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Spectroscopic Characterization of Solubilized Water in Reversed Micelles and Microemulsions: Sodium Bis(2-ethylhexyl) Sulfosuccinate and Sodium Bis(2-ethylhexyl) Phosphate in n-Heptane Naifu Zhou,† Quan Li,† Jinguang Wu,*,† Jing Chen,‡ Shifu Weng,† and Guangxian Xu† Department of Chemistry, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing, 100871, China, and Institute of Precious Metals, Chinese Academy of Sciences, Kunming, 650221, China Received September 12, 2000. In Final Form: May 8, 2001 The states and structure of the solubilized water in reversed micelles and microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane have been characterized by FT-IR and NMR spectroscopic parameters. According to the four-component hydration model, the free, anion-bound, bulklike, and cation-bound water are present in reversed micelles and both of the water-in-oil (W/O) microemulsions formed by AOT and the bicontinuous microemulsions formed by NaDEHP in n-heptane. The observed chemical shifts (δ) of the water protons from NMR spectra were expressed as the weighted average of the in-core anion-bound, bulklike, and cation-bound water. Chemical shifts δ for individual components were evaluated from molar fractions, which were obtained by deconvolution of the O-H stretching vibrational absorption bands, and the observed δ. Results show that in W/O microemulsion of AOT in n-heptane and bicontinuous microemulsion of NaDEHP in n-heptane, the chemical shifts for individual components exhibit constant values, indicating stable microstructure for the given species, which can be considered as the criterion of either W/O or bicontinuous microemulsions. Results also show that in reversed micelles of both AOT and NaDEHP, the O-H bond strength and thereby the microstructure of different hydration species vary with water content, which can be explained by the interaction between electrical double layers. In transition of reversed micelles to microemulsions, the microstructures of water molecules transform from the variable state to a stable state.
Introduction Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) is known as a typical reversed micelle and water-in-oil (W/O) microemulsion-forming surfactant. The aggregation behavior has been widely studied. The spherical reversed micelles of AOT in apolar solvent can solubilize considerable amounts of water in the micellar core, and then the water-swollen reversed micelles can form larger W/O microemulsion droplets.1-6 Bis(2-ethylhexyl) phosphoric acid (HDEHP) is commonly used as an organophosphorus extractant in hydrometallurgical separation of metals which is associated with the aggregation behavior and water solubilization in the extraction process. On the basis of the results of FT-IR, NMR, conductivity, etc. as reported previously,7-9 it has been concluded that the saponification of the acid * Corresponding author. † State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University. ‡ Institute of Precious Metals, Chinese Academy of Sciences. (1) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (2) Martin, C. A.; Magid, L. J. J. Phys. Chem. 1981, 85, 3938. (3) Wong, M., Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730. (4) Haering, G.; Luisi, P. L.; Hauser, H. J. Phys. Chem. 1988, 92, 3574. (5) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480. (6) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. (7) Wu, C.-K.; Kao, H.-C.; Chen, T.; Li, S.-C.; King T.-C.; Hsu, K.-H. Proc. Int. Solvent Extr. Conf., Belgium 1980, Paper No. 80-23. (8) Wu, J.; Gao, H.; Shi, N.; Chen, T.; Xu, G. Proc. Int. Solvent Extr. Conf., Denver 1983, 335. (9) Zhou, N. F.; Wu, J.; Yu. Z.-J.; Neuman. R. D.; Wang, D.; Xu, G. Sci. China, Ser. B 1977, 40, 61.
extractants in a mixed solvent is a process of forming W/O microemulsions or reversed micelles, while the extraction of metal by the saponified extractants is a process of destroying the given aggregates. The aggregation behavior of NaDEHP is rather peculiar in that the cylindrical reversed micelles in n-heptane can transform to a bicontinuous microemulsion under appropriate conditions.10 On the other hand, in the NaDEHP/benzene/water system, NaDEHP can form crystallites at very low water content.11 The aggregation characteristics of AOT and NaDEHP depend not only on their molecular structural difference but also on the hydration states and the microstructure of the solubilized water molecules. Water molecules play a key role in this process. Hydration of the polar headgroup of amphiphilic molecules is a requisite in forming an oriented adsorption monolayer at the air/water or oil/water interface. In a study of the microscopic interfacial phenomena in solvent extraction,12 it has been proposed that the hydration affinity of the phosphate headgroup of NaDEHP can be considered as the driving force of solubilization of water into the core of the NaDEHP aggregates. Although the microstructures of water clusters have been investigated extensively,13-15 the multiple states of water with multiple structures in various circumstances are still not clear. The transition from reversed micelles (10) Yu, Z.-J.; Neuman, R. D. Langmuir 1995, 11, 1081. (11) Feng, K.-I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17207, 17212. (12) Wu, J.; Shi, N.; Zhou, W.; Zhou, N. F.; Gao, H.; Xu, G. Prog. Nat. Sci. 1997, 7, 257. Wu, J.; Zhou, N. F.; Shi, N.; Zhou, W.; Gao, H.; Xu, G. Prog. Nat. Sci. 1997, 7, 385. (13) Cruzan, J. D.; Braly, L. B.; Liu, K.; Brown, M. G.; Loesser, J. G.; Saykally, R. J. Science 1996, 271, 59. (14) Liu, K.; Cruzan, J. D.; Sakally, R. J. Science 1996, 271, 929. (15) Pribble, R. N.; Zwier, T. S. Science 1994, 265, 75.
10.1021/la001313a CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
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to W/O microemulsions has been an unclear and indefinite question. Nevertheless, the aggregate system of NaDEHP provides a reference system of study for its clear-cut transition from reversed micelle to bicontinuous microemulsion under appropriate conditions. In a previous paper, a four-component model of the solubilized water was proposed and the important role of the cation-bound water was discussed.16-17 The aim of the present study is to reveal the microstructural characteristics of the solubilized water in reversed micelles together with W/O microemulsions of AOT and bicontinuous microemulsion of NaDEHP in n-heptane and to define the above categories by combining the results obtained by FT-IR and 1H NMR spectroscopic measurements. Experimental Section Materials. AOT and NaDEHP were purified by the methods described in a previous paper.16 Samples were prepared by adding different amounts of the double-distilled water to 1.0 M AOT and NaDEHP solution in n-heptane, respectively, and a series of samples containing different amounts of water were obtained. Water content (W0) was expressed as the molar ratio of water to surfactant. According to the phase diagram of the tricomponent system NaDEHP-n-heptane-water,10 when W0 is below 4, the samples so prepared are in the reversed micelle region; when W0 is greater than 4, the samples are in the bicontinuous microemulsion region. The samples of AOT and NaDEHP in n-heptane were subjected to the 1H NMR and FT-IR spectroscopic measurements. Spectroscopy. Infrared spectra were recorded in the wavenumber range of 900-4000 cm-1 with a Nicolet Magna IR-750 spectrometer using BaF2 windows. To study the microstructure of the solubilized water, the O-H stretching vibrational absorption spectra in the region of 3050-3750 cm-1 were fitted into four subpeaks according to the indication of the second derivative spectra with the help of the Galactic PeakSolve program. 1H NMR spectra were recorded with the same instruments under the same conditions respectively as reported previously.16,17
Results and Discussion 1. Characteristic FT-IR Spectra of O-H Stretching Vibrational Absorption Bands of Solubilized Water. Representative results of the FT-IR spectra of the O-H stretching vibrational absorption bands of solubilized water and the second derivative spectra for water/AOT/ n-heptane and water/NaDEHP/n-heptane are shown in parts a and b of Figure 1, respectively, suggesting that the absorption band is overlapped by four subpeaks. In previous paper,17 the four subpeaks were assigned to free (component a), anion-bound (component b, for AOT, the hydration sites of headgroup of the surfactant molecules are not only at SO3- but also at CdO), bulklike (component c), and cation-bound (component d) water. The results of deconvolution show the wavenumbers of the four components a, b, c, and d are 3600, 3520, 3400, and 3265 cm-1 (AOT system) and 3600, 3500, 3400, and 3265 cm-1 (NaDEHP system), respectively. The free water, with O-H stretching vibration at 3600 cm-1, is defined as the water species dispersing among long hydrocarbon chains of surfactant molecules.6 It exists as monomers (or dimers) and has no hydrogen bond interaction with its surroundings. However, a small amount water can also dissolve in nonpolar solvent as free water. In a reversed micelle system, component a may contain two types of free water and thus can be defined as “out-core” water, because they (16) Li, Q.; Weng, S.; Wu, J.; and Zhou, N. F. J. Phys. Chem. B 1998, 102, 3168. (17) Li, Q.; Li, T.; Wu, J.; Zhou, N. F. J. Colloid Interface Sci. 2000, 229, 298.
Figure 1. FT-IR spectra (solid line) and second derivative spectra (dotted line) of O-H stretching vibrational absorption band of water in (a) water/AOT/n-heptane at W0 ) 6 and (b) water/NaDEHP/n-heptane at W0 ) 6.
are isolated from the in-core water by the monomolecular films of surfactant molecules. Parts a and b of Figure 2 show the area fractions of each component as a function of water content for the two systems. It is interesting to observe that at lower water content, the cation-bound water is predominant in the NaDEHP system, while anion-bound water is predominant in the AOT system. This finding can be attributed to the different hydration affinities of ions; i.e., in the NaDEHP system, Na+ ions exhibit a stronger affinity than phosphate ions, while in the AOT system, sulfonate ions have a stronger affinity than Na+ ions. 2. 1H NMR Peaks of Water Protons in AOT and NaDEHP Systems as a Function of Water Content. The single peaks (as shown in Figure 3) of the 1H NMR spectra for the solubilized water in both systems indicate the rapid exchange between water protons at various states. Therefore, the observed chemical shift (δ) is considered as the weighted average of those at corresponding states.3,18 (18) Fauer, A.; Ahlnas, T.; Tistshenko, A. M.; Chachaty, C. J. Phys. Chem. 1985, 89, 3373.
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Figure 2. Molar fraction as a function of W0 for various components in water/AOT/n-heptane (4) and water/NaDEHP/n-heptane (0): component a (free water); (B) component b (anion-bound water); (C) component c (bulklike water); (D) component d (Na+-bound water).
Figure 4a shows the variation of the chemical shift to different directions (downfield and increase in δ for AOT system, upfield and decrease in δ for NaDEHP system) with increase in W0. It is noteworthy that for the NaDEHP system, a remarkable decrease in δ with W0 was observed in the reversed micelle region (W0 < 4), followed by a slow decrease with W0 in the bicontinuous microemulsion region (W0 > 4). According to the phase diagram of the ternary component system,10 water/NaDEHP/n-heptane, W0 ) 4 is at the transition point of reversed micelle-bicontinuous microemulsion. Thus, the remarkable change in δ at W0 ) 4 for the present system, can be considered as an indication of the transition. For the AOT system, the observed δ shows a continuous increase when W0 < 8 and then a linear increase with W0 beyond W0 ) 8. This result is quantitatively comparable with the earlier results reported by Wong et al.3 (plotted as 4 in Figure 4a). The break point at W0 ) 8 is in fact a microstructural transition
of water molecule as verified by the chemical shift data calculated in the next paragraph. The change in δ to opposite directions with W0 for the two systems is consistent with the opposite variations of the O-H stretching vibrational wavenumber with W0 (Figure 4b). Evidently, the higher O-H stretching frequency corresponds to a stronger bond strength and therefore a higher electron density around the protons leading the magnetic resonance of water protons to the upfield, i.e., lower values of the chemical shift. 3. Evaluation of Chemical Shift for Various Components of Water. The evaluation is based on the fact that the chemical shift of water protons in the core of aggregates is considered as the weighted average of those of the individual components.3,18 But in present systems, only three components of the solubilized water (except component a) are involved in rapid exchange and dealt with the weighted average, because the free water
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Figure 4. (a) Chemical shift versus W0 plots and (b) wavenumber of O-H stretching vibrational absorption peak versus W0 plots, for water/AOT/n-heptane (b) and water/NaDEHP/ n-heptane (0) systems. Dotted lines represent calculated chemical shifts; (4) data from ref 3. Table 1. Chemical Shifts for the Three In-Core Components of Solubilized Water chemical shift (ppm)
system
Figure 3. Representative 1H NMR peaks of water protons in (a) the water/AOT/n-heptane system and (b) the water/ NaDEHP/n-heptane system.
molecules are isolated by oriented hydrocarbon chains of surfactant molecules from the majority of the in-core hydrated molecules. As such, the chemical shifts δb, δc, and δd for the three in-core components can be evaluated by inserting the observed δ values and molar fractions, Xb, Xc, and Xd (data from Figure 2) into eq 1
δ ) Xbδb + Xcδc + Xdδd
(1)
where b represents the anion-bound water (component b), c represents the bulklike water (component c), and d represents the Na+-bound water (component d). By solving the three simultaneous equations, chemical shifts for individual components can be obtained (Table 1). The evaluation showed that eq 1 can be solved only for the systems at higher water content, i.e., for the AOT system, W0 > 8, and for NaDEHP system, W0 > 4. Results (Table 1) can be treated and discussed as follows. By inserting these evaluated data of δ values and the X values for different components into eq 1, the calculated weighted average δcalc can be obtained. The dotted lines in Figure 4a represent δcalc and are agreeable with the experimental data.
W/O microemulsion of AOT (W0 > 8) bicontinuous microemulsion of NaDEHP (W0 > 4)
δb (anion bound)
δc (bulklike)
δd (Na+ bound)
2.81
5.68
6.09
4.31
5.21
6.13
It can be seen that when W0 > 8 for the AOT system and W0 > 4 for the NaDEHP system, agreements are satisfactory indicating that water molecules at different states have stable microstructures. By considering the bicontinuous microemulsion of NaDEHP in n-heptane as a reference system of study, the constant chemical shifts for various water species can be used to specify the given microemulsion. In a similar manner, one can specify, therefore, the AOT system under this condition of constant chemical shifts for various water species is a W/O microemulsion. Evidently, the existence of stable water microstructures in the “water pool” can be regarded as the structural characteristics of the W/O microemulsion. However, at a lower water content, as shown in Figure 4a, remarkable deviations are observed for both systems, which will be discussed in the following paragraphs. It is significant to compare the results shown in Figure 4, as obtained by 1H NMR and FT-IR spectroscopic measurements for the two systems. (i) For component b, the δ (anion-bound water) shows a lower value, 2.81 ppm, for sulfonate-bound water (AOT system) than, 4.31 ppm, for phosphate-bound water (NaDEHP system), which are qualitatively comparable with the fact that the wave-
Solubilized Water in Reversed Micelles
number equals 3520 ( 10 cm-1 for sulfonate-bound water (AOT system) and is a lower value, 3500 ( 10 cm-1, for phosphate-bound water (NaDEHP system). These findings indicate that the greater hydration affinity of the sulfonate group shifts electron cloud to the water protons through the formation of a hydrogen bond. (ii) For component c, δ (bulklike) values are 5.68 ppm for AOT system and 5.21 ppm for NaDEHP system. Compared to 4.8 ppm for bulk water, it means that the O-H bond strength of the bulklike water molecules is probably weakened to a more or less extent in both systems. However, the identical wavenumbers (bulklike) for the two systems are not quite consistent with the difference between δc (AOT) and δc (NaDEHP). This inconsistency may be due to the deviation introduced in the deconvolution process. (iii) For Na+bound water, results of δd (6.09 and 6.13 ppm for AOT and NaDEHP systems, respectively) and wavenumbers (3250 ( 20 cm-1 for both systems) are agreeable with each other, indicating the identical hydration structure for the cationbound water in both microemulsions. 4. Structural Characteristics of Water in Transition of Bicontinuous Microemulsion-Reversed Micelles of NaDEHP. For the NaDEHP system, when W0 > 4, each component of the solubilized water exhibits a stable microstructure in the bicontinuous microemulsion region. This bicontinuous feature can be explained by the “local dynamic domain” model, as proposed by Yu and Neuman.10 According to this model, there exist W/O and oil-in-water (O/W) microdroplet domains, which are constantly in rapid disintegration, reassembly, or exchange via an intermediate state through surfactant molecular diffusion and thermal fluctuations. The intermediate state, in fact, is similar to the “layered-model” proposed by Shinoda,19 except the former is unstable but the later is in a stable state. In the reversed micelle region, aggregates of NaDEHP are cylindrical in shape. When W0 is greater than 4 (where the molar ratio of water to n-heptane is about 1), it seems likely that water molecules can open the cylindrical NaDEHP aggregates to form a continuous aqueous phase and thus make the microemulsion bicontinuous. From Figure 4a, it is seen that when W0 < 4, δcalc is no longer consistent with the observed data and shows a remarkable lower value than the observed δobs (expressed by 0). This finding for the reversed micelle of NaDEHP can be attributed to the fact that the three components of solubilized water molecules in the cylindrical micellar core with limited size are greatly compressed closer and interact each other. The clear-cut transition at W0 ) 4 between bicontinuous microemulsion and reversed micelles indicates a sharp microstructural transition of the aggregates as well as the solubilized water molecules. With increase in W0, water molecules are transformed from the closed cylindrical reversed micellar core to an opened aqueous layer. In bicontinuous microemulsion, the steady increase of the bulklike water with W0 plays an important role in keeping the gradual decrease of the δ toward 5.21 ppm, a value appreciably higher than that for the bulk water. (19) Shinoda, K. Prog. Colloid Polym. Sci. 1983, 68, 1.
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From the above discussion it is seen that a bicontinuous microemulsion in the case of a NaDEHP-n-heptane system can be characterized by the stable O-H bond strength and the stable microstructures of solubilized water in the continueous aqueous phase. 5. Structural Characteristics of Water in Transition of W/O Microemulsion-Reversed Micelles of AOT Systems. As for the AOT system, the deviation between δcalc and δobs can also be explained by the compression of the diffused double layer at lower water content. As compared to those for the W/O microemulsions, the Na+-bound water molecules are closer to the negatively charged interfacial monolayer of AOT molecules yielding a smaller δd (Na+-bound) and a greater δb (anion-bound), while the bulklike water can also be affected to some extent. This structural dependence of the solubilized water molecules on Wo can be regarded as the characteristics of the reversed micelles. However, these characteristics may not be unique to reversed micelles but may be available to any confined water existing in surfactant aggregates. With increase in water content (W0 ) 8), the reversed micellar solution is transformed to a W/O microemulsion, where the steady increase of the bulklike water plays an important role in keeping the gradual increase of δ. Similarly, the stable O-H bond strength and the stable microstructures of the solubilized water species in the core of the spherical aggregates of AOT in n-heptane can be regarded as the characteristics of W/O microemulsion. Conclusion The four hydration states of the solubilized water in reversed micelles and microemulsions, i.e., free water, anion-bound water, bulklike water, and cation-bound water, have been characterized by FT-IR and 1H NMR techniques. The free water is defined as out-core water, because it is isolated from the in-core water by the oriented monolayers formed by surfactant molecules. The other three species are defined as in-core water, which exists in the core of reversed micelles, and characteristic in the variation of the O-H bond strength and the microstructure of the water molecules for each component depending on the water content. In the W/O microemulsion of the AOT system and the bicontinuous microemulsion of the NaDEHP system, each in-core component of the solubilized water has stable O-H bond strength and microstructure which are independent of the water content. Therefore, the W/O microemulsion can be characterized as an aggregate with a stable O-H bond strength and stable microstructures of the solubilized water in the “water pool”. This characteristic holds also true for the bicontinuous microemulsions. Acknowledgment. Projects are supported by the State Key Project of Fundamental Research of China (G1998061907), the Collaboration Project of Yunnan Province-Peking University (B9808-K), and NSF of China (Grant No. 39730160 and 20023005). LA001313A
