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Agencia Postal Nro. 3, Rio Cuarto 5800, Argentina. Received September 29, 2000. In Final Form: December 13, 2000. Solubilization of 1,2,3-propanetriol...
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Langmuir 2001, 17, 1847-1852

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Solubilization of Pure and Aqueous 1,2,3-Propanetriol by Reverse Aggregates of Aerosol-OT in Isooctane Probed by FTIR and 1H NMR Spectroscopy Omar A. El Seoud,*,† N. Mariano Correa,‡ and Luzia P. Novaki† Instituto de Quı´mica, Universidade de Sa˜ o Paulo, C.P. 26.077, 05513-970 Sa˜ o Paulo, S.P., Brazil, and Departamento de Quı´mica y Fı´sica, Univ. Nacional de Rio Cuarto, Agencia Postal Nro. 3, Rio Cuarto 5800, Argentina Received September 29, 2000. In Final Form: December 13, 2000 Solubilization of 1,2,3-propanetriol, PT, and its aqueous solution, PT-W, (W ) water), by reverse aggregates of sodium bis(2-ethylhexyl) sulfosuccinate, Aerosol-OT or AOT, in isooctane has been studied by FTIR and 1H NMR spectroscopy, and the results were compared with those of solubilization of 1,2ethanediol, ED, and W by the same surfactant. Curve fitting of the IR νOD band of aggregate-solubilized PT or PT-W showed the presence of a main peak and a smaller one. Dependence on [solubilizate]/[AOT] of 1H NMR chemical shifts, δproton, of solubilized PT, PT-W, as well as the surfactant protons showed monotonic increase or decrease. Both spectroscopic techniques indicated the following order of interaction with AOT: PT > ED > W. Over the entire range of [solubilizate]/[AOT], the main νOD peak areas correspond to 88 ( 2%, and 90 ( 2% of the total peak areas for PT, and PT-W, respectively. This result shows that aggregate-solubilized PT or PT-W do not seem to coexist in “layers” of different structures, as suggested by the multistate solubilization model.

Introduction Surfactants aggregate in water and other protic solvents to form a variety of organized assemblies, e.g., normal micelles and lyotropic liquid crystals. Only relatively structured organic solvents capable of forming threedimensional H-bonded networks are effective in causing micellization. Examples are 1,2,3-propanetriol, PT, 1,2ethanediol, ED, and formamide. Replacement of water by these solvents leads to a reduced tendency for surfactant aggregation and structure formation. This has been interpreted in terms of reduced “solvophobic” interactions between the surfactant hydrophobic tail and the nonaqueous protic solvent.1,2 Recently, effects of glycols on the self-assembly of amphiphilic block copolymers have been reported.3 Several surfactants, including anionic sodium bis(2ethylhexyl) sulfosuccinate, aerosol-OT, AOT, and cationic cetyltrimethylammonium bromide aggregate in both protic solvents to form normal micelles and in nonpolar aprotic oils (e.g., alkanes and halogenated alkanes) to form reverse micelles (for brevity, we use the term oil to indicate the continuous, aprotic pseudophase). One of the most important properties of reverse aggregates is their capacity to solubilize water. Designation of the aggregates present * Fax: +55-11-3818-3874. E-mail: [email protected]. † Universidade de Sa ˜ o Paulo. ‡ Univer. Nacional de Rio Cuarto. (1) (a) Rico, I.; Lattes, A. J. Phys. Chem. 1986, 90, 5870. (b) Cantu`, L.; Corti, M.; Degiorgio, V.; Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1987, 116, 384. (c) Fletcher, P. D. I.; Gilbert, P. J. J. Chem. Soc., Faraday Trans. 1, 1989, 85, 147. (d) Wa¨rnheim, T.; Jo¨nsson, A.; Sjo¨berg, M. Prog. Colloid Polym. Sci. 1990, 82, 271. (e) Bakshi, M. S. J. Chem. Soc., Faraday Trans., 1993, 89, 4323. (f) Palepu, R.; Gharibi, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir, 1993, 9, 110. (2) (a) Palepu, R.; Gharibi, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1993, 9, 110. (b) Binana-Limbele, W.; Zana, R. Colloid Polym. Sci. 1989, 267, 440. (c) Lee, D. J.; Huang, W. H. Colloid Polym. Sci. 1996, 274, 160. (d) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424. (e) Nagarajan, R.; Wang, C.-C. Langmuir 2000, 16, 5242. (3) (a) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 3660; (b) 3676.

either as wet reverse micelles, RMs, or water-in-oil microemulsion (W/O µE) depends on the [water]/[surfactant], W/S, ratio. In RMs, the amount of solubilized water is less than or equal to the amount necessary to hydrate the surfactant head-ions. Solubilization of water over and above this threshold results in the formation of an isotropic and thermodynamically stable W/O µE (under a specified set of experimental conditions).4 Reverse aggregates dissolve other protic solvents, e.g., PT, ED, and formamide as well as their binary mixtures with water. This capacity has prompted intense research aiming at determination of the physicochemical properties of the aggregates thus formed, e.g., their hydrodynamic radii, aggregation numbers, as well as interdroplet interactions.5-7 These systems are interesting per se and because the stabilizing effect of bulk PT on proteins is also extended, e.g., to AOT-solubilized R-chymotrypsin and lipases.8 Rapid mixing of reagents, which is a nuisance in bulk PT (viscosity at 25 °C ) 934 mPa s),9 is easy in (4) (a) Eicke, H.-F.; Kvita, P. In Reverse Micelles: Biological and TechnologicalRelevance of Amphiphilic Structures in Apolar Media; Luisi, L. P., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 21. (b) El Seoud, O. A. In Organized Assemblies in Chemical Analysis; Hinze, W. L.; JAI Press: Greenwich, 1994; Vol. 1, p 1. (c) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Adv. Colloid Interface Sci. 1999, 82, 189. (5) (a) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3307; (b) 81, 2053. (c) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727. (6) (a) Aveyard, R.; Binks, B. P.; Fletche, P. D. I.; Kirk, A. J.; Swansbury, P. Langmuir 1993, 9, 523. (b) Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (c) Hayes, D. G.; Gulari, E. Langmuir 1995, 11, 4695. (d) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (7) (a) Martino, A.; Kaler, E. W. Langmuir 1995, 11, 779. (b) Nees, D.; Wolff, T. Langmuir 1996, 12, 4960. (c) Laia, C. A. T.; Lo´pez-Cornejo, P.; Costa, S. M. M.; d’Oliveira, J.; Martinho, J. M. G. Langmuir 1998, 14, 3531. (c) Rariy, R. V.; Bec, N.; Klachko, N. L.; Levashov, A. V.; Balny, C. Biotechnol. Bioeng. 1998, 57, 552. (d) Atay, N. Z.; Robinson, B. H. Langmuir 1999, 15, 5056. (8) (a) DiPaola, G.; Belleau, B. Can. J. Chem. 1978, 56, 848. (b) Gekko, K.; Timasheff, S. N. Biochemistry 1981, 20, 4667. (c) Huang, P.; Dong, A. C.; Caughey, W. S. FASEB J. 1992, 6, A475. (d) Hayes, D. G.; Gulari, E. Biotechnol. Bioeng. 1992, 40, 110. (e) Yang, C. L.; Gulari, E. Biotechnol. Prog. 1994, 10, 269. (f) Rariy, R. V.; Bec, N.; Klachko, N. L.; Levashov, A. V.; Balny, C. Biotechnol. Bioeng. 1998, 57, 552.

10.1021/la001378d CCC: $20.00 © 2001 American Chemical Society Published on Web 02/20/2001

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AOT-solubilized PT because the reaction “medium” within the aggregate has the low viscosity of the oil pseudophase (viscosity of heptane at 25 °C ) 0.378 mPa).6d,9 Potential novel applications of these micellar systems require a clear understanding, on the molecular level, of, e.g., the mechanism and strength of surfactant-protic solvent interactions, structure of the polar pseudophase and so on. Recently, we have studied solubilization of ED and ED-W by AOT reverse aggregates by FTIR and 1H NMR spectroscopy, and the data obtained were compared with those of AOT-solubilized water. Both techniques indicated that both solubilizates interact with the surfactant by solvating its head-ions, which interact more strongly with ED than with water.11 We have now extended this study to solubilization of PT and PT-W by AOT in isooctane. Glycerol is an interesting solubilizate because rotation of the two CH2OH groups around the C-C bonds gives rise to three staggered conformations, leading to six possible backbone structures.10 These possibilities, which are not open to ED, may be manifested in PT-AOT interactions. The following are objectives of the present study: (i) investigation of protic solubilizate-surfactant interactions; (ii) comparison of solubilization data with those of ED, ED-W, and W; (iii) determination of the structure of solubilized protic pseudophase. Our IR and 1H NMR data indicated the following order of solubilizate-surfactant interactions: PT > ED > W. These data also showed that aggregate-solubilized PT or PT-W does not seem to coexist in “layers” of different structures, as suggested by the multistate water solubilization model. Experimental Section Materials. Chemicals were obtained from Aldrich or Merck. Isooctane and PT were purified as given elsewhere.12 AOT was dried under reduced pressure, over P2O5 until constant weight. The UV-vis of 1-methyl-8-oxyquinolinium betaine (a solvatochromic probe) in the presence of AOT reverse aggregates in isooctane showed that the surfactant is free of acidic impurities, which would have greatly reduced the intensity of the probe solvatochromic B1-band at 502 nm.11,13 Deuterated PT was prepared by stirring 5 mL of PT with 6 mL of D2O at room temperature. Water was then distilled off, and the procedure was repeated twice. The deuterium content of PT, 97.1%, was determined by 1H NMR by using toluene as an internal reference. In the following, we use the symbol PT-d3 to denote DOCH2CHODCH2OD. Methods. The following solubilizate solutions were prepared by volume: FTIR Experiments. Water (8% D2O in H2O), PT (40% PT-d3 in PT), aqueous PT (40% PT (containing 4% PT-d3) in H2O (containing 4% D2O)). 1H NMR Experiments. Water (80% D2O in H2O), PT (80% PT-d3 in PT), aqueous PT (30% PT (containing 80% PT-d3) in H2O (containing 80% D2O)). The reason for using partially deuterated solubilizates is explained in the Discussion. Stock solutions of the surfactant or solubilizate/surfactant were prepared by weight. Every effort has been made to ensure that pairs of solutions which are to be mixed in order to obtain a certain [solubilizate]/[AOT] (e.g., those of dry AOT and AOT at W/S ) 20.0) contained the same amount of surfactant. Solutions (9) CRC Handbook of Chemistry and Physics, 73 ed.; CRC Press: Boca Raton, FL, 1992. (10) (a) Blackmore, P. F.; Williams, J. F.; Clark, M. G. J. J. Chem. Educ. 1973, 50, 555. (b) van Den Enden, L.; van Alsenoy, C.; Scarsdale, J. N.; Scha¨fer, L. J. Mol. Struct. (THEOCHEM) 1983, 104, 471. (c) Bastiansen, O. Acta Chem. Scand. 1949, 3, 471. (11) Novaki, L. P.; Correa, N. M.; Silber, J. J.; El Seoud, O. A. Langmuir 2000, 16, 5573. (12) Perrin, D. D.; Armarego, W. L. F. In Purification of Laboratory Chemicals, 3rd. ed.; Pergamon Press: New York, 1988. (13) (a) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1995, 172, 71. (b) Falcone, R. D.; Correa, N. M.; Biasutti, A. A.; Silber, J. J. Langmuir 2000, 16, 3070.

El Seoud et al. with variable [solubilizate]/[AOT] were prepared as follows: stock solutions were pipetted into 1 mL volumetric vials, and these were weighed after each addition; i.e., the final [solubilizate]/ [AOT] was determined by weight. FTIR. The surfactant solution was 0.20 M in isooctane. The following cells from Wilmad Glass (Buena, NJ) were used: CaF2 (1.02 mm), KRS-5 (0.49 mm), and Irtran-2 (0.21 and 0.11 mm). The exact path length was determined by the fringe method.14 IR spectra were recorded with a Bomem MB 100-C26 FTIR spectrophotometer. Transmission spectra were obtained by coadding 18 spectra at 1 cm-1 resolution. The νOD spectral band of HOD, (partially deuterated) PT, or aqueous PT is superimposed on a finite background which could be approximated with the spectrum of pure H2O, PT, or PT-W in the νOD spectral region.15,16 Therefore the reference sample, at each solubilizate concentration, was a surfactant solution containing matched [solubilizate]/[AOT], adjusted with H2O, PT, or PT-W. Deconvolution of the νOD spectral band was carried out with a commercial software (GRAMS/32 program package, Galactic Industries Co., Salem, NH). 1H NMR. The surfactant solution was 0.20 M in isooctane. A Bruker DRX-500 NMR spectrometer (operating at 500.13 MHz for protons) was used. The spectra were recorded at a digital resolution of 0.06 Hz/data point. The spectrometer probe temperature, 30.0 °C, was periodically monitored by measuring the chemical shift difference between the two singlets of a methanol reference sample. The probe thermal stability was assured by the observation that successive measurements of the sample chemical shift (after 10 min in the probe for thermal equilibration) were within digital resolution limit. A capillary tube containing D2O was used as a frequency “lock”. Chemical shifts were measured relative to internal TMS.

Results 1H NMR of Solubilized PT and Aqueous

FTIR and PT. In what follows, IR and 1H NMR data for AOTsolubilized ED, ED-W, and W were taken from our recent publication.11 IR frequencies and 1H NMR chemical shifts were calculated for matched [solubilizate]/[S] by interpolation from experimentally obtained data for solubilized PT, ED and W, respectively. Regression coefficients of the equations that gave the best data fit for AOT-solubilized PT and PT-W are collected in Table 1, along with corresponding correlation coefficients, c.c. Figure 1 shows typical νOD peaks for solubilized (partially deuterated) PT (1A, [PT]/[AOT] ) 4.0), and aqueous PT (1B, [PT-W]/[AOT] ) 21.2), along with the corresponding band deconvolution. Although curve fitting was carried out by considering contribution from Gaussian and Lorentzian components, our calculations showed that the bands are essentially Gaussian, in agreement with published IR data on bulk PT-d3,15 bulk HOD,16 as well as ED and W solubilized in reverse aggregates of AOT and cetyltrimethylammonium bromide.11,17 As a function of increasing [solubilizate]/[AOT], the frequency (position of maximum absorption of the main peak) decreases and the full width at half-height, fwhh, increases. At [PT]/ [AOT] ) 4.0, we measured 2504 and 172 cm-1 for νOD and (14) Compton, S. V.; Compton, D. A. In Practical Sampling Techniques in Infrared Analysis; Coleman, P. B., Ed.; CRC Press: Boca Raton, FL, 1993; p 217. (15) (a) Beaudoin, J. L. J. Chim. Phys. 1977, 74, 268. (b) Solomons, J. E. J. Phys. Chem. 1977, 81, 1492. (c) Beaudoin, J. L.; Eloundou, J. P. J. Chem. Phys. 1979, 71, 47. (16) (a) Waldron, R. D. J. Chem. Phys. 1957, 26, 809 (b) Wall, T. T.; Hornig, D. F. J. Chem. Phys. 1965, 43, 2079. (c) Wall, T. T.; Hornig, D. F. J. Chem. Phys. 1967, 47, 784. (d) Walrafen, G. E. J. Chem. Phys. 1968, 48, 244. (e) Schiffer, J.; Hornig, D. F. J. Chem. Phys. 1968, 49, 4150. (f) Lucas, M.; De Trobriand, A.; Ceccaldi, M. J. Phys. Chem. 1975, 79, 913. (g) Kristiansson, O.; Eriksson, A.; Lindberg, J. Acta Chem. Scand. 1984, A38, 609. (17) (a) Novaki, L. P.; El Seoud, O. A. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 928. (b) Novaki, L. P.; El Seoud, O. A. J. Colloid Interface Sci. 1998, 202, 391.

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Table 1. Regression Analysis Results for the Dependence on [Solubilizate]/[AOT] of the IR Stretching Frequencies and 1H NMR Chemical Shiftsa parameter νODd νODd δOH-prime δOH-seconde δH1f δH1′ f δOHf δH1f δH1′ f

system

equationb

A

B

PT/S PT-W/S PT/S PT/S PT/S PT/S PT-W/S PT-W/S PT-W/S

polynom/2 expon/1 linear linear linear linear polynom/3 expon/1 expon/1

2540.58 2511.75 2156.67 2300.17 2187.41 1601.79 1837.97 2116.38 1580.47

-16.75 42.92 112.18 86.89 -18.68 -8.00 83.46 82.05 25.17

C

D

1.95 7.26

-6.59 3.20 1.48

0.20

c.c.c 0.9953 0.9952 0.9989 0.9996 0.9984 0.9992 0.9950 0.9826 0.9873

a Abbreviations: PT ) 1,2,3-propanetriol; S ) surfactant or AOT, and W ) water. Regression analyses were carried out for [solubilizate]/ [AOT] from 0.44 to 4.0 and 2.47 to 21.2, for PT, and PT-W, respectively. b Equation that gave the best regression analysis, followed by its order; e.g., for PT-S, polynom/2 means the following second-order polynomial equation: νOD ) A + B ([PT]/[S]) + C ([PT]/[S])2. For PT-W/S, expon/1 refers to the following first-order exponential decay: νOD ) A + B exp(-[PT-W/S]/C). c Regression correlation coefficient. d ν e 1H NMR chemical shift of the primary OD ) stretching frequency of the O-D band of solubilized (partially deuterated) PT or PT-W. (δOH-prim) and secondary (δOH-second) OH group of PT. f δOH, δH1, and δH1′ refer to the 1H NMR chemical shift of the OH group of solubilized PT-W, and the H1 and H1′ of the surfactant, respectively.

Figure 1. Representative band deconvolution of the νOD peak of solubilizates in the presence of AOT/isooctane, showing pure PT (1A, [PT]/[AOT] ) 4.0) and PT-W (1B, [PT-W]/[AOT] ) 21.2). The solid-line peak is experimental, whereas the symbols (- - -), (‚‚‚), and (-..-..-) are used to denote the two peaks calculated by curve fitting and their sum, respectively. Abbreviations: PT ) 1,2,3-propanetriol, AOT ) sodium bis(2ethylhexyl) sulfosuccinate, and W ) water.

fwhh, respectively. At [PT-W]/[AOT] ) 20.0, the corresponding values were found to be 2515 and 167 cm-1 for νOD and fwhh, respectively. The latter values are similar to those which we measured for bulk PT-W, 2498 and 165 cm-1, respectively. Figure 2A shows the dependence on [solubilizate]/[AOT] of ∆νOD ()νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 2.0) for PT, ED, and W, respectively, whereas Figure 2B shows the corresponding plots for solubilized PT-W, ED-W, and W, respectively, where ∆νOD ) νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 20.0). The dependence of main peak areas of the νOD bands (obtained by deconvolution, Figure 1) on [solubilizate]/ [AOT] are linear, as shown in Figure 3 for PT (3A) and PT-W (3B), respectively. These areas correspond to 88 ( 2% and 90 ( 2% of the total peak areas for solubilized PT and PT-W, respectively. Figures 4 and 5 show the dependence on [solubilizate]/ [AOT] of 1H NMR chemical shifts, where Figure 4A is for the OH protons of pure solubilizates, ∆δOH ()δOH at any [solubilizate]/[AOT] - δOH at [solubilizate]/[AOT] of 2.0), and parts B and C of Figure 4 are for the AOT protons that shift most, namely, H1 and H1′ (see part A of Scheme

Figure 2. Dependence of ∆νOD of the main peak on [solubilizate]/ [surfactant]. (A) Pure protic substances (∆νOD ) νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 2.0) (B) Aqueous binary mixtures (∆νOD ) νOD at any [solubilizate]/ [AOT] - νOD at [solubilizate]/[AOT] of 20.0). The symbols are 2, PT or PT-W; b, ED or ED-W; and 9, W. Abbreviations are as given in Figure 1; ED ) 1,2-ethanediol.

Figure 3. Dependence of areas of the main νOD peaks on [solubilizate]/[AOT] for pure PT (A) and PT-W (B). Abbreviations are as given in Figure 1

1); e.g., Figure 4B shows ∆δH1 ()δH1 at any [solubilizate]/ [AOT] - δH1 at [solubilizate]/ [AOT] of 2.0). Finally, Figure 5 shows the corresponding plots for binary mixtures up to [solubilizate]/[AOT] ) 15.0, where Figure 5A is for the

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Figure 4. Dependence on [solubilizate]/[surfactant] of ∆δproton ) δproton at any [solubilizate]/[AOT] - δproton at [solubilizate]/ [AOT] of 2.0. (A) Solubilizate OH group. (B) Protons H1 and (C) H1′ of AOT. The symbols are the same as in Figure 2, except that 2 and 4 stand for PT primary and secondary OH groups, respectively. Abbreviations are as given in Figure 1.

Figure 5. Dependence on [solubilizate]/[surfactant] of ∆δproton ) δproton at any [solubilizate]/[AOT] - δproton at [solubilizate]/ [AOT] of 15.0, where ∆δproton ) ((∆δprotonPT-W + ∆δprotonED-W + ∆δprotonW)/3). (A) Solubilizate OH group. (B) Protons H1 and (C) H1′ of AOT. The symbols are the same as in Figure 2. Abbreviations are as given in Figure 1.

OH group and parts B and C of Figure 5 are for AOT H1 and H1′, respectively. In comparison to Figure 4, the chemical shift differences, ∆δproton, depend much less on the nature of organic solubilizate. Rather than plotting three closely spaced curves for each proton (corresponding to solubilized PT-W, ED-W, and W, respectively), we have found it more convenient to plot, in each case, a single (i.e., mean) curve that goes through the points. Each curve was simply calculated by regression analysis of an averaged ∆δproton versus [solubilizate]/[AOT], where ∆δproton ) ((∆δprotonPT-W + ∆δprotonED-W + ∆δproton W) /3). The equations that describe these plots and the corresponding c.c. were found to be in Figure 5A, a third-order polynomial, 0.9999, in Figure 5B, first-order exponential decay, 0.9872, and in Figure 5C, first-order exponential decay, 0.9997. Discussion Comparison of the Interactions of Polar Protic Solubilizates With AOT. First, we comment on the interpolation range that we have employed in order to obtain data at matched [solubilizate]/[AOT]. As previously stated, the limiting [solubilizate]/[AOT] ratios in isooctane

El Seoud et al.

are 4.0, 2.0, and 60.0 for PT, ED, and W, respectively. Therefore, we restricted data regression and subsequent interpolation to the range employed with ED, i.e., [solubilizate]/ [AOT] ) 0.5 to 2.0. In principle, we could have extended the concentration ratio to that possible for PT, namely, 4.0. Because of uncertainty in data extrapolation, we have decided to take a more conservative approach, i.e., limit our comparison to the range of solubility of ED in the micellar system. Figures 2A (IR) and 4A (1H NMR) show that the order of interaction of AOT with solubilized protic solvents is PT > ED > W. 1H NMR spectra of PT in bulk DMSO-d6 and in AOT reverse aggregates in isooctane show two peaks for the OH groups, corresponding to the primary hydroxyl groups, i.e., C1-OH and C3-OH, and the secondary one, C2-OH, respectively. As shown in Figure 4A, the former OH groups interact with the surfactant more than the secondary one, whose strength of interaction is similar to that of ED. The dependence on [solubilizate]/[AOT] of ∆νOD (Figure 2A) and ∆δOH (Figure 4A) is a clear indication that all solubilizates interact with AOT by a similar mechanism, namely, by solvating its head-ions. This occurs via formation of H-bonds with the oxygen atoms of the sulfonate group, and charge-dipole interactions between the solubilizate OH groups and the sodium counterion.4 This results in the shifting of νOD to lower frequencies and of δOH to lower fields (i.e., away from TMS). With regard to solubilizate-AOT interactions, the following is relevant: (i) The strength of interaction depends on the number of OH groups present in the molecule2c,18 (this agrees with the above-mentioned order). Indeed, it has been concluded that primary solvation of AOT head-ions requires 2-3 molecules of PT per surfactant molecule,8a and the corresponding number for solubilized water is ca. 6-10 molecules per AOT; i.e., each PT molecule is equivalent to ca. 3 molecules of water. (ii) Counterion solvation by polyols may be more efficient than that by water because they act as bidentates. The probability of bidentate formation by PT is higher than that of ED because the former solvent has one secondary and two primary OH groups.10,18,19 (iii) PT and ED differ in several properties that influence their solvation of ions (e.g., AOT head-ions), as can be seen from the following ratios of (property of PT)/(corresponding property of ED): dielectric constant, 1.13; dipole moment, 1.17; polarizability, 1.42; molar heat of vaporization, 1.40; and molar heat capacity at constant pressure, 1.48.18a Compared to ED, PT has a higher H-bond donor acidity and a lower H-bond acceptor basicity.18d As a consequence, the free energies of transfer from water to PT or ED (a measure of ion-solvent interactions) are noticeably different.18c (v) The above-mentioned order of interaction with AOT agrees with negative virial coefficients, calculated from dynamic light scattering of AOT-solubilized PT and ED. Solubilization leads to a reduced tendency for adsorption of surfactant molecules at the oil/protic solubilizate interface, leading to a more fluid interface (relative to that when water is the solubilizate) and a lower threshold of phase separation.5a,6a,7c Similar conclusions can be reached by examining the dependence of chemical shifts of AOT H1 and H1′ on (18) (a) Marcus, Y. In Ion Solvation; Wiley: Chichester, 1985; p 130 and references therein. (b) Krestov, G. A. In Thermodynamics of Solvation; Ellis Horwood, New York, 1991; p 151 and references therein. (c) Marcus, Y. Pure Appl. Chem. 1990, 62, 899 and references therein. (d) Fonrodona, G.; Ra`flos, C.; Bosch, E.; Rose´s, M. Anal. Chim. Acta 1996, 335, 291. (19) (a) Kuhn, L. P.; Bowman, R. E. Spectrochim. Acta. 1961, 17, 650. (b) Suzuki, M. J. Electroanal. Chem. 1994, 372, 39.

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Scheme 1. Structure (A), and Rotational Isomers of AOT (B)

[solubilizate]/[AOT], Figure 4B,C. These plots show that the order of ∆δ is not the same for both protons, being ED > PT > W and PT > ED > W for H1 and H1′, respectively. Part B of Scheme 1 shows the three conformations of AOT at the water/oil interface. In all conformers, H1 is in the oil pseudophase, whereas the two H1′ protons are mostly in the aqueous pseudophase because rotational isomer III predominates (I/II/III ) 0.21:0.04:0.75).20 As discussed in the preceding paragraph, the mechanism of solubilizate-AOT interactions appears to be the same for all three protic solvents. Therefore, it is expected that the conformers shown in part B of Scheme 1 also apply to AOTsolubilized PT and ED: i.e., H1′ is mostly in contact with the protic solvent, whereas H1 is in the oil pseudophase. The virial coefficient calculated for ED-AOT is more negative than that for PT-AOT5a,6c,7c and has been interpreted in terms of penetration of the diol into the interface (i.e., ED acts as a cosurfactant). This explanation also agrees with the following results: (i) Interfacial tensions (in mN/m) at protic solvent/dodecane (a model oil) interfaces are 17 and 30 for ED and PT, respectively (i.e., penetration of ED into the oil phase is less energetic1d). (ii) For the system protic solvent/AOT/toluene, the interfacial area of the surfactant at the ED/oil interface is 2.8 times greater than the corresponding one at the PT/oil interface. (iii) ED penetration into the interface also explains experimentally determined AOT concentrations in toluene, which were found to be much higher for solubilized ED than those for solubilized PT or water.6a Therefore, the chemical shift of H1 is affected by a combination of (1) changes in its electron density due solvation of the sulfonate head-ion and (2) a “medium” effect due to penetration of ED into the interface. That is, the above-mentioned inversion of order of ∆δ can be attributed to a rapid change in the properties (e.g., H-bonding and mean dielectric constant) of the microenvironment of H1 as a function of increasing [ED]/[AOT]. Interactions of Aqueous PT and ED with AOT. AOT-solubilized PT-W and ED-W show a single (averaged) 1H NMR peak for the OH groups present. Figure 5A-5C shows that the dependence of ∆δproton on the nature of solubilized polyol is much less than that observed for pure protic solvents; see Figure 4. Because of the high mole fraction of water, χW, in the starting aqueous mixtures, 0.937 and 0.90 for PT-W, and ED-W, respectively, it dominates the chemical shifts. Consequently, the behavior and magnitude of ∆δproton are not very different for all solubilizates. The reason for employing χpolyol e 0.1 is that bulk binary mixtures behave ideally within this mole fraction range;21 thus, complications due to departure from ideality are avoided. Interestingly, data (20) Yoshino, A.; Sugiyama, N.; Okabayashi, H.; Taga, K.; Yoshida, T.; Kamo, O. Colloid Surf. 1992, 67, 67. (21) (a) Huot, J.-Y.; Battistel, E.; Lumry, R.; Villeneuve, G.; Lavalle´, J. F.; Anusiem, A.; Jolicoeur, C. J. Solution Chem. 1988, 17, 601. (b) Marcus, Y. J. Chem. Soc., Faraday Trans. 1990, 86, 2215. (c) Chang, Y.; Page´, M.; Jolicoeur, C. J. Phys. Chem. 1993, 97, 7, 7359. (d) Taniewska-Osinska, S.; Woznicka, J.; Bartel, L. Thermochim. Acta 1981, 47, 65. (e) To, E. C. H.; Davies, J. V.; Tucker, M.; Westh, P.; Trandum, C.; Suh, K. S. H.; Koga, Y. J. Soln. Chem. 1999, 28, 1137.

regression of the three solubilizates by a single curve may indicate that binary mixture ideality is maintained within the reserve aggregate. The observed “leveling” effect of water precludes further discussion on the strength of interactions of the aqueous binary mixtures with AOT. FTIR Investigation of the Structure of Reverse Aggregate-Solubilized PT and PT-W. At the outset, we discuss the reason for employing deuterated solubilizates in the FTIR experiments. Conclusion with regard to the types of solubilizate present within RMs and W/O µE’s is usually arrived at from the number of peaks obtained by curve fitting, e.g., of νOH (of solubilized H2O) or νOD (of solubilized D2O). However, the basic premise involved in this assumption, i.e., that each band obtained by curve fitting may be attributed to a different type of water, is open to question because these bands possibly originate from coupled water molecule vibrations, and from a bending overtone often reported in the spectrum of liquid water.22 On the other hand, deconvolution of νOH or νOD vibrations of HOD is straightforward because both frequencies are essentially decoupled, provided that D2O e 10%.23 This advantage has been recognized for both the bulk aqueous phase16,23 and reverse aggregates.17,22a,24 Deconvolution of the νOD band of solubilized PT and PT-W can be employed to probe their microstructures within the pool, i.e., whether they are present in “layers” of different structures. IR spectroscopy is a particularly suitable technique to detect these layers, if present, because its time scale (10-12-10-14 s) is faster than the time scales, e.g., on which water molecules are expected to interchange between discrete layers (between 10-7 s and 10-11 s).22d,23a In analyzing νOD, we bear in mind principals that are central to curve fitting of spectroscopic data,25,26 and indeed to any problem whose solution relies on curve fitting.27 Basically, the model employed to explain the origin of vibrational dynamics of the system should fit the data accurately, and agree with chemistry. In principle, a protic solubilizate may be present in layers if its concentration within the reverse aggregate is more than sufficient to solvate the surfactant head-ions. This is the case of AOT-solubilized PT-W. On the other hand, if primary solvation of AOT head-ions requires 2-3 molecules of PT per AOT,8a then presence of a second layer (22) (a) Pacynko, W. F.; Yarwood, J.; Tiddy, G. J. T. Liq. Cryst. 1987, 2, 201. (b) Christopher, D. J.; Yarwood, J.; Belton, P. S.; Hills, B. J. Colloid Interface Sci. 1992, 152, 465. (c) Scherer, J. R. In Advances in Infrared Raman Spectroscopy; Clark, R. J., Hester, R. E., Eds.; Wiley: New York, 1980; Vol. 5, p 149. (d) Tiddy, G. J. T. Nucl. Magn. Reson. 1979, 8, 174 (23) (a) Senior, W. A.; Verrall, R. E. J. Phys. Chem. 1969, 73, 4242. (b) Mundy, W. C.; Gutierrez, L.; Spedding, F. H. J. Chem. Phys. 1973, 59, 2173. (24) Amico, P.; D’Angelo, M.; Onori, G.; Santucci, A. Nuovo Cimento 1995 17D, 1053. (25) Wills, H. A.; van der Maas, J. H.; Miller, R. G. J. Laboratory Methods in Vibrational Spectroscopy, 3rd ed.; Wiley, New York, 1987; p 188. (26) (a) Maddams, W. F. Appl. Spectrosc. 1980, 34, 245. (b) Clark, R. J.; Hester, R. E.; Wiley: New Yodeginste, B. G. M.; De Galan, L. Van Infrared Raman Spectroscopy. Anal. Chem. 1975, 47, 2124. (27) Gandour, R. D.; Coyne, M.; Stella, V. J.; Schowen, R. L. J. Org. Chem. 1980, 45, 1733.

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of PT inside the micellar pool cannot be ruled out because maximum [PT]/[AOT] is 4.0. AOT-solubilized water has been pictured as present in several layers. The first one, Wbound, is at the periphery of the pool and is made of water molecules tightly bound to the surfactant headgroup. The second, intermediate “layer”, Wintermediate, refers to distorted H-bonded water species, e.g., cyclic dimers or higher aggregates with unfavorable H-bonds. The third, central “layer” contains bulklike water, Wbulklike, and its formation coincides with formation of the W/O µE.28 Other models depicting the presence of only two or four types of water have also been suggested.4b Our results, and those of others on ionic and nonionic reverse aggregates,11,17,22b,29,30 related micellar systems,31-33 and theoretical calculations on the molecular dynamics of water,34 indicate that treatment of experimental spectroscopic data in terms of the coexistence of structurally different water layers within the pool is probably an oversimplification. Solubilized water is present as a pseudophase whose properties change continuously as more water is added. There are two alternative interpretations for our IR results (for simplicity, we base subsequent discussion on results shown in Figure 1): (i) The peaks obtained by curve fitting correspond to two types of solubilizates in the pool: namely, (PT)bound at cm-1 2502.7, (PT)bulklike at 2338.8 cm-1, for PT, and (PT-W)bound at cm-1 2515.3, and (PT-W)bulklike at 2358.1 cm-1, for PT-W. (ii) There is one type of solubilizate present, PT or PT-W, this gives rise to the observed main peak. The additional small peak need not be associated with solubilizate molecules present in layers of different structure, as implied by the multistate solubilization model. Independent of interpretation of the peak origin, the dependence of their areas on [PT]/[AOT] or [PT-W]/[AOT] should agree with chemistry. According to the multistate solubilization model, [(PT)bound] and [(PT-W)bound] should (28) (a) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. (b) Maitra, A.; Jain, T. K.; Shervani, Z Colloids Surf. 1990, 47, 255. (c) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5340. (d) D’Angelo, M.; Onori, G.; Santucci, A. J. Phys. Chem. 1994, 98, 3189; (29) (a) El Seoud, O. A.; El Seoud, M. I.; Mickiewicz, J. A. J. Colloid Interface Sci. 1994, 163, 87. (b) El Seoud, O. A.; Okano, L. T.; Novaki, L. P.; Barlow, G. K. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1147. (30) (a) El Seoud, O. A. J. Mol. Liquids 1997, 72, 85 and references therein. (b) El Seoud, O. A.; Novaki, L. P. Prog. Colloid Polym. Sci. 1998, 109, 42. (c) Novaki, L. P.; Pires, P. A. R.; El Seoud, O. A. Colloid Polym. Sci. 2000, 278, 143. (31) (a) Belleteˆte, M.; Durocher, G. J. J. Colloid Interface Sci. 1989, 134, 289. (b) Belleteˆte, M.; Lachapelle, M.; Durocher, G. J. J. Phys. Chem. 1990, 94, 5337. (32) Enders, H.; Nimtz, G. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 512. (33) Lis, L. J.; McAlister, M.; Fuller, N.; Rand, R. P.; Parsegian, V. A. Biophys. J. 1982, 37, 657. (34) Teleman, O.; Joensson, B.; Engstroem, S. Mol. Phys. 1987, 60, 193.

El Seoud et al.

level off after completion of solvation of the headgroup. Consequently, it is expected that areas of the peaks at 2338.8 and 2358.1 cm-1 that we assigned (for sake of argument) to bound solubilizates should reach a limiting value at [PT]/[S] g 2 and [PT-W]/[S] g 6, respectively. On the other hand, areas of the peaks that were attributed to bulklike solubilizates (at 2338.8 cm-1, and 2358.1, respectively) should continuously increase after the abovementioned thresholds of [PT]/[S] and [PT-W]/[S], respectively. Figure 3A,B shows, however, that this is not the case for the following reasons: (i) Areas of the main peaks (and those of the smaller ones; graphs not shown) increase linearly as a function of [solubilizate]/[AOT]. That is, the ratio between the area of each peak and the total peak area is independent of [solubilizate]/[AOT]. (ii) Even within the µE domain, the main peaks are those which presumably correspond to bound solubilizates, not bulklike ones! Therefore, the small Gaussian peaks that were introduced in order to achieve a good curve fit need not be associated with layers of structurally different solubilizates in the pool. Their use is necessary because the νOD peak is asymmetric, in agreement with results of studies of bulk PT-d3 and bulk HOD.15,16 That is, our data are best explained without resorting to the coexistence of layers of PT or PT-W of different structures within the pool. Conclusions Noninvasive techniques, FTIR, and 1H NMR have been employed to investigate solubilization of PT and PT-W by AOT reverse aggregates, and the results were compared with those of solubilization of ED, ED-W, and W by the same surfactant. All solubilizates interact similarly with AOT, namely, by solvating the surfactant head-ions, and the order of solubilizate-AOT interaction is PT > ED > W. Chemical shifts of (water-rich) binary mixtures are dominated by water, and this precludes a clear-cut conclusion on the strength of AOT-binary mixture interactions. Curve fitting of the νOD band requires use of two peaks because the band is asymmetric, not because the solubilizate molecules are present in layers of different structure. This conclusion agrees with those previously reached for solubilization of W and ED by reverse aggregates. Acknowledgment. L.P.N. thanks FAPESP for a postdoctoral fellowship. O.A.E.S. thanks the FAPESP for financial support and the CNPq for a research productivity fellowship. N.M.C. thanks Prof. J. J. Silber for her help, the FAPESP for travel and lodging funds in Sa˜o Paulo, and the CONICET for a postdoctoral fellowship. We thank Prof. P. R. Olivato for making the FTIR spectrophotometer available to us, P. A. R. Pires for acquisition of the 1H NMR spectra, and E. B. Tada for help with the figures. LA001378D