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Effect of Hydrophobic Chain Length of Surfactants on Enthalpy

It is well-known that there exists a minimum critical micelle concentration (cmc) in the cmc-temperature curve. It is found that the temperature of mi...
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J. Phys. Chem. B 1998, 102, 4350-4356

Effect of Hydrophobic Chain Length of Surfactants on Enthalpy-Entropy Compensation of Micellization Li-Jen Chen*,† Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 106, Republic of China

Shi-Yow Lin*,‡ and Chiung-Chang Huang Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei, Taiwan 106, Republic of China ReceiVed: December 5, 1997; In Final Form: March 26, 1998

It is well-known that there exists a minimum critical micelle concentration (cmc) in the cmc-temperature curve. It is found that the temperature of minimum cmc, Tmin, for both nonionic and ionic surfactants increases as the hydrophobicity of surfactants decreases. The temperature dependence of cmc is used to calculate the enthalpies and entropies of micelle formation for six different homologous series of surfactants. The enthalpyentropy compensation plot exhibits an excellent linearity. It is found that all the compensation lines for surfactants in a homologous series are parallel to one another and the intercept of these compensation lines is a linear function of the hydrophobic chain length of surfactants.

I. Introduction A variety of processes of certain solutes in aqueous solution, such as oxidation-reduction, hydrolysis, protein unfolding, etc., exhibit a linear relationship between the entropy change and the enthalpy change.1-3 This phenomenon is known as enthalpy-entropy compensation. The micellization process of surfactants has been shown also to exhibit such a compensation phenomenon.4-8 The hydrophobic effect due to the interaction between the hydrocarbon tail of the surfactant and water plays an important role in micelle formation.9 According to the working scheme of Lumry and Rajender10 for a compensation phenomenon, the micellization can be described as consisting of two-part processes: (a) the “desolvation” part, i.e., the dehydration of the hydrocarbon tail of surfactant molecules, and (b) the “chemical” part, i.e., aggregation of the hydrocarbon tails of surfactant molecules to form a micelle. In general, the compensation phenomenon between the enthalpy change ∆H°m and the entropy change ∆S°m in the various processes can be described in the form of

∆H°m ) ∆H* m + Tc∆S° m

(1)

The slope Tc, known as compensation temperature, can be interpreted as a characteristic of solute-solute and solutesolvent interactions, i.e., proposed as a measure of the “desolvation” part of the process of micellization. The intercept ∆H* m characterizes the solute-solute interaction, i.e., considered as an index of the “chemical” part of the process of micellization. By use of a phase separation model11 or a mass action model,12 thermodynamic functions, Gibbs free energy change ∆G°m, entropy change ∆S°m, and enthalpy change ∆H°m, for the process of micelle formation can be obtained through determin* To whom correspondence should be addressed. † E-mail address: [email protected]. ‡ E-mail address: [email protected].

ing the temperature dependence of the critical micelle concentration (cmc).13 In our previous work,14 we found that the compensation plots for three nonionic surfactants C12Ejs with j ) 4, 6, and 8 in aqueous solution all coincide into a straight line, which implies that both the “desolvation” and “chemical” part of the process of micellization are practically independent of the size of hydrophilic group of surfactants. This observation then increases our interest in further exploring the effect of hydrophobic chain length of surfactants on the enthalpy-entropy compensation. In this study, we re-examined the effect of a hydrophilic head group on the entropy-enthalpy compensation behavior and then further examined the hydrophobic chain length effect on the compensation plot resulting from the micellization process by using different alkyl chain lengths of nonionic surfactant CiEj. First, the cmc’s of C10E4, C10E8, and C14E8 were experimentally determined over the temperature range 10-80 °C using the Wilhelmy plate technique. The experimental procedure and results are described in the next section. It is well understood that the critical micelle concentration (cmc) varies as a function of temperature and there exists a minimum cmc in the cmc versus temperature curve for both ionic and nonionic surfactants in aqueous solutions.13 Only some of the experimental data of the temperature of minimum cmc, Tmin, have been reported and discussed in the literature.15,16 There is, to the best of our knowledge, no systematic study on the relation between Tmin and the molecular structure of the surfactants. On the basis of our experimental cmc data14 and the available data in the literature,8,16-26 the dependence of Tmin on surfactant hydrophilic-lipophilic balance is discussed in section III. In section IV, the cmc data treatment of determining ∆G°m, ∆H°m, and ∆S°m is described for both nonionic and ionic surfactants. It is found that the compensation lines of different alkyl chain length of CiEj are all consistent with the same compensation temperature, Tc, however, they have different intercepts, ∆H°m. In addition, it is intriguing to note that the intercept ∆H* m is a linear function

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of the hydrophobic chain length of the surfactant. We further examined the available cmc data of ionic surfactants in the literature.8,16,18-26 The intercept ∆H*m of compensation lines is also found as a function of hydrophobic chain length of surfactant for several homologous series of ionic surfactants in aqueous solutions. II. Experimental Section Nonionic surfactants C10E4, C10E8, and C14E8 were purchased from Nikko Chemicals Co. and used without further purification. Water was purified in a Barnstead NANOpure II system with the resistance around 18 MΩ/cm. The surface tension measurement was performed by using a surface tensiometer (CBVP-A3, Face) and a dynamic contact angle analyzer (DCA322, Cahn), along with a sandblasted platinum plate of dimensions 1.95 × 1.00 × 0.02 cm. Sample solution was kept in a double-walled Pyrex vessel thermostated at a prescribed temperature. To prevent the contamination of the solution from dust in the air during the operation, the vessel had a cover with a hole only allowing a thin wire attached to the platinum plate to go through, and the whole vessel was placed inside a closed sample chamber of the surface tensiometer or of the dynamic contact angle analyzer. The surface tension was measured as a function of surfactant concentration at various temperatures, and the results are given in Figure 1. The cmc was then taken as the concentration at the sharp break, as listed in Table 1. It is interesting to note the well-known behavior13 that at a constant temperature the cmc reduces to about 1/10 as the hydrophobic group of surfactant CiEj increases by two methylene units. III. Temperature of Minimum cmc In general, the cmc initially decreases and then increases as the system temperature increases. The initial decrease of the cmc is a direct consequence of the decrease in hydrophilicity of surfactant molecules, which is simply due to a smaller probability of hydrogen bond formation at higher temperatures. That is, the increase in temperature causes the decrease in hydration of the hydrophilic head group, which is in favor of the formation of micelles. Therefore, the onset of micellization occurs at lower concentrations as the temperature increases. In contrast, while the surfactant molecules dissolve in water, the hydrophobic tail group distorts the water structure. The increase in temperature also causes the increase in breakdown of the structured water molecules surrounding the hydrophobic alkyl group, which is in disfavor of the formation of micelles. Consequently, the onset of micellization tends to occur at higher concentrations as the temperature increases. Therefore, as the temperature further increases, the effect of hydrophobic groups begins to exert its influence and finally predominates as the cmc reaches a minimum value and finally increases with temperature. The existence of a minimum cmc in the cmctemperature curve is thus an outcome of these two opposing effects. It was found14 that the temperatures of minimum cmc, Tmin, for surfactants C12E4, C12E6, and C12E8 are 46, 49, and 52 °C, respectively. Obviously, the Tmin systematically increases as the oxyethylene chain length increases, while the values of Tmin for surfactants C10E8, C12E8, and C14E8 with increasing alkyl chain length are found to be 61, 52, and 46 °C, respectively. Thus, Tmin systematically decreases as the number of methylene groups increases. Consequently, Tmin is smaller for less hydrophilic surfactants. This tendency of Tmin as a function of hydrophilicity, as given in Table 2, is also consistently observed

Figure 1. Variation of surface tension as a function of surfactant concentration in aqueous solution: (a) C10E4, (b) C10E8, and (c) C14E8 at various temperatures: 10 °C (O), 15 °C (0), 20 °C (b), 25 °C (4), 30 °C (*), 35 °C (]), 40 °C (f), 45 °C (9), 50 oC (g), 60 °C (+), 70 °C (2), and 80 °C (×).

TABLE 1: Experimental Results of Critical Micelle Concentration (mol/cm3) for Surfactants C10E4, C10E8, C12E8, and C14E8 T, °C

C10E4

10 15 20 25 30 35 40 45 50 60 70 80

9.4 × 8.1 × 10-7 7.3 × 10-7 6.7 × 10-7

a

10-7

5.7 × 10-7 5.2 × 10-7 4.9 × 10-7 4.8 × 10-7 5.0 × 10-7 5.1 × 10-7

C10E8

C12E8a

C14E8 1.2 × 10-8

1.5 × 10-6

9.7 × 10-8

1.1 × 10-6 9.8 × 10-7

8.4 × 10-8 7.6 × 10-8

9.4 × 10-9 8.8 × 10-9 8.0 × 10-9

8.1 × 10-7 7.1 × 10-7 6.2 × 10-7 6.5 × 10-7 6.7 × 10-7 7.2 × 10-7

6.8 × 10-8

7.4 × 10-9

6.5 × 10-8 6.9 × 10-8 7.2 × 10-8 9.0 × 10-8

7.3 × 10-9 7.5 × 10-9 9.0 × 10-9 1.0 × 10-8

Data from Chen et al.14

for the homologous series of octylphenoxyethoxyethanol,17 abbreviated as OPEi, where subscript i stands for the number of oxyethylene groups, although the values of Tmin of OPE9 and OPE10 are slightly smaller than that of OPE8, in contrast to the observation in the system of CiEj: higher Tmin for more

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TABLE 2: Temperature of Minimum cmc, Tmin, for Various Surfactants compound nonionics C10E4 C10E8 C12E4 C12E6 C12E8 C14E8 OPE5 OPE6 OPE7 OPE8 OPE9 OPE10 cationics 1-dodecyl-4-methoxypridinium bromide 1-dodecyl-4-methoxypridinium chloride dodecylpyridinium iodide dodecylpyridinium bromide decyl-R-picolinium bromide dodecyl-R-picolinium bromide tetradecyl-R-picolinium bromide nonyltrimethylammonium bromide decyltrimethylammonium bromide dodecyltrimethylammonium bromide tetradecyltrimethylammonium bromide dodecylbenzyldimethylammonium bromide anionics sodium n-decyl sulfate sodium n-dodecyl sulfate sodium n-tetradecyl sulfate sodium 2-decyl sulfate sodium 2-tetradecyl sulfate sodium 4-tetradecyl sulfate sodium p-(3-nonyl) benzene sulfonate sodium p-(2-decyl) benzene sulfonate sodium p-(3-decyl) benzene sulfonate sodium p-(5-decyl) benzene sulfonate zwitterionics n-decylbetaine n-undecylbetaine

Tmin °C

ref

58 61 46 49 52 46 37 48 49 54 50 49

this work this work 14 14 14 this work 17 17 17 17 17 17

17 27 0a 15 22 20 15 30 24 21 10 18

18 18 19 20 21 21 21 22 23 23 35 23

29 26 21a 38 27a 32 24 0) and becomes exothermic (∆H°m < 0) at high temperatures. Note that ∆G°m < 0 in Figure 2 is mainly due to entropy effects, especially at low temperatures. This observation implies that the micellization is an entropy-driven

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Figure 4. Variation of the intercept ∆H*m as a function of the number of oxyethylene groups for surfactants C12Ej (4) and OPEj (O).

Figure 5. Enthalpy-entropy compensation plots for surfactants OPE1 (O), OPE2 (0), OPE3 (4), OPE4 (]), OPE5 (+), OPE6 (×), OPE7 (b), OPE8 (2), OPE9 (0), and OPE10 (f). Figure 2. Variation of ∆G°m, ∆H°m, and -T∆S°m as a function of temperature for surfactants (a) C10E4, (b) C10E8, and (c) C14E8 in aqueous solutions.

Figure 3. Enthalpy-entropy compensation plots for surfactants C12E4 (+, dotted line), C12E6 (0, dashed line), and C12E8 (O, solid line).

process. The results of the temperature dependence of ∆H°m and ∆S°m in Figure 2 are applied to construct the compensation plot. Consider three nonionic surfactants, C12E4, C12E6, and C12E8 in aqueous solutions, to examine the effect of hydrophilic head group on the compensation phenomenon. Figure 3 shows that the compensation plots for these three surfactants exhibit a linear

character closely obeying eq 1. The intercept ∆H*m is a very weak function of oxyethylene chain length. ∆H*m increases slightly as the number of oxyethylene groups increases, as shown in Figure 4. We further examine this compensation phenomenon for another nonionic surfactant, p-tert-octylphenoxyethoxyethanols (OPEjs), in aqueous solution with the number of oxyethylene groups ranging from 1 to 10. The cmc data are taken from Crook et al.17 All the compensation lines for OPEjs, shown in Figure 5, are closely packed and hard to distinguish from one another. The inset in Figure 5 enlarges the most packed region for clarity. All these compensation lines are almost parallel to one another for different oxyethylene chain lengths. The intercept ∆H* m increases slowly and levels off as the number of oxyethylene groups increases, as shown in Figure 4, in accord with the finding of Bedo et al.5 for nonylphenoxyethoxyethanols. It is a general phenomenon that the tendency of the increase in ∆H* m diminishes as the number of oxyethylene groups increases for both OPEj and C12Ej systems, as shown in Figure 4. Note that the intercept ∆H*m stands for the enthalpy effect under the condition ∆S°m ) 0. The increase in the intercept ∆H* m thus corresponds to a decrease in the stability of the structure of micelles. In other words, the effect of chemical part of the process in micellization diminishes as the oxyethylene chain length increases. Consider the effect of alkyl chain length on the compensation plot at a fixed hydrophilic group. The homologous series of surfactant CiE8s is examined with the alkyl chain length i varying from 10 to 15. The experimental data of the cmc of C11E8, C13E8, and C15E8, are adopted directly from literature,30

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Chen et al. mass action model.12 The latter is preferable for ionic surfactants since the effect of counterions can be explicitly considered. For the mass action model, the standard Gibbs free energy of micelle formation per mole of monomer is given by

∆G°m ) (2 - β)RT ln xcmc

Figure 6. Enthalpy-entropy compensation plots for surfactants: C10E8 (O), C11E8 (0), C12E8 (4), C13E8 (]), C14E8 (g), and C15E8 (+).

Figure 7. Variation of ∆H*m as a function of the number of carbon atoms i in the straight-chain hydrophobic group of surfactant CiE8s.

and the temperature range of this set of data is relatively narrow, 15-40 °C. The enthalpy-entropy compensations for micellization of CiE8 with various lengths of alkyl chain i in water are parallel to one another and shifted downward by increasing the alkyl chain length i, as shown in Figure 6. In other words, the intercept ∆H*m decreases with an increase in the alkyl chain length i. It is intriguing to note that the intercept ∆H*m is a linear function of the alkyl chain length i, as one can see in Figure 7. This fact implies that the desolvation part of the process of micellization is practically independent of the alkyl chain length of surfactants in a homologous series, since the compensation temperature is constant. The increase in ∆H* m corresponds to a decrease in the stability of the structure of the micelles. It is interesting to observe that the value of ∆H* m decreases by ∼2.9 kJ/mol, as shown in Figure 7, when the alkyl chain length increases by one methylene group. On the other hand, the value of ∆H* m decreases by ∼0.24 kJ/mol on average, as shown in Figure 4, when the hydrophilic head group decreases by one oxyethylene group. In other words, the effect of the chemical part of the process of micellization enhances more pronouncedly with an increase in the alkyl chain length than with a decrease in the oxyethylene chain length. An increase by one methylene unit reduces the intercept ∆H* m to about one-tenth the value due to a decrease by one oxyethylene unit. These findings are in good agreement with the previous theoretical prediction of Poland and Scheraga.31 These authors pointed out that the interaction of hydrocarbon portions of the surfactant in water, known as hydrophobic bonding, is essential in accounting for the stability of micelles. The enthalpy-entropy compensation phenomenon exists not only in nonionic surfactants but also in ionic surfactants. The temperature dependence of the cmc of ionic surfactants can also be applied to calculate the enthalpy and entropy of micelle formation according to the phase separation model11 and the

(7)

where β is the degree of dissociation of micelles. For a completely ionized micelle, β ) 1; for a neutral micelle, β ) 0. Experimentally, the degree of dissociation β can be determined from the ratio of slopes above and below the cmc in the conductance versus concentration plot. Similar to the case of nonionic surfactants, the cmc data are also correlated by the polynomial equation (4). On the other hand, the experimental data of the degree of dissociation of micelles, β, are correlated by a linear function of temperature. Thus, the Gibbs free energy of micelle formation is determined from eq 7. The enthalpy and the entropy of micelle formation can be obtained from the Gibbs-Helmholtz eq 3 and eq 6, respectively. It should be pointed out that the calculation of the Gibbs free energy of micelle formation for ionic surfactants requires both the experimental data of cmc and of the degree of dissociation of micelles, β. From the experimental data available in literature, only four different homologous series of ionic surfactants are found to have relatively complete data of cmc and of the degree of dissociation β over a wide temperature range. They are sodium alkyl sulfate,32 sodium p-(3-alkyl)benzenesulfonate,26 alkyl R-picolinium bromide,21 and alkyltrimethylammonium bromide.8,23,33,35 The corresponding compensation plots are given in Figure 8. In Figure 8, all these compensation lines for surfactants in a homologous series are parallel to one another. It is interesting to consistently observe that the intercept ∆H* m is a linear function of the hydrophobic chain length for surfactants in a homologous series, as shown in Figure 9. Note that all the homologous series of ionic surfactants demonstrate very good linear behavior of the intercept ∆H* m as a function of hydrophobic chain length. It should be pointed out that there exists an inconsistency between the different sources8,23,33,35 of experimental data for surfactant alkyltrimethylammonium bromide, especially the data for the degree of dissociation β. In Figure 8d only the experimental data of Adderson and Taylor23,35 are used for consistency. As one can see in Figure 9, all the curves of the intercept ∆H*m vs the number of carbon atoms in a straight chain hydrophobic group are closely packed together, except the one for the homologous series of sodium p-(3-alkyl)benzenesulfonate. As mentioned above, the intercept ∆H* m characterizes the solute-solute interaction and stands for an index of the effectiveness of chemical part of the process of micellization. Under the condition of a fixed number of carbon atoms in a straight chain hydrophobic group, the surfactants with different hydrophilic head groups exhibit a small variation in the intercept ∆H* m due to the relatively small effects of the hydrophilic group on the chemical part of the process of micellization. However, the relatively large variation in the intercept ∆H*m for the homologous series of sodium p-(3-alkyl)benzenesulfonate is simply due to the introduction of a hydrophobic branch. The effect of shifting the hydrophilic group toward the middle of the alkyl chain is to decrease the intercept ∆H*m, as one can see for the surfactant sodium p-(3-alkyl)benzenesulfonate in Figure 8. This can be interpreted to mean that the introduction of a hydrophobic branch leads to an increase in steric hinderance in the formation of micelles (or an increase in hydrophilic

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Figure 8. Enthalpy-entropy compensation plots for surfactants in four different homologous series: (a) sodium octyl sulfate (O), sodium decyl sulfate (0), and sodium dodecyl sulfate (4); (b) sodium p-(3-nonyl)benzenesulfonate (O), sodium p-(3-decyl)benzenesulfonate (0), and sodium p-(3-dodecyl)benzenesulfonate (4); (c) decyl-R-picolinium bromide (O), dodecyl-R-picolinium bromide (0), and tetradecyl-R-picolinium bromide (4); (d) decyltrimethylammonium bromide (O), dodecyltrimethylammonium bromide (0), and tetradecyltrimethylammonium bromide (4).

TABLE 3: Compensation Temperature Tc for Surfactants in Different Homologous Series

Figure 9. Variation of ∆H*m as a function of the number of carbon atoms in the straight hydrophobic chain for various surfactants: alkyltrimethylammonium bromide (4), alkyl-R-picolinium bromide (]), sodium p-(3-alkyl)benzenesulfonate (0), and sodium alkyl sulfate (O).

character) that manifests itself in a decrease in the value of the intercept ∆H*m. It is interestingly found that when the alkyl chain length of ionic surfactants increases by one methylene unit the value of ∆H*m decreases by ∼3.9 kJ/mol, which is larger than that for nonionic surfactants CiE8, ∼2.9 kJ/mol. This observation somehow implies that an increase in the alkyl chain length of ionic surfactants by one methylene unit enhances the stability of micelles more effectively than that of nonionic surfactants. Since the compensation temperature Tc characterizes the “chemical” part of the process, Tc serves as the basis of comparison for differing examples of compensation behavior. The Tc value lying in the range 270-300 K has been used as a diagnostic test for the participation of water in the protein reaction.10 It should be pointed out that exceptionally high Tcs are also reported in the literature for certain processes. For example, the Tc for interfacial behavior between organic liquids and water has been found as high as 585 K, which is about double what is normally associated with processes occurring in bulk aqueous systems.36 The thermodynamic properties of

surfactant

Tc, K

CiEj OPEj sodium alkyl sulfate alkyl-R-picolinium bromide alkyltrimethylammonium bromide sodium p-(3-alkyl)benzenesulfonate

322 ( 6 328 ( 8 304 ( 3 308 ( 1 308 ( 4 314 ( 8

interfacial water are likely to be much different from those of the structured water in the bulk. Consequently, the Tc is inexplicably high. The compensation temperatures for various homologous series of surfactants in this study are listed in Table 3. It is interesting to observe that both nonionic surfactants CiEj and OPEj consistently have the same compensation temperature around 325 K within experimental uncertainty. This fact implies that the desolvation part of the process of micellization is practically independent of surfactant type within the homologous series. Comparably, the other four ionic surfactants listed in Table 3 also consistently have the same compensation temperature, around 308 K. Note that the compensation temperatures for nonionic surfactants are slightly larger than those of ionic surfactants. It is believed that this variation in compensation temperature is due to the dramatic difference in the nature of the hydrophilic group between the ionic and nonionic surfactants. V. Conclusion In this study, we carefully examined the temperature of minimum cmc and the enthalpy-entropy compensation behavior in connection with the hydrophobicity of surfactants. The temperature of minimum cmc for both nonionic and ionic surfactants decreases as the hydrophobic character of a surfactant enhances. This observation suggests that the temperature of

4356 J. Phys. Chem. B, Vol. 102, No. 22, 1998 minimum cmc would be an index, just like hydrophiliclipophilic balance, for the hydrophobicity of a surfactant. The enthalpy-entropy compensation plots for over 30 different surfactants are examined and found to exhibit a linear character. All the compensation lines for surfactants in a homologous series are almost parallel to one another. The intercept ∆H*m of compensation line is a linear function of the straight-chain length of the hydrophobic group of a surfactant molecule, as shown in Figures 7 and 9. The value of ∆H*m decreases as the hydrophobicity of a surfactant increases. Acknowledgment. This work was supported by the National Science Council of Taiwan, Republic of China. References and Notes (1) Hammett, L. P. Physical Organic Chemistry: Reaction Rates, Equilibrium, and Mechanism, 2nd ed.; MaGraw-Hill: New York, 1970. (2) Lumry, R.; Gregory, R. B. In The Fluctuating Enzyme; Welch, G. R., Ed.; Wiley: New York, 1986; pp 1-190. (3) Lumry, R. In Methods in Enzymology; Ackers, G. K., Johnson, M. L., Eds.; Academic: New York, 1995; Vol. 259, pp 628-720. (4) Jolicoeur, C.; Philip, P. R. Can. J. Chem. 1974, 52, 1834. (5) Bedo, Zs.; Berecz, E.; Lakatos, I. Colloid Polym. Sci. 1992, 270, 799. (6) Singh, H. N.; Saleem, S. M.; Singh, R. P.; Birdi, K. S. J. Phys. Chem. 1980, 84, 2191. (7) Goto, A.; Takemoto, M.; Endo, F. Bull. Chem. Soc. Jpn. 1985, 58, 247. (8) Lee, D. J. Colloid Polym. Sci. 1995, 273, 539. (9) Tanford, C. The Hydrophobic Effect, 2nd ed.; John-Wiley: New York, 1980. (10) Lumry, R.; Rajender, S. Biopolymers 1970, 9, 1125. (11) Matijevic, E.; Pethica, B. A. Trans. Faraday Soc. 1958, 54, 587. (12) Philips, J. N. Trans. Faraday Soc. 1955, 51, 561.

Chen et al. (13) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; JohnWiley & Sons: New York, 1989. (14) Chen, L.-J.; Lin, S.-Y.; Huang, C.-C.; Chen, E.-M. Colloid Surf., A, in press. (15) Kresheck, G. C. In Water: A ComprehensiVe Treatise; Franks, F. Ed.; Plenum Press: New York, 1975; Vol. 4, Chapter 2. (16) La Mesa, C. J. Phys. Chem. 1990, 94, 323. (17) Crook, E. H.; Trebbi G. F.; Fordyce, D. B. J. Phys. Chem. 1964, 68, 3592. (18) Stead, J. A.; Taylor, H. J. Colloid Interface Sci. 1969, 30, 482. (19) Mukerjee, P; Ray, A. J. Phys. Chem. 1966, 70, 2150. (20) Adderson, J. E.; Taylor, H. J. Colloid Sci. 1964, 19, 495. (21) Adderson, J. E.; Taylor, H. J. Pharm. Pharmacol. 1970, 22, 523. (22) Musbally, G. M.; Perron, G.; Desnoyers, J. E. J. Colloid Interface Sci. 1976, 50, 80. (23) Adderson, J. E.; Taylor, H. J. Pharm. Pharmacol. 1971, 23, 311. (24) Flockhart, B. D. J. Colloid Sci. 1961, 16, 484. (25) van Os, N. M.; Daane, G. J.; Bolsman, T. A. B. M. J. Colloid Interface Sci. 1987, 115, 402. (26) van Os, N. M.; Daane, G. J.; Bolsman, T. A. B. M. J. Colloid Interface Sci. 1988, 123, 267. (27) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937. (28) Chen, L.-J.; Hsu, M.-C.; Lin, S.-T.; Yang, S.-Y. J. Phys. Chem. 1995, 99, 4687. (29) Corkill, J. M.; Goodman, J. F.;. Harrold, S. P Trans. Faraday Soc. 1964, 60, 202. (30) Meguro, K.; Ueno, M.; Esumi, K. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 109. (31) Poland, D. C.; Scheraga, H. A. J. Phys. Chem. 1965, 69, 2431. (32) Goddard, E. D.; Benson, G. C. Can. J. Chem. 1957, 35, 986. (33) Evans, D. F.; Wightman, P. J. J. Colloid Interface Sci. 1982, 86, 515. (34) Swarbrick, J.; Daruwala, J. J. Phys. Chem. 1969, 73, 2627. (35) Adderson, J. E.; Taylor, H. In Proceedings of the IVth Congress of Surface ActiVity 1964; Gordon and Breach: New York, 1967; pp 613620. (36) Aveyard, R.; Saleem, S. M. J. Chem. Soc. Faraday Trans. I 1977, 73, 896.