Water

Langmuir 2000, 16, 3633-3635

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Apparent Molar Volume of Solubilized Water in AOT/ Isooctane/Water Reverse Micellar Aggregates Yurika Yoshimura,*,† Ikuo Abe,† Mitsuo Ueda,‡ Kanji Kajiwara,‡ Teruo Hori,§ and Zoltan A. Schelly| Department of Organic Chemistry, Osaka Municipal Technical Research Institute, Osaka, Japan, Department of Design Engineering & Management, Kyoto Institute of Technology, Kyoto, Japan, Department of Biochemistry and Chemical Engineering, Fukui University, and Center for Colloidal and Interfacial Dynamics, Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019 Received September 9, 1998. In Final Form: January 5, 2000 States of the solubilized water in reverse micellar solution of aerosol-OT (AOT, bis(2-ethylhexyl) sodium sulfosuccinate) in isooctane were investigated by determining the apparent molar volume of water, Vw, through density measurements at different temperatures and water contents wo ([H2O]/[AOT]) of the solution. The low Vw values in the range up to wo ) ∼ 2 are due to the strong interaction of H2O molecules with the ionic headgroup of AOT. Then, Vw exhibits a steep rise with water content in the range of wo ) 2-6 owing to the swelling of the spherical reverse micelle, which is caused by the weaker binding of the additional water molecules to the ester group of AOT. In the range of wo ) 6-12, Vw exhibits a complex behavior as a function of the water content with a distinct minimum in the vicinity of wo ) 10. This nonmonotonic course of Vw as a function of water content in this range cannot be rationalized in terms of monotonic changes of the characteristics of the solubilized water in the micelle, or in terms of simple swelling of the reverse micelle. The minimum in the molar volume of the solubilized water is interpreted in terms of a structural rearrangement of the micellar aggregate from spherical to another (probably cylindrical) geometry, thus providing a larger interface for strong interaction between H2O molecules and the compact ionic headgroups of the surfactant molecules. Beyond the minimum (10 < wo e 40), Vw increases and displays slight undulations with increasing wo, which raises the possibility of further structural changes in the aggregate.

Introduction Characteristics of reverse micellar aggregates in organic media have been a long-term focus of interest in the utilization of these systems in a variety of applications such as reactivity control, tertiary oil recovery, solar energy conversion, and enzyme-mediated synthesis.1-4 Fundamental for improved applications of reverse micellar systems is the thorough understanding of the state of solubilized water, and substantial efforts have been focused on its study.5-7 In this paper, we report results of investigations of the solubilized water in reverse micelles of aerosol-OT (bis(2-ethylhexyl) sodium sulfosuccinate) (AOT) in isooctane, obtained by determining the apparent molar volume of water through density measurements of the solution at different water contents and temperatures. Experimental Section AOT (Nacalai Tesque Co. Ltd.) was purified by the method of Luisi et al.8 and was dried and kept over P2O5. The purified and dried AOT was found to contain 350 ppm (w/w) water (determined * To whom correspondence should be addressed. † Osaka Municipal Technical Research Institute. ‡ Kyoto Institute of Technology. § Fukui University. | The University of Texas at Arlington. (1) Fendler, J. H. In Reverse Micelles, Luisi P. L., Straub B. E., Eds.; Plenum Press: New York, 1984; p 305. (2) Shield, J. W.; Ferguson, H. D.; Bommarius, A. S.; Hatton, T. A. Ind. Eng. Chem. Fundam. 1986, 25, 603. (3) Walde, P.; Mao, Q.; Bru, R.; Luisi, P. L.; Kuboi, R. Pure Appl. Chem. 1992, 64, 1771. (4) Barbaric, S.; Luisi, P. L. J. Am. Chem. Soc. 1981, 103, 4239. (5) Ueda, M.; Schelly, Z. A. Langmuir 1988, 4, 653. (6) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869. (7) Martin, C. A.; Magid, L. J. J. Phys. Chem. 1981, 85, 3938.

by Karl Fischer titration; Mitsubishi Chemical Co., model CA07) which corresponds to wo ≡ [water]/[AOT] ) 0.2. Isooctane as the solvent and the other chemicals used were of reagent grade. Reverse micellar solutions of various amounts of solubilized water were prepared by injecting the requisite amount of water with a microsyringe into the dry AOT/isooctane solution, and the original water content of AOT was taken into account for the calculation of the resulting wo value. At a given temperature, the apparent molar volume of the solubilized water (Vw) in the reverse micellar system was obtained by measuring the density (ds) of the AOT/isooctane/water solution in the presence of various amounts of water with a density meter (DMA60/602, Anton Paar Co.), and calculated using the following equations:9

Vw ) [(1 - Vo)/Cw]Mw

(1)

Vo ) (ds - Cw)/dAOT-IO

(2)

where Vo is the volume fraction of AOT plus isooctane in the sample of density ds; Cw and Mw are the concentration (g/cm3) and molecular weight of water, respectively, and dAOT-IO is the density of AOT/isooctane solution without water added. The density measurements were repeated several times, and the length of error bars in the figures indicates the difference between highest and lowest values of molar volume calculated from the measured density data.

Results and Discussion Figure 1 shows the variation in density (ds) of AOT/ isooctane/water solutions with water content wo at temperatures 10, 15, 24, and 40 °C. The precision level of the (8) Luisi, P. L.; Meier, P.; Imre, V. E.; Pande, A. In Reverse Micelles, Luisi P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 323. (9) Gekko, K.; Noguchi, H. Macromolecules 1974, 7, 224.

10.1021/la981209h CCC: $19.00 © 2000 American Chemical Society Published on Web 03/11/2000

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Yoshimura et al.

Figure 3. Apparent molar volume of solubilized water Vw in AOT/isooctane/water reverse micellar solution as a function of water content wo at 15 °C. Concentration of AOT: 0.1 M. Dashed line: molar volume of bulk water.

Figure 1. Density (ds) of AOT/isooctane/water reverse micellar solution as a function of water content (wo) and temperature. Concentration of AOT: 0.1 M. Density of bulk water (g/cm3): 0.9997000 (10 °C), 0.9991010 (15 °C), 0.9982030 (24 °C), and 0.9924090 (40 °C).



density measurements is better than 5 × 10-5 g/cm3. As can be seen in Figure 1, the density of the solution increases with the wo value at each temperature. Detailed examination of the density curves reveals that they slightly deviate from linearity in the range of wo ) 0-16, and that the deviation is most obvious at the lowest temperature, 10 °C. The calculated apparent molar volume of water (Vw) as a function of wo (Figures 2-5), which reflects the solubilization state of H2O in the AOT reverse micelle, exhibits a steep rise with water content in the range of wo ) 2-6, followed by a distinct minimum at wo ) 10 at all temperatures tested. Above wo ) 12, Vw approaches a constant value. Note that the nonmonotonic course of Vw as a function of water content in the range of wo ) 6-12 cannot be rationalized in terms of monotonic changes of the characteristics of the solubilized water in the micelle, or in


terms of simple swelling of the reverse micelle. Alternative explanations that involve change of the state of solubilized water, and/or change of aggregation state of the surfactant with increasing amount of water present, must be invoked. This will be aided by the implicit (and necessary) as-

Solubilized Water in AOT/Isooctane/Water

sumption involved in the use of eqs 1 and 2 for the calculation of Vw, namely, that dAOT-IO is independent of the amount of water added. In fact, the nonmonotonic course of Vw thus obtained seems to reflect a change in dAOT-IO, as elaborated below. The shape of AOT reverse micelle in the presence of a small amount of solubilized water is known to be spherical.10 The low values of Vw in this region (wo ∼ 2) are due to the strong interaction of H2O molecules with the ionic headgroup of AOT.11 Hydration of the headgroup consumes all the water present (∼ 2H2O/1AOT), hence no water structure as such can establish. The water molecules are strongly bound through ion-dipole interaction to the Na+ and -SO3- ions of AOT (instead of to other water molecules through hydrogen bonding), resulting in an apparent molar volume less than that of bulk water. In the range of wo ) 2-6, the increase of Vw seems to be due to the weaker binding of the additional water molecules to the ester groups of AOT.12 The hydrogen bonds formed between water molecules as well as between H2O and the ester groups may be viewed as the advent of formation of a bulk water-like structure, which leads to the swelling of the spherical reverse micelle and to the increase of the molar volume of the solubilized water observed. With further addition of water (wo ) 6-12), the almost discontinuous variation in apparent molar volume of the solubilized water in the AOT reverse micelle may be due to structural changes of the aggregate, where the spherical micelle rearranges to another shape, and at least two types of aggregates coexist in equilibrium. The thermodynamic instability that leads to the shape transition may originate from the driving force that tends to expose the maximum number of free water molecules to the ionic headgroup of the surfactant, which is the least possible situation for spherical geometry. In this respect, more advantageous structures are ellipsoidal or cylindrical aggregates which provide a larger interfacial area for the energetic interaction between water and the AOT headgroup than is possible in the primary spherical reverse micelle. If the structural rearrangement initiates around wo ) 6, it would bring about the depression of the molar volume of solubilized water, with Vw ultimately reaching the observed minimum at wo ) 10. Because Vw is the sum of the molar volumes of solubilized water in both the primary and secondary micelles, the shift of their equilibrium toward the secondary structure ceases when the binding sites of AOT become saturated by water. This occurs around wo ∼10,12 which coincides with the location of the minimum we observe. The structural rearrangement suggested above is supported by several other experimental findings. For instance, with increasing water content, the profile of smallangle X-ray scattering10 of AOT reverse micellar solution drastically changes at wo ) 10 which, again, is in accord (10) Hirai, M.; Kawai-Hirai, R.; Yabuki, S.; Takizawa, T.; Hirai, T.; Kobayashi, K.; Amemiya, Y.; Oya, M. J. Phys. Chem. 1995, 99, 6652. (11) Christopher, D. J.; Yarwood, J.; Belton, P. S.; Hills, B. P. J. Colloid Interface Sci. 1992, 152, 465. (12) Derecskei, B.; Derecskei-Kovacs, A.; Schelly, Z. A. Langmuir 1999, 15, 1981.

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with the location of distinct minimum in the molar volume observed in the present work. Similarly, a clear minimum in the IR absorption bandwidth of hydrogen bond stretching was found at wo ) 6-13, which indicates that a change may occur in the state of solubilized water in the AOT reverse micelle in this region.11 The molar volume of water in AOT reverse micelle was previously investigated by D’Aprano et al.13 in another solvent (n-heptane) at 25 °C, without detecting a minimum in Vw around wo ) 10. If their and our systems are supposed to behave similarly, they might have missed the narrow minimum either because they increased wo in too large steps (9.4, 11.54), or because of their method of calculating Vw. Namely, to calculate the ratio of empirical constants (A and B) involved in their eq 3, they had to resort to the use of boundary conditions (wo f 0 and wo f ∞) at which, however, the assumed spherical reverse micelles should not exist. Beyond the transition region, with increasing amount of solubilized water (wo ) 10-12), Vw increases continuously owing to the increase of free water in the rearranged micelle and approaches the molar volume of bulk water (at wo > 12). In the range of wo ) 20-40, Vw exhibits slight undulations with increasing amount of water added (Figures 2-5), which suggests the possibility of further structural rearrangements of the reverse micelle. A related Raman spectroscopic analysis of solubilized water in AOT reverse micelle supports this possibility.14 Namely, whereas the ratio between free and bound water was found to generally increase with wo in the range of wo ) 4-20 (measured only at wo ) 4, 10, and 20), the ratio was found to be essentially constant for wo ) 20-40. The constancy of the ratio between free and bound water implies the existence of a mechanism that consumes added free water, i.e., the existence of one or more equilibria between different aggregation structures. From the considerations described above, it can be concluded that the mode of aggregation of AOT in the reverse micelle changes from spherical to another shape (most likely cylindrical) when the amount of solubilized water is increased in the range of wo ) 0-20, and that there may be further structural transition equilibria between different types of aggregates when even more water is added (wo ) 20-40). Although the shape and structure of the aggregates involved is not clear at present, elucidation of these structures and the state of the solubilized water would be important for understanding the microenvironment provided by such systems. Acknowledgment. This work was supported in part by the National Science Foundation and the Welch Foundation. LA981209H (13) D’Aprano, A.; Lizzio, A.; Turco Liveri, V. J. Phys. Chem. 1987, 91, 4749. (14) D’Aprano, A.; Lizzio, A.; Turco Liveri, V.; Aliotta, F.; Vasi, C.; Migliardo, P. J. Phys. Chem. 1988, 92, 4436.

Water

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