDeS. According to eq 6, the different behaviors of NaDec and NaDeS can be due to the loss of hydrophilic hydration of the polar head, which is more important in the case of NaDec. This is expected on the basis of the solutesolvent interaction contribution to the volume for the -COONa (-3.1 cm3 mol-1) and -SO4Na (6.1 cm3 mol-1) groups. The latter were obtained from the standard partial molar volumes of sodium alkyl sulfates25 and sodium alkyl carboxylates33 in water and the geometric contributions (24.7 and 37.2 cm3 mol-1 for -COONa34 and SO4Na,35 respectively). Acknowledgment. The authors are grateful to the Ministry of University and of Scientific and Technological Research (Cofin MURST 97 CFSIB) for financial support. Supporting Information Available: Tables of conductivity, concentration, density, and apparent molar volumes of L64 in water and in the aqueous mixed solvents examined in this study. Table of the critical micelle concentration and the degree of dissociation of sodium decyl sulfate micelles. This material is available free of charge via the Internet at http://pubs.acs.org. LA991586+
(29) Rodenas, E.; Sierra, M. L. Langmuir 1996, 12, 1600, and references therein. (30) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 65. (31) Meunier, L.; Ballerat-Busserolles, K.; Roux-Desgranges, G.; Roux, A. H. J. Therm. Anal. Calor. 1998, 54, 271. (32) Perron, G.; Francoeur, J.; Desnoyers, J. E.; Kwak, J. C. Can. J. Chem. 1987, 65, 990.
(33) De Lisi, R.; Milioto, S.; Pellerito, A.; Inglese, A. Langmuir 1998, 14, 6045. (34) Caponetti, E.; Chillura Martino, D. F.; Floriano, A.; Triolo, R. Langmuir 1993, 9, 1193. (35) Caponetti, E.; Chillura Martino, D. F.; Floriano, A.; Triolo, R.; Wignall, G. D. Langmuir 1995, 11, 246.
