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Articles Physicochemical Studies on Reverse Micelles of Sodium Bis(2-ethylhexyl) Sulfosuccinate at Low Water Content K. Murali Manoj, R. Jayakumar,*,† and S. K. Rakshit*,‡ Biotechnology Research Centre, Department of Chemical Engineering, Indian Institute of Technology, Madras 600036, India, and Chemical Laboratory, Central Leather Research Institute, Madras 600036, India Received April 6, 1995. In Final Form: March 15, 1996X Studies on aggregation of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane at low water content were carried out. The critical micelle concentration (cmc) thermodynamic parameters were determined at various temperatures. The observations suggested that the micellization process was endothermic in nature and that it is mainly an entropic process. The cmc was also determined at various R values (R < 3) and it was found that cmc increases with increasing R. The aggregation number was determined at the R value of 1.6. From the pyrene and ANS binding studies, the microviscosity and micropolarity changes on the onset of micellization are well characterized. Spectral studies with ANS indicate a decrease in microviscosity on increase in R value. ANS fluorescence was monitored in the presence of additives like acrylamide, KI3, and tetrabutylammonium iodide. The iodide-based additives lead to quenching of fluorescence of ANS with concomitant red shift in the λem suggesting decreased microviscosity and micropolarity. Acrylamide was shown to increase the polarity by the ANS spectral features. These findings led to the conclusions that the positive ∆Hm° values arise mainly from the dismantling of hydrated ions in the quasi-lattice of AOT interior. With an increase in temperature, immobilized structured states of water get destructured, which ultimately leads to favoring the aggregation process.
Introduction Reverse(d) micellar systems (RMS), isotropic solutions of water in oil (microemulsion) stabilized by surfactants, are powerful models that have found applications in biological compartmentalization studies,1 enzymatic catalysis,2-9 and separation of biomolecules.10-16 Among the surfactants that form reversed micelles (RM), the best known are the systems derived from AOT, which has a wedge-shaped molecular geometry that enables it to form stable RM without cosurfactants. The driving force for * Corresponding authors. † Central Leather Research Institute. ‡ Indian Institute of Technology. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Fendler, J. H. In Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Luthi, Peter; Luisi, Pier Luigi J. Am. Chem. Soc. 1984, 106, 7285. (3) Luisi, P. L.; Giemini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 209. (4) Shield, John W.; Ferguson, Holly D.; Bommarius, Andreas S.; Hatton, T. Alan Ind. Eng. Chem. Fundam. 1986, 25, 603. (5) Eggers, D. K.; Blanch, H. W. Biopro. Eng. 1988, 3, 83. (6) Ruckenstein, Eli; Karpe, Prakash J. Colloid Interface Sci. 1990, 408. (7) Smolders, A. J. J.; Pinheiro, H. M.; Norrhha, P.; Cabral, J. M. S. Biotechnol. Bioeng. 1991, 38, 1210. (8) Chang, Pahn Shick; Rhee, Joon Shick; Kim, Jae-Jin Biotechnol. Bioeng. 1991, 38, 1159. (9) Prazeres, D. M. F.; Garcia, F. A. P.; Cabral, J. M. S. Bioproc. Eng. 1994, 10, 21. (10) Dekker, M.; Van’triet, K.; Weijers, S. R.; Battussen, J. W. A.; Laane, C.; Bijsterbosch, B. H. J. Chem. Eng. 1986, 33, B27. (11) Lesser, Martin E.; Luisi, Pier Liugi; Palmieri, Sandro Biotechnol. 1989, 34, 1140. (12) Ayala, Guadalupe A.; Kamat, Sanjay; Beckman, Eric J.; Russell, Alan J. Biotechnol. Bioeng. 1992, 39, 806. (13) Krei, Georg A.; Hustedt, Helmut Chem. Eng. Sci. 1992, 47, 99. (14) Ichikawa, S.; Imai, M.; Shimizu, M. Biotechnol. Bioeng. 1992, 39, 20. (15) Goklen, K. E.; Hatton, T. A. Biotechnol. Prog. 1985, 1, 69. (16) Fornay, Craig E.; Glatz, Charles E. Biotechnol. Prog. 1994, 10, 499.
S0743-7463(95)00279-4 CCC: $12.00
aggregate formation in apolar medium is attributable to the highly solvophobic sodium sulfosuccinate group of the AOT molecule. The size and number of molecules that aggregate may be controlled by manipulating the amount of water (or R ) [H2O]/[AOT]) and by the addition of cosurfactants. In recent years, extensive work has been done in the field of application of RMS2-20 and the characterization of the systems has also been attempted.21-25 New techniques using small angle neutron scattering,26 photon correlation spectroscopy,27 and positron annihilation28 are proving to be useful in studying RMS. In most cases, the properties of the solubilized water pool have been the center of focus.29-32 (17) Hedstrom, Gun; Slotte, J. Peter; Molander, Ove; Rosenholm, Jarl B. Biotechnol. Bioeng. 1992, 39, 218. (18) Komives, C.; Osborne, D.; Russell, A. J. Biotechnol. Prog. 1994, 10, 340. (19) Kelley, Brian D.; Wang, Daniel I. C.; Hatton, T. Alan Biotechnol. Bioeng. 1993, 42, 1199. (20) Hagen, Anna J.; Hatton, T. Alan; Wang, Daniel I. C. Biotechnol. Bioeng. 1990, 35, 955. (21) Hasegawa, Masatoshi; Sugimura, Tokuko; Kuraishi, Kazuhiro Chem. Lett. 1992, 1373. (22) Lianos, Panaglotis; Lang, Jacques; Zana, Raoul J. Phys. Chem. 1982, 86, 4809. (23) Tamura, Katsuhiro; Nii, Nobumasa J. Phys. Chem. 1989, 93, 4825. (24) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (25) Tamamushi, B.; Watanabe, N. Colloid. Polym. Sci. 1980, 258, 174. (26) Shen, E.; Goklen, K. E.; Hatton, T. A.; Chen, S. H. Biotechnol. Prog. 1986, 2, 175. (27) Zulauf, Martin; Eicke, Hans-Friedrich J. Phys.Chem. 1979, 83, 480. (28) Jean, Yan-Ching; Ache, Hans J. J. Am. Chem. Soc. 1979, 100, 984. (29) Huang, J. S.; Sung, J.; Wu, X. L. J. Colloid Interface Sci. 1989, 132, 34. (30) Eicke, Hans-Friedrich; Christen, Heinz Helv. Chim. Acta 1978, 61, 2258.
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The literature available indicates differences in opinion among the researchers regarding several features of this system. The very existence of a critical micelle concentration (cmc) in apolar solvents has been questioned.33 In our earlier studies, we have confirmed the micellization of peptides in apolar solvents.34-37 The characteristics of the AOT RM are also known to be critically dependent on the surfactant purity.38-39 Therefore, our aim in undertaking this work was to characterize the AOT/isooctane RMS with respect to cmc, aggregation number (N), interior microviscosity and micropolarity, thermodynamcis of micellization, etc., so that the system may be applied for bioseparation and biocatalysis with a fair amount of understanding and predictability. Experimental Section Hemi-Mg salt of 8-anilino-1-naphthalenesulfonic acid (ANS), N-cetylpyridinium chloride (CPC), tert-butylphenol (tBP), tetrabutylammonium iodide (TBAI), pyrene, and acrylamide were procured from Sigma Chemicals (USA). AOT was purchased from S.D. Fine Chemicals (India) and purified by the method of Shinoda.38 Water content was estimated by Karl-Fischer titration. The UV absorption and fluorescence emission experiments were carried out on a Shimadzu UV-VIS 1601PC spectrophotometer and Aminco-Bowman spectrofluorometer, respectively. For details regarding the determination of cmc in nonaqueous medium, refer to our earlier work.35,36 The differential signal ∆A/∆C or ∆I/∆C was plotted against the AOT concentration for determination of cmc value. For the determination of aggregation number, ANS was used as the external fluorescence probe and CPC was employed as the quencher.37,40 This technique assumes that the number of both probe and quencher molecules per micelle obeys Poisson’s distribution, which leads to the following expression:
I0 N(Q) ) I Cs - cmc
ln
(1)
where I0 and I are the emitted light intensities with quencher concentration a zero and [Q], respectively, N is the mean surfactant aggregation number, and Cs is the total AOT concentration. N is calculated from the slope of the plot of ln(I0/ I) against initial [Q] for a fixed Cs ) 15 mM. The probe ANS was used at a concentration small enough (1 × 10-6 mol/dm3, fixed) to prevent excimer formation The excitation wavelength was 346 nm, and fluorescence emission was measured at 455 nm.
Results and Discussion A plot of absorbance (at λmax) of tBP shows a discontinuous and abrupt change at the cmc point. This indicates the existence of a totally different microenvironment around the tBP molecule on the onset of micellization. In (31) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730. (32) Wong, M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. Soc. 1976, 98, 2391. (33) Ruckenstein, E.; Nagarjan, R. J. Phys. Chem. 1980, 84, 1349. (34) Mandal, A. B.; Dhathathreyan, A.; Jayakumar, R.; Ramasami, T. J. Chem. Soc., Faraday Trans. 1993, 89, 3075. (35) Jayakumar, R.; Mandal, A. B.; Manoharan, P. T. J. Chem. Soc., Chem. Commun. 1993, 10, 853. (36) Jayakumar, R.; Jeevan, R. G.; Mandal, A. B.; Manoharan, P. T. J. Chem. Soc., Faraday Trans. 1994, 90, 2725. (37) Mandal, Asit Baran; Jayakumar, Rajadoss J. Chem. Soc., Faraday Trans. 1994, 90, 161. (38) Kunieda, Hironobu; Shinoda, Kozo J. Colloid Interface Sci. 1979, 70, 577. (39) Jean, Yan-Ching; Ache, Hans J. J. Am. Chem. Soc. 1978, 100, 6320. (40) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951.
Figure 1. (a) Plot of differential signal of UV absorbance, (∆A/ ∆C) of tBP in AOT/isooctane solution at various concentrations of AOT: R ) 1.6, T ) 298 K, [tBP] ) 1 × 10-5 mol/dm3. (b) Plot of differential signal of fluorescence emission (∆I/∆C) intensity of tBP in AOT/isooctane solution at various concentrations of AOT. R ) 1.6, T ) 298, [tBP] ) 1 × 10-5 mol/dm3, λex ) 262 nm. Table 1. I1/I3 of Pyrene Fluorescence Emission Spectrum, cmc, and ∆Gm° at Different R Values (at 298 K) R
I1/I3
cmc/mM
∆Gm°/kJ
1.6 1.93 2.4 2.9
0.78 0.81 0.84 0.78
1.7 1.9 2.2 2.4
-15.80 -15.52 -15.16 -14.56
the premicellar concentration region, both absorbance maxima (276 and 215 nm) show increase in absorbance on increasing concentration of AOT. This shows that even at premicellar concentrations, AOT molecules tend to aggregate. However, at the cmc, the premicellar aggregates begin to form reverse micelles which ultimately leads to the compartmentalization of the interior polar core and nonpolar outer layer. In all probability, the -OH group of tBP would prefer a location in close proximity to the bound water molecules. This would lead to a reduction in the fluorescence and absorbance intensity at the cmc (Figure 1) because the pKa of tBP changes with respect to different environments. Similar reduction in intensity of absorption and fluorescence emission of the tBP probe was also observed in the micellization of peptides in chloroform.35 The pKa of p-nitrophenol was also found to change on binding to RMs of AOT/hydrocarbon.41 The cmc’s of AOT at various R values (R < 3) were determined and it was found that the cmc increases on increasing R (Table 1). (41) Menger, F. M.; Saito, G. J. Am. Chem. Soc. 1978, 100, 4376.
4070 Langmuir, Vol. 12, No. 17, 1996
Manoj et al.
Micropolarity and Microviscosity
a
Generally, fluorescence emission of ANS in chloroform is observed at 430 nm. On addition of surfactant (above cmc) the fluorescence maximum is shifted to 410 nm, which was interpreted to be due to enhanced microviscosity of the environment.36,42 However, solubility of ANS in isooctane is poor and it is solubilized only on the addition of AOT. The λmax of emission in the premicellar solutions was 458 nm suggesting the local ANS environment to be like that of alcohols (Figure 2a). On attaining cmc, the ANS emission intensity increased indicating a decreased polarity at the ANS binding site. Note that there is about a 10 nm blue shift indicating the increased microviscosity of the micellar core. However, on further increment of R, the λmax of the spectrum shows a red shift (Figure 2b). This shows that increasing the water content decreases the microviscosity of the micellar interior. The quantum yield decreases with increase in R implying an increase in polarity at the ANS binding site. Similar observations were reported in AOT microemulsions using 1-aminonaphthylene-4-sulfonic acid.41 Further studies on the nature of micelles were carried out using pyrene as a fluorescent probe. The vibronic structure of the fluorescence spectrum of pyrene is very sensitive to the environment polarity.43,44 A slight increase in intensity of pyrene spectrum indicates that the pyrene binding site is relatively more nonpolar than the bulk solvent (Figure 2c). The ratios of intensity of the first vibronic peak to the third at both premicellar and postmicellar concentrations were compared. The I1/I3 value at premicellar region is 0.75, suggesting the environment to be similar to that of aromatic solvents. In the postmicellar phase, the I1/I3 value is 0.78, which shows that the pyrene molecules may be bound to the nonpolar domain of the micelles. With an increase of R, the I1/I3 ratio attains a maximum value of 0.84 at R ) 2.4 and decreases to 0.78 at R ) 2.9 (Table 1). This may be due to a change in the pyrene binding site at R ) 2.9. A similar observation of change in binding constant of p-nitrophenol in AOT reversed micelles at R ) 3 has been reported earlier.45
b
Thermodynamic Parameters Neglecting activity effects and using a biphasic micellar model, the standard Gibb’s energy change for micelle formation of AOT in isooctane has been calculated from the following equation
∆Gm° ) RT ln cmc
(2)
The standard enthalpy change of micelle formation is determined from the slope of the plot of ln cmc vs. T (Figure 3) using the following equation:
∆Hm° ) -RT2
d ln cmc dT
(3)
The standard entropy change and heat capacity change were also obtained using the following equations:
∆Gm° ) ∆Hm° - T∆Sm°
(4)
∆Cp ) d∆Hm°/dT
(5)
The values calculated for the thermodynamic parameters are given in Table 2. For the interpretation of these values, a consideration of the intermolecular forces prevailing in the hydrophilic interior of the micelles is required. (42) Slavik, J. Biochim. Biophys. Acta 1982, 694, 1.
Figure 2. (a) Fluorescence spectra of ANS at (1) premicellar (0.5 mM) and (2) postmicellar (5 mM) concentrations of AOT in isooctane. λex ) 346 nm, R ) 1.6, T ) 298 K, [ANS] ) 5 × 10-6 mol/dm3 (fixed). (b) Fluorescence spectra of ANS at various R values. λex ) 346 nm, [ANS] ) 5 × 10-6 mol/dm3, T ) 298 K. (c) Fluorescence spectra pyrene in ANS at (1) premicellar (0.5 mM) and (2) postmicellar (5 mM) concentrations of AOT in isooctanme. λem ) 335 nm, R ) 1.6, T ) 298 K, [pyrene] ) 1 × 10-6 mol/dm3.
Previous investigations on the dissolution state of AOT RMs at low R suggested that the AOT molecules are arranged in a compact structure with their alkyl chains
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Figure 3. Plot of ln cmc and ∆Hm° vs T at R ) 1.6. Table 2. Cmc and Some Thermodynamic Parameters of AOT/Isooctane RMs at Various Temperatures, R ) 1.6 temp/K cmc/mM ∆Gm°/kJ ∆Hm°/kJ ∆Sm°/J K-1 ∆Cp/J K-1 293 298 303 308 318
2 1.7 1.5 1.1 0.6
-15.14 -15.80 -16.38 -17.44 -19.61
38.07 39.35 40.70 42.04 44.81
184.60 185.06 188.35 193.13 202.60
270
forming a distinct array by themselves.46 A hydrogen bonded trimer model was also proposed to explain RM formation.30 Earlier studies indicated the presence of at least two molecules of water bound to an AOT molecule.47 When water molecules are introduced into the quasilattice or the assembly of ion pairs, two enthalpic features need to be considered.48-50 The first arises from the ion-water interactions which probably make a negative contribution to ∆Hm°. The second results from the expansion of dismantling of the quasilattice which makes a positive contribution to ∆Hm°. The endothermic process of water dissolution into the anhydrous RMs was reported earlier.51 The calculated positive ∆Hm° suggests that the second process dominates during micelle formation. With an increase in R, the cmc increases, indicating that a further addition of water molecules contributes to the dismantling effect of quasilattice which will inhibit the aggregation process. The positive value of ∆Sm° may be due to the release of free isooctane molecules at the onset of micellization to some extent. The interaction between sodium counterions and solubilized water also seems to play an important role. The Na+-H2O interaction energy is of the order of 100 kJ/mol of H2O.52-54 Hence, it is expected that water (43) Llanos, P.; Lang, L.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 1019. (44) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 2039. (45) Jean, Yan-Ching; Ache, Hans J. J. Am. Chem. Soc. 1978, 6320. (46) Martin, C. A.; Magid, L. J. J. Phys. Chem. 1981, 85, 3938. (47) Hauser, H.; Hearing, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869. (48) Shinoda, K. In Solubilization and solubility; Maruzen: Tokyo, 1986; p 229. (49) Goto, A.; Harade, S.; Fugita, T.; Micra, Y.; Yoshida, H.; Kishimoto, H. Langmuir 1993, 9, 86. (50) Ray, S.; Bisal, S. R.; Moulik, S. P. Langmuir 1994, 10, 2507. (51) D’Aprano, A.; Lizzio, A.; Leveri, V. T. J. Phys. Chem. 1987, 10, 2507. (52) Arshadi, M.; Yamdagni, R.; Kebarle, P. J. Phys. Chem. 1970, 74, 1475. (53) Dzidic, I.; Kebarle, P. J. Phys. Chem. 1970, 74, 1466.
Figure 4. Results of ANS quenching experiments. ln Io/I vs [CPC] for micellar solutions of AOT in isooctane at 298 K. [AOT] ) 15 × 10-3 mol/dm3 (fixed), [ANS] ) 1 × 10-6 mol/dm3 (fixed), λex ) 346 nm, λem ) 455, R ) 1.6 and T ) 298 K.
is bound extremely firmly up to the completion of the Na+ hydration shell. Evidently, by increasing temperature, water molecules will be energetically rich enough to participate in quasi-cooperative water nucleation thereby favoring aggregation of AOT molecules around such a nucleus. Zendel et al.55 and Eicke and Christen30 reported that the energy concerning the hydration of counterions and oxygen atom in the sulfonate moiety of the surfactant are weaker than hydrogen bridges in bulk water. The specific heat capacity change, ∆Cp (derived from the slope of the plot of ∆Hm° vs T (Figure 3)) of the RM was found to be 0.65 J/(g K). This suggests the involvement of structured water molecules in the micellization process. Thus, these studies indicate that the aggregation of AOT in isooctane at low water content is basically an entropically driven process. Aggregation Number The aggregation number, N, determined by fluorescence quenching was found to be 120 ( 20 (Figure 4). Ueda et al.,56 using vapor pressure osmometry, found the aggregation number for AOT RM at different R to be
