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Thermodynamic Study on Surface Adsorption and Micelle Formation of Poly(ethylene glycol) Mono-n-tetradecyl Ethers Md. Nazrul Islam and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received March 18, 2003. In Final Form: June 10, 2003 We present the temperature dependence of adsorption and micelle formation of oxyethylenated nonionic surfactants of the general formula C14En with n ) 1, 2, 3, 4, and 8 by surface tension measurements. It is found that for the surfactants with a longer ethylene oxide (EO) chain, the critical micelle concentration (cmc) decreases, and the surface excess concentration (Γmax) increases with increasing temperature. But for C14E1, which bears only one EO unit, the cmc values initially increase and then decrease while the Γmax values gradually decrease with increasing temperature. Thus, the cmc-temperature data can be defined as a Λ-shaped curve, which is in striking contrast to the usual behavior of nonionic surfactants. This behavior is just the opposite trend of the usual behavior of ionic surfactants. The initial increases in the cmc and gradual decreases in Γmax values with increasing temperature are attributed to the thermal solubility of the molecules in the bulk and the thermal motion of the adsorbed molecules at the interface, respectively. For C14E2, both the cmc and Γmax values are found to be almost insensitive over the studied temperature range. It is concluded that for C14E2 the repulsive interactions between the headgroups is offset by the dehydration effect. The values of the changes in free energy, enthalpy, and entropy associated with both adsorption and micellization were calculated to understand the thermodynamic nature of the processes. The thermodynamic data suggest that the dehydration effect is more pronounced in both adsorption and micellization for the surfactants with longer EO chains compared to that of one with a shorter EO chain over the studied temperature range.
Introduction Surfactants have a characteristic molecular structure consisting of a hydrophilic group that has a strong affinity for water together with a hydrophobic group that does not. This unique duality of surfactants toward an aqueous environment leads to a wide variety of complex selfassociation phenomena, which simple molecules or water cannot exhibit. When a surfactant is dissolved in water, the hydrophobic groups occupying the cavities in the hydrogen-bonding network of water molecules distort the water structure. This distortion of the water structure increases the free energy and decreases the entropy of the system. To avoid contact with water molecules, the individual surfactant molecules self-organize to form a variety of structures1-3 in the bulk of the solution known as micelles. Depending on the nature of the surfactant, the micellar behavior of the surfactant is found to respond differently with temperature. According to the results of ionic surfactants, the cmc versus temperature data fit on a U-shaped curve with a minimum around room temperature.4-9 On the other hand, nonionic surfactants show a gradual trend of decreases in the cmc with increasing * Author to whom all correspondence should be addressed. Phone: 81-028-689-6170. Fax: +81-28-689-6179. E-mail: teiji@ cc.utsunomiya-u.ac.jp. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989. (2) Puvvada, S.; Blankschtein, D. J. Chem. Phys. 1990, 92, 3710. (3) Nagarajan, R. Langmuir 2002, 18, 31. (4) Swarbrick, J.; Daruwala, J. J. Phys. Chem. 1969, 73, 2627. (5) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320. (6) Mukerjee, P.; Korematsu, K.; Okawauchi, M.; Sugihara, G. J. Phys. Chem. 1985, 89, 5308. (7) Callaghan, A.; Doyle, R.; Alexander, E.; Palepu, R. Langmuir 1993, 9, 3422. (8) Sesta, B.; Mesa, C. L. Colloid Polym. Sci. 1989, 267, 748. (9) Mesa, C. L. J. Phys. Chem. 1990, 94, 323.
temperature.10-20 However, increases in the cmc values with increasing temperature21 and the observation of a minimum22 in the cmc-temperature curve have also been reported, in striking contrast to the usual behavior of the nonionic surfactants. Unlike micellization, surfactant molecules dissolved in the bulk of the aqueous solution can form monolayers being adsorbed preferentially at the air-water interface. In this process, the surface pressure increases as a result of the gradual increase in the surface concentration of the adsorbed molecules. At a definite temperature, the adsorbed molecules can undergo a pressure-induced phase transition in the adsorbed monolayers, showing a variety of condensed phase domains.23-30 For surfactant solutions, (10) Schick, M. J.; Atlas, S. M.; Eirich, F. R. J. Phys. Chem. 1962, 66, 1326. (11) Schick, M. J. J. Phys. Chem. 1963, 67, 1796. (12) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X.-Y.; J. Phys. Chem. 1982, 86, 541. (13) Ray, A.; Nemethy, G. J. Phys. Chem. 1971, 75, 809. (14) Megeru, K.; Takasawa, Y.; Kawahashi, N.; Tabata, Y.; Ueno, M. J. Colloid Interface Sci. 1981, 83, 50. (15) Corkill, J. M.; Goodman, J. F.; Harrold, S. P. Trans. Faraday Soc. 1964, 60, 202. (16) Sharma, B.; Rakshit, A. K. J. Colloid Interface Sci. 1989, 129, 139. (17) Motomura, K.; Iwanaga, S.; Uryu, S.; Matsukiyo, H.; Yamamaka, M.; Matuura, R. Colloids Surf. 1984, 9, 19. (18) Schick, M. J. J. Colloid Sci. 1962, 17, 801. (19) Wongwailikhit, K.; Ohta, A.; Seno, K.; Nomura, A.; Shinozuka, T.; Takiue, T.; Aratono, M. J. Phys. Chem. 2001, 105, 11462. (20) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J. J. Phys. Chem. 1994, 98, 6557. (21) Crook, E. H.; Trebbi, G. F.; Fordyce, D. B. J. Phys. Chem. 1964, 68, 3592. (22) Oda, H.; Nagadome, S.; Lee, S.; Ohseto, F.; Sasaki, Y.; Sugihara, G. J. Oil Chem. Soc. Jpn. 1997, 46, 559. (23) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (24) Melzer, V.; Vollhardt, D.; Brezesinski, D.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591.
10.1021/la030112e CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003
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although the surface pressure develops directly from the affinity of the dissolved molecules to adsorb spontaneously from the bulk to the surface, the surface density of the adsorbed molecules is governed by a delicate balance between the van der Waals interactions in the alkyl chains, which favor high-density states, and the repulsive interactions in the headgroups, which favor less-ordered and more-expanded states. In particular, greater dipolar repulsions and a higher degree of hydration make it more difficult to achieve high-density states because soluble surfactants generally have larger headgroups than the insoluble ones of the same chain length. Although this behavior has been known for a long time, there are many controversial issues, which are still unresolved. For example, there are controversies associated with changes in the cmc,21,22,30 equilibrium surface tension at gcmc (γcmc), and surface concentration (Γmax) values12,19-21,30 with temperature for ethoxylated nonionic surfactants. One of the key issues is, therefore, to address the reason and draw an unambiguous conclusion regarding the temperature dependency of surface adsorption and micelle formation of the surfactants, which can resolve these standing controversies. This is particularly important from the viewpoint of both fundamental and applied aspects in selecting a suitable surfactant, which could show desirable performance properties under a set of conditions. Ethoxylated surfactants are used in a wide variety of industrial applications and fundamental research. But there is little information regarding the thermodynamic nature of their bulk and interfacial properties. Here, it will be shown that the following thermally controlled factors must be taken into account to explain the temperature dependence of the surface adsorption and bulk micellization of surfactants containing ethylene oxide (EO) units in the headgroup. These are (1) the dehydration around the EO chain and its effects on the hydrophobic character and size of the headgroups, (2) the effect of the thermal solubility on the surface concentration and micelle formation, and (3) the effect of the thermal motion and chain flexibility on the packing of the molecules in the adsorbed film. All these factors are the function of temperature and EO chain length, and the relative magnitude of these effects governs adsorption and micellization of the surfactants in the course of temperature change. Keeping these facts in mind, we have endeavored to present a detailed account of the temperature-dependent micellization and adsorption behavior of the surfactants. The thermodynamic parameters of micellization and adsorption were calculated to understand the nature of the processes. Experimental Section Materials. Ethylene glycol mono-n-tetradecyl ether (C14E1), diethylene glycol mono-n-tetradecyl ether (C14E2), triethylene glycol mono-n-tetradecyl ether (C14E3), tetraethylene glycol mono-n-tetradecyl ether (C14E4), and octaethylene glycol monon-tetradecyl ether (C14E8) were supplied by Nikko Chemical Co., Tokyo, Japan, with a purity of >99%, and were used as received. The chemical structures of the surfactants are given in Chart 1. The solutions of the surfactants were prepared in ultrapure water of resistivity 18 MΩ‚cm (Elgastat UHQ-PS) for the present study. Method. The surface tensions of the solutions of different concentrations were measured by a surface tensiometer (Kru¨ss (25) Pollard, M. L.; Rennan, P.; Steiner, C.; Maldarelli, C. Langmuir 1998, 14, 7222. (26) Hossain, M. M.; Yoshida, M.; Kato, T. Langmuir 2000, 16, 3345. (27) Hossain, M. M.; Kato, T. Langmuir 2000, 16, 10175. (28) Islam, M. N.; Yanzhi, R.; Kato, T. Langmuir 2002, 18, 9422. (29) Islam, M. N.; Okano, T.; Kato, T. Langmuir 2002, 18, 10068. (30) Islam, M. N.; Kato, T. J. Phys. Chem. B 2003, 107, 965.
Islam and Kato
Figure 1. Surface tension versus logarithm of the concentrations of aqueous solutions of C14E1 at different temperatures. Chart 1. Chemical Structures of the Surfactants
K 10) equipped with a platinum plate. The solutions were transferred into a vessel that was thermostated by circulating water of the desired temperature. The surface-tension measurements were started with a dilute solution, and the subsequent concentrated solutions were prepared by adding a previously prepared dense stock solution into the vessel. Establishment of equilibrium was checked by taking a series of readings after 15-min intervals until no significant changes occurred. To eliminate the evaporation losses beyond 25 °C, the vessel was covered with a lid. The accuracy of the measurements was within (0.1 mN/m.
Results and Discussion Adsorption and Micellar Behavior of the Surfactants. Spontaneous adsorption of surfactant molecules from the bulk to the surface leads to decreases in the surface tension (γ) with increasing the bulk concentration (C) of the solution. At a definite concentration, the γ versus logarithm of bulk concentration (log C) plots show a distinct break point corresponding to the critical micelle concentration (cmc) of the surfactant. Figure 1 shows the representative γ-log C plots of C14E1 at different temperatures. It is seen that the break points initially move to higher concentrations and then to a lower concentration, while the surface activity gradually decreases showing increases in the surface tension at gcmc (γcmc) with increasing temperature. To examine the adsorption behavior, we have applied the Gibbs adsorption equation dγ ) -2.303RTΓmax d log C and calculated the Γmax values of the surfactants at different temperatures from the linear portion of the γ-log C plots before the break points (the plots for the other surfactants are not shown). Figure 2 shows the γ-log C plots of the surfactants at 25 °C. It clearly shows that the break points corresponding to the cmc move to higher concentrations as the
Poly(ethylene glycol) Mono-n-tetradecyl Ethers
Figure 2. Effect of headgroup size on the equilibrium surfacetension reduction capability of the surfactants at 25 °C: (1) C14E1, (2) C14E2, (3) C14E3, (4) C14E4, and (5) C14E8.
Figure 3. Dependence of the cmc values on temperature: (1) C14E1, (2) C14E2, (3) C14E3, (4) C14E4, and (5) C14E8.
number of EO units increases. This is because of increases in both the hydration and the higher degree of repulsive interactions with increasing the number EO units in the headgroups, which inhibits micellization. Figure 3 shows the variation of the cmc values of the surfactants with temperature. Although all the surfactants bear the same hydrophobic alkyl chain, their cmc values are found to respond differently with temperature, which is indicative of different modes of interactions of the headgroups with water. There are two opposing thermally controlled effects, which must be considered simultaneously to explain the temperature dependence of the cmc. These are (1) an increase in the dehydration of the headgroup that results in an increase in the hydrophobic character of the molecules and (2) the thermal solubility of the molecules, which tends to break the micelles. These two factors oppose each other depending on a number of factors,31 and the relative magnitude of the two effects determines whether the cmc will increase or decrease in the course of temperature change. The cmc values of C14E1 initially increase with increasing temperature, attain the maximum at 37 °C,30 and then decrease gradually with further increases in temperature. Thus, the cmc-temperature plot can be shown as a Λ-shaped curve. Because the headgroup of C14E1 bears only one EO unit, the dehydration effect cannot be so pronounced at lower temperatures. Crook et al.21 reported that the increase in the cmc of ethoxylated nonionic surfactants having shorter EO chains is the result of the predominance of the thermal solubility over the dehydration effect of the (31) Tanford, C. J. Phys. Chem. 1974, 78, 2469.
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Figure 4. Equilibrium surface tension at and above the cmc (γcmc) versus temperature plots for the surfactants: (1) C14E1, (2) C14E2, (3) C14E3, (4) C14E4, and (5) C14E8.
headgroup with increasing temperature. The initial increase in the cmc for C14E1 is, therefore, the predominance of the thermal solubility over the dehydration of the molecules. When the thermal solubility exerts its limiting influence, the cmc values attain the maximum. With further increases of temperature, the cmc values start to decrease. At higher temperatures, the dehydration effect probably dominates over the solubility effect and facilitates the molecules conveniently to form micelles in the bulk. On the other hand, the cmc values of C14E3, C14E4, and C14E8 decrease gradually with increasing temperature, which is the usual behavior of nonionic surfactants. Because the headgroups of these surfactants are much larger than that of C14E1 and hold a large number of water molecules, the dehydration effect should be an important factor in governing the temperature dependency of the cmc. As the temperature increases, dehydration around the headgroup leads to an increase in the hydrophobic character of the molecules. As a result, repulsive interactions between the headgroups decrease and micellization becomes favorable with increasing temperature. Let us take an interest in the temperature dependency of the cmc values of C14E2. As is shown in Figure 3, the cmc values of the surfactant are found to be almost insensitive to changes in the temperature. The cmc value of a surfactant is governed by the balanced forces between the van der Waals interactions in the hydrophobic alkyl chains that tend to stabilize the micelles and the opposing hydration of the headgroups that tends to break up the micelles. In the present case, the van der Waals interactions between the hydrophobic alkyl chains should be the same because the chains are the same for all the surfactants. Therefore, it is the headgroup whose relative extent of interaction in the aqueous medium will govern the various modes of changes in the cmc values with temperature. Because C14E2 bears only two EO units in its headgroup, neither the dehydration effect nor the solubility effect should be as pronounced. In other words, C14E2 being intermediary in nature marks a boundary line between the increasing and the decreasing trends of the cmc values of the surfactants. Therefore, the observed behavior of the cmc values of C14E2 is because of the compensation of the thermal solubility effect by the dehydration effect within the studied temperature range. Figure 4 shows the equilibrium surface tension (γcmc) values of the surfactants at different temperatures. The γcmc values of C14E1 increase rapidly with increasing temperature, suggesting the decrease in the Γmax values of the adsorbed molecules. Increased molecular motion
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surfactants. The free energy (∆Gm°), enthalpy (∆Hm°), and entropy (∆Sm°) of micellization were calculated using the following expressions:1,12
Figure 5. Dependence of the surface excess concentration of the surfactants on temperature: (1) C14E1, (2) C14E2, (3) C14E3, (4) C14E4, and (5) C14E8 on temperature.
and chain flexibility with increasing temperature result in a poorer packing of the molecules in the adsorbed monolayers; consequently, the area per molecule and γcmc increase. On the other hand, the γcmc values of C14En with n ) 3, 4, and 8 decrease with increasing temperature. This result clearly suggests the surface activity of the molecules increase with increasing temperature. An increase in the surface activity with increasing temperature seems contradictory because a temperature increase leads to increases in the kinetic energy of the adsorbed molecules. However, increases in the surface activity of the adsorbed molecules with increasing temperature have been reported previously,20 which is controversial to other observations.12 In the present study, we observed that the surface concentration can either increase, decrease, or remain almost steady with increasing temperature. To find out the reason, we need to take into account the following thermally controlled effects, which govern the surface concentration of the adsorbed molecules: first, dehydration of the hydrophilic headgroup and second, thermal motion of the adsorbed molecules at the airwater interface. The first effect tends to provide the molecules of closer packing by reducing the headgroup size, while the second effect tends to disorganize the monolayer by imparting kinetic energy to the molecules. Therefore, the relative magnitude of these two antagonistic effects will determine whether the Γmax values will increase or decrease with increasing temperature. As is shown in Figure 5, the Γmax values of C14E3, C14E4, and C14E8 increase appreciably with increasing temperature. This indicates that the dehydration effect shrinks the headgroup size and increases appreciably the hydrophobicity of the molecules, which overshadows the effect of thermal motion in the adsorbed molecules. Consequently, adsorbed molecules gain closer packing in the monolayers, and the surface concentration increases with increasing temperature. Because C14E1 bears only one EO unit in its headgroup, the dehydration effect cannot reduce appreciably its head size. Rather, increases in temperature perturb the adsorbed molecules, which dominates over the dehydration effect, decreasing the surface concentration of the molecules. For C14E2, the headgroup being comparatively large, the decrease in its size caused by the dehydration effect is probably offset by the combined effect of molecular motion and chain flexibility within the studied temperature range. As a result, like the cmc, both the Γmax and the γcmc values of C14E2 remain almost unchanged over the studied temperature range. Thermodynamics of Adsorption and Micelle Formation. Table 1 shows the thermodynamic parameters involved in the adsorption and micellization of the
∆Gm° ) RT ln cmc
(1)
∆Sm° ) -{∂(∆Gm°)/∂T}p
(2)
∆Gm° ) ∆Hm° - T∆Sm°
(3)
For all the surfactants, the free energy terms for micellization (∆Gm°) at different temperatures are found to be negative. The interaction between the hydrocarbon chains associated with the transfer of the alkyl chain from the aqueous medium to the micelle core along with the interactions responsible for the removal of water molecules in the form of icebergs around the alkyl chains contribute to a large negative free energy of micellization. The ∆Sm° values at 25 °C were calculated from the slopes of ∆Gm° versus temperature (T) plots. For C14E1, the plot is found to be curvilinear, for which a tangent was drawn through the required point and the slope of the tangent was taken to be ∂(∆Gm°)/∂T and was used to calculate ∆Sm°. However, for the other members the slopes were taken directly from ∆Gm° versus T plots because of their almost linear nature. Except for C14E1, the ∆Hm° values calculated by using eq 3 at 25 °C are found to be all positive and increase gradually with increasing the number of EO units in the headgroup. The destruction of the hydrogen bonds in the iceberg around the alkyl chain gives a positive enthalpy change. On the other hand, the destruction of the higher degree of orderly arrangement of water molecules in the iceberg gives a positive entropy change. Despite this fact, for C14E1 both ∆Hm° and ∆Sm° are negative at 25 °C. A negative enthalpy value can arise when a substantial number of water molecules surrounding the tiny headgroup become more important than the destruction of the icebergs around the hydrophobic alkyl chains, particularly at a lower temperature.21 On the other hand, a negative entropy change arises probably because of the ordering of randomly oriented monomeric units in a micellar structure, which outweighs the disordering effect of the destruction of icebergs around the alkyl chains.30 However, for other amphiphiles, both ∆Hm° and ∆Sm° are positive and increase with increasing the number of EO units in the headgroup. The increases in the ∆Hm° values suggest that a greater number of hydrogen bonds with the EO units and water molecules are broken during micellization as the number of EO units increases. The slight increase in the ∆Sm° values with increasing the EO chain length are probably because of the desolvation of EO units nearer to the alkyl chains during micellization.12,21 Table 1 also lists the standard free energies, ∆Gad°, enthalpies, ∆Had°, and entropies, ∆Sad° of adsorption of the surfactants. The ∆Gad° values were calculated from the following expression1,12
∆Gad° ) ∆Gm° - (πcmc/Γmax)
(4)
where πcmc and Γmax are the equilibrium surface pressure and maximum surface concentration of the adsorbed molecules, respectively, at and above the cmc. The ∆Had° and ∆Sad° values were calculated from the relationships corresponding to eqs 2 and 3. The free energy of adsorption is the energy required to transfer 1 mol of surfactant in solution to the surface at unit surface pressure. For all the surfactants, the ∆Gad° values are negative and become more negative with increasing temperature, suggesting that adsorption becomes more spontaneous with increas-
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Table 1. Thermodynamic Parameters of Adsorption and Micellization of the Surfactantsa surfactant
T (°C)
∆Gm° (kJ/mol)
∆Gad° (kJ/mol)
C14E1
15 25 35 45 15 25 35 45 15 25 35 45 15 25 35 45 15 25 35 45
-40.9 -40.4 -42.6 -43.6 -39.8 -41.0 -42.4 -44.0 -38.7 -40.6 -42.4 -44.3 -38.3 -40.1 -42.0 -43.6 -36.9 -38.8 -40.5 -42.4
-46.8 -48.5 -48.9 -50.4 -47.2 -48.2 -49.1 -50.5 -47.9 -48.7 -49.4 -50.5 -48.6 -49.3 -50.3 -50.7 -49.6 -50.2 -50.8 -51.5
C14E2
C14E3
C14E4
C14E8
a
∆Hm° (kJ/mol)
∆Had° (kJ/mol)
∆Sm° [J/(K‚mol)]
∆Sad° [J/(K‚mol)]
-35.1
-16.2
-17
112
2.0
-20.3
131
97
10.4
-23.4
171
88
11.3
-25.7
178
82
12.7
-32.1
190
63
The cmc values were taken in mole fractions for the calculation of the thermodynamic parameters.
ing temperature. This result is consistent with increases in the hydrophobicity of the molecules caused by the dehydration of the headgroup with increasing temperature. At a given temperature, the ∆Gad° values are found to be more negative than the corresponding ∆Gm° values, suggesting that when micelles are formed, work has to be done to transfer the surfactant molecules from the monomeric form at the surface to the micellar state through the solvent medium. The ∆Had° values are all negative, which is in line with previous observations7,12 and become more negative with increasing the number of EO units in the headgroup. In all cases, the ∆Had° values are more negative than the corresponding ∆Hm° values except that for C14E1, suggesting that EO units in the monolayer remain much more hydrated compared to those in the micelles. This result implies stronger van der Waals interactions between the alkyl chains during micelle formation, which make the headgroup more cramped than in monolayer formation. On the other hand, the ∆Sad° values of the surfactants decrease with increasing the number of EO units. This suggests that the movement of the adsorbed molecules at the solution surface becomes more restricted as the size of the headgroup increases. From computer simulation, Collazo et al.32 suggested that for molecules with a headgroup larger than the diameter of the tail, the molecules are more ordered in the headgroup region than at the tail end. Because the headgroups of the surfactants bear EO chains, it is expected that the diameter of the headgroup should be larger than that of the hydrocarbon tail. Therefore, the higher degree of orderly arrangement of the molecules in the headgroup region also contributes to decreases in ∆Sad° values with increasing the EO chain length. According to the concept of entropy of solution,33 which assigns largely the change in entropy to solute rather than to solvent, the amplitude of rotational and translational motions of the molecules becomes restricted because of close contact with neighboring solvent molecules. With increasing the number of EO units, the headgroups being surrounded by a greater number of water molecules lose the freedom of motion. At (32) Collazo, N.; Shin, S.; Rice, S. A. J. Chem. Phys. 1992, 96, 4735. (33) Wertz, D. H. J. Am. Chem. Soc. 1980, 102, 5316.
the same time, the motion of the water molecules bound to the EO chains, to some extent, becomes restricted. All these effects collectively decrease the ∆Sad° values of the surfactants with increasing the number of EO units in the headgroup. Conclusions In this work, we have shown that the cmc, Γmax, and γcmc can either increase, decrease, or remain almost constant depending on the number of EO units in the headgroup of the ethoxylated surfactants. The Γmax values increase while the cmc values decrease with increasing temperature for the surfactants with a longer EO chain in the headgroup. For C14E1, which bears only one EO unit, the Γmax values gradually decrease while the cmc values initially increase and then decrease with increasing temperature. In the case of C14E2, the cmc, γgcmc, and Γmax values remain almost constant over the studied temperature range, marking a boundary between the increases and the decreasing trends of the surface and bulk properties of the surfactants. This is probably because of the cancellation of the dehydration effect by the thermokinetic motion of the molecules at the interface within the studied temperature range. The ∆Gad° values are found to be more negative than the corresponding ∆Gm° values and decrease with increasing both the temperature and the EO chain length. The ∆Sm° values are found to increase with increasing the number of EO units, suggesting that a greater number of hydrogen bonds with oxygen atoms of the EO groups and water molecules are broken up during micellization. On the other hand, the ∆Sad° values of the surfactants decrease with increasing the number of EO units, indicating the restriction in the freedom of motion of the adsorbed surfactant molecules at the air-water interface. Acknowledgment. We thank Dr. K. Iimura and Professor N. Suzuki of this laboratory for helpful discussions. Part of this work was supported by the Satellite Venture Business Laboratory of Utsunomiya University. LA030112E
