FTIR and 1H NMR Studies of the Solubilization of Pure and Aqueous 1

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FTIR and 1H NMR Studies of the Solubilization of Pure and Aqueous 1,2-Ethanediol in the Reverse Aggregates of Aerosol-OT Luzia P. Novaki,† N. Mariano Correa,‡ Juana J. Silber,‡ and Omar A. El Seoud*,† 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, Universidad Nacional de Rio Cuarto, Agencia Postal Nro. 3, Rio Cuarto 5800, Argentina Received November 17, 1999. In Final Form: April 3, 2000 Solubilization of 1,2-ethanediol, ED, and its aqueous solution, ED-W, by the reverse aggregates of sodium bis(2-ethylhexyl) sulfosuccinate, Aerosol-OT or AOT, in n-heptane and isooctane has been studied by FTIR and 1H NMR. Curve fitting of the νOD bands of the aggregate-solubilized (partially deuterated) ED and ED-W showed the presence of a main peak and a smaller one. The frequency of the former peak decreases, whereas its full width at half-height increases as a function of increasing [solubilizate]/[AOT]. The dependence on the later ratio of 1H NMR chemical shifts, δproton, of solubilized water, ED, ED-W as well as the surfactant discrete protons showed monotonic increase or decrease. Results of both techniques indicated that ED and/or water interact with the surfactant by a similar mechanism, i.e., by solvating its head-ions. The magnitudes of |∆νOD| and |∆δproton| showed, however, that AOT interacts more strongly with ED than with water. Over the entire range of [solubilizate]/[surfactant], the main νOD peak area corresponds to 85 ( 2% (ED), and 88 ( 2% (ED-W) of the total peak area. These results show that the aggregate-solubilized ED or ED-W does not seem to coexist in “layers” of different structures, as suggested by the multi-state water solubilization model.

Introduction One of the most important properties of reverse micelles (surfactant aggregates in nonpolar organic solvents) is their capacity to solubilize water. For example, a clear solution containing 10.80% water and 4.45% of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AerosolOT or AOT) in isooctane can be readily obtained. In the presence of solubilized water, the designation of the aggregates present 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 e the amount necessary to hydrate the surfactant headgroup. Solubilization of water over and above this threshold results in the formation of isotropic and thermodynamically stable W/O µEs (under a specified set of experimental conditions). In addition to water, reverse aggregates solubilize a large variety of substrates, including acids, bases, mono- and polyhydric alcohols, dyes, monomers, polymers, and biopolymers. The use of the micellar “core” as a “micro-reactor” whose size and properties can be tailored to one’s needs has generated much interest.1,2 The (average) solubilization site of a substrate in the reverse aggregate depends on its hydrophilic/hydrophobic character and charge. Thus sulfophthalene dyes adsorb at the W/O interface of AOT, whereas potassium 2-naphthol-6,8-disulfonate is solubilized within the micellar * To whom correspondence should be addressed. Fax: 55-113818-3874. E-mail: 〈[email protected]〉. † Universidade de Sao Paulo. ‡ Universidad Nacional de Rio Cuarto. (1) Eicke, H.-F.; Kvita, P. In Reverse Micelles: Biological and Technological Relevance of Amphiphilic Structures in Apolar Media; Luisi, L. P., Straub, B. E., Eds.; Plenum Press: New York, 1984; p 21. (2) (a) El Seoud, O. A. In Organized Assemblies in Chemical Analysis; Hinze, W. L., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, p 1; (b) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Adv. Colloid Interface Sci. 1999, 82, 189.

water “pool”.3,4 The need to solubilize hydrophobic substrates in the center of the reverse aggregate, and the fact that aqueous solvents are extensively used in organic syntheses and in mechanistic studies of, e.g., acyl transfer reactions, have increased the interest in studying aspects of solubilization of polar solvents other than water, e.g., 1,2-ethanediol (hereafter referred to either as diol or ED), glycerol, or formamide.5-7 Much work is still needed, however, before we clearly understand solubilizatesurfactant interactions and the effects of solubilization on the physicochemical properties of these solvents. We have investigated the solubilization of ED in AOT reverse aggregates in n-heptane and isooctane by FTIR and 1H NMR spectroscopy. The maximum solubility of this diol in AOT aggregates is rather low, two molecules per surfactant monomer. Cosolubilization of water and/ or dodecanoic acid increases the solubility of the diol, but the latter system is complex (5 components), and we have limited our study to 3 component (ED/AOT/alkane), and 4 component systems (ED/W/AOT/alkane). Both spectroscopic techniques have been used to investigate the interactions of ED and/or aqueous diol with the surfactant, whereas FTIR has also been employed to investigate the structure of ED and aqueous ED within the pool. Our results indicate that the ED-AOT interactions are (3) Bardez, E.; Monier, E.; Valeur, B. J. Phys. Chem. 1985, 89, 5031. (4) El Seoud, O. A. Adv. Colloid Polym. Sci. 1989, 30, 1. (5) (a) Fletcher, P. D. I.; Freedman, R. B.; Robinson, B. H.; Rees, G. D.; Schoma¨cker, R. Biochim. Biophys. Acta 1987, 912, 278; b) Schubert, K. V.; Hayes, D. G.; Gulari, E. Langmuir 1995, 11, 4695. (6) (a) Bergenstahl, B.; Jo¨nsson, A.; Sjo¨blom, J.; Stenius, P.; Wa¨rnheim, T. Prog. Colloid Polym. Sci. 1987, 74, 108; (b) Schubert, K. V.; Strey, R.; Kahlweit, M. Prog. Colloid Polym. Sci. 1992, 89, 263; (c) Schubert, K. V.; Busse, G.; Strey, R.; Kahlweit, M. J. Phys. Chem. 1993, 97, 248. (7) (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) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292; (d) Laia, C. A. T.; Lo´pez-Cornejo, P.; Costa, S. M. M.; d’Oliveira, J.; Martinho, J. M. G. Langmuir 1998, 14, 3531.

10.1021/la991503p CCC: $19.00 © 2000 American Chemical Society Published on Web 05/23/2000

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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

System

equationb

A

B

νODd νODd νODd νODd,e areaf

ED/S W/S ED-W/S W/S ED/S W/S ED-W/S ED/S ED/S ED/S ED-W/S ED-W/S ED-W/S W/S W/S W/S

linear linear exponential polynomial linear linear linear polynomial polynomial polynomial polynomial polynomial exponential polynomial polynomial exponential

2534.47 2570.66 2510.54 2569.93 93.31 22.50 12.99 2066.13 2202.08 1628.71 1793.43 2205.76 1607.14 1792.76 2191.52 1577.39

-4.09 -3.45 41.20 -3.45

areaf areaf δWg δH1g δH1′g δEDg δH1g δH1′g δED-Wg δH1g δH1′g

C

166.13 -39.47 -6.97 87.72 -20.70 22.92 80.89 -19.88 36.35

2.42 0.062

-47.70 4.70 -3.88 -7.93 1.96 0 -7.38 1.82 0

D

E

7.86

9.73 1.61 0.37 -0.085 2.27 0.35 -0.079 3.09

-0.0064 0.0014 -0.0061 0.0013

c.c. or ΣQc 0.9988 0.9995 0.793 58.9 0.9996 0.9991 0.9992 11.34 0.17 0.095 250 92.6 0.35 167 0.84 25.3

a Unless stated otherwise, regression analyses were carried out for [solubilizate]/[AOT] from 0.05 to 2.0, 4.0, and 20.0 for ED, W, and ED-W, respectively. b Equations which gave the best regression analyses: exponential ) Exponential decay of the type: Parameter ) A + B exp((-([Solubilizate]/[S])-C)/D); polynomial ) Polynomial equation of the type: Parameter ) A + B ([Solubilizate]/[S]) + C ([Solubilizate]/[S])2 + D ([Solubilizate]/[S])3 + E ([Solubilizate]/[S])4 . c c.c. ) correlation coefficient (for linear correlations); ΣQ ) sum of the squares of the residues (for exponential and polynomial correlations). d νOD ) Stretching frequency of the O-D band of solubilized water, diol, or aqueous diol. e Covering W/S from 0.05 to 20.0, data taken from ref 14b. f Area of the main peak of the O-D band obtained by curve fitting, e.g., Figure 1. g δW, δED, δED-W refer to the 1H NMR chemical shift of the OH group of solubilized water, diol, and aqueous diol, respectively. δH1, δH1′, refer to the 1H NMR chemical shift of protons H1 and H1′ of the surfactant, Figure 5A.

stronger than their water-AOT counterpart, and that the reverse aggregate-solubilized diol and/or aqueous diol do not seem to coexist in layers of different structures. Experimental Section Materials. All chemicals were obtained from Aldrich or Merck. n-Heptane and isooctane were distilled from CaH2, then kept under nitrogen over activated type 4 Å molecular sieves.8 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 showed that the surfactant is free of acidic impurities, which would have greatly reduced the intensity of the solvatochromic B1-band at 502 nm.9 Deuterated 1,2-ethanediol was prepared by stirring 5 mL ED with 6 mL D2O, at room temperature. Water was then distilled off, and the procedure was repeated twice. Its deuterium content, 97.1%, was determined by 1H NMR by using toluene as an internal standard. In the following, we use the symbol ED-d2 to denote DOCH2CH2OD. Methods. The following solubilizate solutions were prepared by volume. FTIR experiments: Water (8% D2O in H2O); pure diol (8% ED-d2 in ED); aqueous diol: 40% diol (containing 4% ED-d2) in H2O (containing 4% D2O). 1H NMR experiments: 80% ED-d2 in ED; 80% D2O in H2O; aqueous diol: 20% diol (containing 80% ED-d2) in H2O (containing 80% D2O). The reason for using partially deuterated solubilizates is explained in the Discussion Section. Stock solutions of the surfactant and/or solubilizate/surfactant were prepared by weight. 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 with variable [solubilizate]/[AOT] were prepared as follows: Stock solutions were pipetted in 1 mL volumetric flasks, these were weighted after each addition, so that the final [solubilizate]/ [AOT] was determined by weight. FTIR. The surfactant solution in n-heptane was 0.20 M. 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). (8) Perrin, D. D.; Armarego, W. L. F. in Purification of Laboratory Chemicals, 3rd. ed.; Pergamon Press: New York, 1988. (9) (a) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1995, 172, 71; (b) Schowen, K. B. In Transition States for Biochemical Processes; Gandour, R. D., Schowen, R. L., Eds.; Plenum Press: New York, 1978, p 225.

The exact path length was determined by the fringe method.10 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 diol or aqueous diol is superimposed on a finite background which could be approximated with the spectrum of pure H2O, ED, or ED-H2O in the νOD spectral region.11 Therefore the reference sample, at each solubilizate concentration, was a surfactant solution containing matched [solubilizate]/ [AOT], adjusted with H2O, ED, or ED-H2O. Deconvolution of the νOD spectral band was carried out by the GRAMS/32 curve-fitting program (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 30.0 °C, by using acquisition parameters which were adjusted to achieve a digital resolution of 0.1 Hz/data point. The spectrometer probe temperature 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 the digital resolution limit. A capillary tube containing D2O was used as a frequency “lock”. Chemical shifts were measured relative to internal TMS.

Results Comparison of the FTIR and 1H NMR Data at Matched [Solubilizate]/[Surfactant]. We wish to compare the results of solubilization of ED, ED-W, and W at the same [solubilizate]/[AOT]. Mixing of the stock solutions to give matched [solubilizate]/ [surfactant] would have been laborious and time-consuming. Instead, we have collected data (e.g., IR frequencies and 1H NMR chemical shifts) for solutions of similar concentration ratios. Appropriate equations were then employed to recalculate the data at the desired (i.e., matched) [solubilizate]/ (10) 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. (11) (a) Mundy, W. C.; Gutierrez, L.; Spedding, F. H. J. Chem. Phys. 1973, 59, 2173; (b) Wiafe-Akenten, J.; Bansil, R. J. Chem. Phys. 1983, 78, 7132; (c) Mikenda, W. Monatshefte Chem. 1986, 117, 977; (d) Lindgren, J.; Hermansson, K.; Wo´jcik, M. J. J. Phys. Chem. 1993, 97, 5254.

FTIR and NMR of Ed-W in AOT Reverse Aggregates

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Figure 1. Representative IR spectra and band deconvolution of the νOD peak of solubilizates in the presence of reverse aggregates of AOT in n-heptane. The figure shows solubilized HOD (part A, W/S ) 2.48), diol (part B, ED/S ) 0.99), and aqueous diol (part C, ED-W ) 17.80).

[surfactant]. The equations employed were those which gave the best data fit, e.g., the smallest sum of the squares of the residues, ∑Q, see Table 1. Note that ∑Q depends on the magnitude of variation of the property that is being measured, i.e., IR frequency or 1H NMR chemical shift. Finally, determination of effects of solubilization on these spectroscopic properties requires comparison of data obtained at two ratios of [solubilizate]/ [surfactant]; we chose 0.05 as the lower limit for this ratio. FTIR and 1H NMR of Solubilized Water, Diol, and Aqueous Diol. Figure 1 shows typical νOD peak of aggregate-solubilized HOD (part A, W/S ) 2.48), pure diol (part B, ED/S ) 0.99), and aqueous diol (part C, ED-W/S) 17.80) along with the corresponding band deconvolution. Although curve fitting was done by considering the contributions from Gaussian and Lorentzian components, our calculations showed that the bands are essentially Gaussian, in agreement with previous work on bulk HOD, bulk ED-d2,11-13 as well as HOD solubilized in RMs and W/O µEs of AOT and of cetyltrimethylammonium bromide.14 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. The corresponding values at [solubilizate]/ [AOT] ) 2.0 were found to be: νOD ) 2564 cm-1 and 2526 cm-1, fwhh ) 137 cm-1 and 146 cm-1 for HOD and the diol, respectively. At [solubilizate]/[AOT] ) 20.0 the corresponding values were found to be: νOD ) 2525 cm-1 and 2515 cm-1, fwhh ) 185 cm-1 and 167 cm-1 for HOD and ED-W, respectively. The latter values are similar to those obtained outside the micellar domain, νOD 2515 ( 10 cm-1 and 2505 cm-1, fwhh 170 ( 10 cm-1 and 168 cm-1, (12) (a) Ha, T.-K.; Frei, H.; Meyer, R.; Gu¨nthard, H. sH. Theor. Chim. Acta 1974, 34, 277; (b) Frei, H.; Ha, T.-K.; Meyer, R.; Gu¨nthard, H. sH. Chem. Phys. 1977, 25, 271. (13) (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. A 1984, 38, 609.

Figure 2. Dependence on [solubilizate]/[surfactant] of ∆νOD () νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 0.05) of the main peak for solubilized HOD (9, curve A) and aqueous diol (2, curve B).

for HOD (in electrolyte solutions)11 and bulk aqueous diol (present work), respectively. Figure 2 shows the change in ∆νOD () νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 0.05), part A ) HOD, part B ) aqueous diol. The data for pure diol are given in Table 2. The linear correlations between [solubilizate]/[AOT] and the corresponding areas of the main peaks are shown in Figure 3 for ED (part A), and aqueous diol (part B). These areas correspond to 85 ( 2%, and 88 ( 2% of the total peak areas, for ED and aqueous diol, respectively. The corresponding graph for HOD is that published elsewhere.14b Figure 4 shows the dependence on [solubilizate]/[AOT] of ∆δproton () δproton at any [solubilizate]/[AOT] - δproton at [solubilizate]/[AOT] of 0.05). Part A shows ∆δ for the OH (14) (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 2. Regression Analysis Results for the Dependence on [ED]/[AOT] or [W]/[AOT] of the IR Stretching Frequencies and 1H NMR Chemical Shiftsa ∆νOD, cm-1( b

[solubilizate]/[AOT] 0.5 1.0 1.5 2.0

∆δOH, Hzc

∆δH1, Hzd

∆δH1′ Hzc

W

ED

W

ED

W

ED

W

ED

-2 -3 -5 -7

-2 -4 -6 -8

34.6 69.8 101.8 130.9

64.2 120.0 166.5 211.1

-8.5 -17.1 -25.0 -32.1

-16.6 -32.8 -46.7 -58.2

-4.9 -9.5 -13.4 -16.7

-3.9 -8.9 -13.4 -16.3

a The symbols used here are those of Table 1. b ∆ν c OD () νOD at any [solubilizate]/[AOT] - νOD at [solubilizate]/[AOT] of 0.05). ∆δproton () δproton at any [solubilizate]/[AOT] - δproton at [solubilizate]/[AOT] of 0.05).

Figure 3. Dependence on [solubilizate]/[surfactant] of the area of the main peak for solubilized diol (part A) and aqueous diol (part B).

Figure 4. Dependence on [solubilizate]/[surfactant] of ∆δproton () δproton at any [solubilizate]/[AOT] - δproton at [solubilizate]/[AOT] of 0.05) of: water (9) and aqueous diol (*, part A), AOT proton H1 (water (9) and aqueous diol (*), part B) and H1′ (water (9) and aqueous diol (*), part C).

groups of the solubilizates, whereas parts B and C show ∆δ for protons H1 and H1′ of the surfactant (see Figure 5A). These surfactant protons showed the largest change in δ. For example, the corresponding ∆δ for H3 and H3′ are -3.6 Hz and -5.4 Hz, respectively, for a change of [ED-W]/[AOT] from 0.52 to 22.1. Discussion Choice of the Solubilizate and System Composition. We used ED because it has been examined as an

analogue to water.15a Indeed, the Kirkwood-Buff integrals for ED-W mixtures (which bear on the strength of waterwater, water-diol, and diol-diol interactions) showed that these two solvents are mutually compatible in terms of their H-bonding structure. The deviation from the “quasi(15) (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, 7359; (d) Prabhu, P. V. S. S.; Ramanamurti, M. V. Bull. Chem. Soc. Japan 1992, 65, 1716.

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Figure 5. (A) Structure of AOT; (B) Rotational isomers of AOT.

ideal” dependence of these integrals on the mole fraction of ED, χED, is only observed in the water-rich composition region, 0 < χED e 0.1.15b,c Additional evidence for the association of ED and water comes from the dependence of the dielectric constant of their binary mixtures on χED.15d This compatibility means that it is safe to discuss the solubilization of ED and aqueous diol in terms of what is known about solubilization of water.2 Co-solubilization of water increases the amount of solubilized diol, and this increase depends on the volume fraction of ED in the starting binary mixture. To determine whether the interactions of ED-W with AOT are noticeably dependent on the composition of the binary mixture, we used two volume fraction compositions, namely, 40% and 20% in the FTIR and 1H NMR experiment, respectively. We now address the reason for employing partially deuterated solubilizates in the FTIR experiments. Conclusions with regard to the types of water present within RMs and W/O µEs are usually based on the number of peaks obtained by curve fitting 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, seems possibly suspect because these bands may originate from coupled water molecule vibrations, and from a bending overtone often reported in the spectrum of liquid water.16a-d 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%.17 This advantage has been recognized both for the bulk aqueous phase,13,17 and for reverse aggregates.14,16,18 Finally, we employed isooctane as a solvent in the 1H NMR experiment because its peaks spread over a narrower δ range than those of n-heptane. Comparison Between the Interactions of Water, ED, and ED-W with AOT. The dependence on [solubilizate]/[AOT] of ∆νOD (Figure 2) and of ∆δOH (Figure 4) is a clear indication that both solubilizates interact similarly with AOT, 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 and the sodium counterion.2 This results in shift of νOD to lower frequencies, and of δOH to lower fields (i.e., away from TMS). Our results show, however, that ED-AOT and ED-W-AOT interactions are stronger than their water-AOT counterparts. At first glance, this (16) (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 Adv. 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; (e) Yoshino, S. N.; Okabayashi, H.; Taga, K.; Yoshida, T.; Kamo, O. Colloid Surf. 1992, 67, 67. (17) (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. (18) Amico, P.; D’Angelo, M.; Onori, G.; Santucci, A. Nuovo Cimento 1995, 17D, 1053.

is a surprising result because ED is less polar and has a lower dielectric constant than water,19 however, the following factors should also be taken into account: (i) The Gibbs free energies of transfer of the sodium ion from water to ED and to ED-W are negative (i.e., this transfer is energetically favorable).19,20 (ii) The dipole moment of ED is greater than that of water,19 and the former solvent has two independent OH groups to bind to the surfactant. (iii) The solvatochromic parameters π*solv (solvent dipolarity/polarizability), Rsolv (solvent “acidity” or H-bond donation ability), and βsolv (solvent “basicity” or H-bond acceptance ability) which are obtained by solving the TaftKamlet-Abboud equation for a single solute in a series of solvents21a are (solvatochromic parameter of water, followed by its counterpart of ED): 1.09, 0.895; 1.17, 0.882; and 0.47, 0.748 for π*solv, Rsolv, and βsolv, respectively, i.e., ED is less acidic and more basic than water.21b-d Therefore, its interaction (as an acid) with the sulfonate group of AOT should be weaker than that of water, whereas the inverse is true for its interaction (as a base) with the surfactant counterion. In solubilization by AOT, the latter interaction is more important than the former,1,2 i.e., the strength of interaction of AOT with a solubilizate depends more on its basicity than on its acidity. (iv) Light scattering measurements have indicated that ED may be acting as a cosurfactant; this leads to a micellar interface which is more fluid than that formed when only water is present.5b,7c,7d The result of (i to iv) seems to favor ED-AOT, and ED-W/AOT interactions over their W-AOT counterparts, as shown by the relative magnitudes of |∆ν| and |∆δ|, vide Table 2 and Figures 2 and 4. Figure 4B and 4C deserves a comment because it shows that H1′ of the surfactant is more sensitive to the nature of solubilizate (i.e., ED-W or water) than H1. The inverse may have been expected because the latter proton is closer to the surfactant head-ions. Figure 5B shows three conformations of AOT at the oil/water interface. In all conformers H1 is in the oil pseudo-phase, whereas the two H1′ protons are mostly in the aqueous pseudo-phase because rotational isomer III predominates (I: II: III ) 0.21:0.04:0.75).16e Therefore, the chemical shift of the former proton may be essentially affected by changes in the electron density of the sulfonate group due to its solvation. Inductive effects also influence the chemical shift of H1′, but there may be a contribution (not possible for H1) from changes in its H-bonding to aqueous diol as [solubilizate]/[AOT] increases. This difference probably (19) Krestov, G. A. In Thermodynamics of Solvation; Ellis Harwood, New York, 1991; pp 151, 153. (20) Marcus, Y. Pure Appl. Chem. 1990, 62, 899. (21) (a) Kamlet, M. J.; Abboud, J.-L.; Taft, R. W. Prog. Phys. Org. Chem. 1981, 13, 485; (b) Bosch, E.; Rose´s, M.; Herodes, K.; Koppel, I.; Leito, I.; Taal, V. J. Phys. Org. Chem. 1996, 8, 403; (c) Rose´s, M.; Buhvestov, U.; Ra`fols, C.; Rived, F.; Bosch, E. J. Chem. Soc., Perkin Trans. 2 1997, 1341; (d) Buhvestov, U.; Rived, F.; Ra`fols, C.; Bosch, E.; Rose´s, M. J. Phys. Org. Chem. 1998, 11, 185.

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makes H1′ a more sensitive probe to the nature of the solubilizate. Structure of Reverse Aggregate-Solubilized Diol and Aqueous Diol. The following question now arises: What is the structure of the micelle solubilized ED and/or ED-W? Before addressing this point it is worthwhile to consider the following which is central to curve fitting of spectroscopic data,22,23 and indeed to any problem whose solution relies on curve fitting:24 quantitative treatment of IR experimental data requires some a priori hypothesis on the origin of the vibrational dynamics of the system under analysis. The suggested model should fit the data accurately (i.e., with the least possible error), and agrees with chemistry. As shown in Figure 1A and 1B, two bands were employed in the curve fitting, however, it is difficult to envisage the existence of 2 layers of water or ED of different structures when the maximum [solubilizate]/ [AOT] is 2.0! The question of presence of layers of solubilizate becomes relevant, however at higher [solubilizate]/[AOT], i.e., when there is sufficient ED and water within the aggregate to hydrate the surfactant head-ions (Figures 2 and 4). In this case, layers of different structures, if present, should be detected by IR spectroscopy because the time scale of this technique (10-12-10-14 s) is faster than the time scale on which water molecules are expected to interchange with each other (between 10-7 and 10-11 s).16d,17a In principle, for W/O µEs, there can be up to three layers within the micellar water pool. 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 Hbonded 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.18,25 Other models depicting the presence of only two, or four types of water have also been suggested.2 Our results,14,26,27 and those of others on ionic and nonionic reverse aggregates,16b,28 and related micellar systems,29,30 and theoretical calculations on the molecular dynamics of water,31 indicate that treatment of experimental data in terms of the coexistence of structurally different water layers within the pool is probably an oversimplification. Solubilized water is present as a pseudo-phase whose properties change continuously as more water is added. There are two alternative interpretations for the results shown in Figure 1C: (22) 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. (23) (a) Maddams, W. F. Appl. Spectrosc. 1980, 34, 245; (b) Vandeginste, B. G. M.; De Galan, L. Anal. Chem. 1975, 47, 2124. (24) Gandour, R. D.; Coyne, M.; Stella, V. J.; Schowen, R. L. J. Org. Chem. 1980, 45, 1733. (25) (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. (26) (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. (27) (a) El Seoud, O. A. J. Mol. Liquids 1997, 72, 85; (b) El Seoud, O. A.; Novaki, L. P. Prog. Colloid Polym. Sci. 1831, 109, 42. (28) (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. (29) Enders, H.; Nimtz, G. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 512. (30) Lis, L. J.; McAlister, M.; Fuller, N.; Rand, R. P.; Parsegian, V. A. Biophys. J. 1982, 37, 657. (31) Teleman, O.; Joensson, B.; Engstroem, S. Mol. Phys. 1987, 60, 193.

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(i) The peaks obtained by curve fitting correspond to different types of solubilizates in the pool, e.g., (ED-W)bound, (or (ED-W)intermediate, at ca. 2530 cm-1) and (ED-W)bulklike (at ca. 2370 cm-1); (ii) There is one type of solubilizate present, hydrated ED; this gives rise to the observed main peak at ca. 2530 cm-1. The additional peak at ca. 2370 cm-1 need not be associated with solubilizate molecules present in a layer of different structure, as implied by the multi-state solubilization model. Independent of the interpretation of the origin of the peaks, the dependence of their areas on ED-W/S should, however, agree with chemistry. According to the multi-state solubilization model, [(ED-W)bound] should level off after completion of solvation of the headgroup, i.e., at ca. ED-W/S ≈ 6. Consequently, it is expected that the area of the peak at ca. 2530 cm-1 should reach a limiting value at ED-W/S ≈ 6. On the other hand, the area of the peak due to (EDW)bulklike at ca. 2370 cm-1 should continuously increase after the above-mentioned ED-W/S threshold. Figure 3B shows, however, that this is not the case because: (i) The area of the main peak (and that of the smaller one, graph not shown) increases linearly as a function of ED-W/S; (ii) A corollary to (i) is that the ratio between the area of each peak and the total peak area is practically independent of ED-W/S; (iii) Even well within the µE domain, the main peak is that at ca. 2530 cm-1, i.e., which presumably corresponds to (ED-W)bound, not (ED-W)bulklike! Therefore, the small Gaussian peak which has been introduced in order to achieve a good curve fit need not be associated with a layer of structurally different solubilizate in the pool. Its use is necessary because the νOD peak is asymmetric, in agreement with results of studies of bulk HOD and ED-d2.11-13 That is, our data are best explained without resorting to the coexistence of layers of ED-W of different structures within the pool. Conclusions We have used noninvasive techniques, FTIR and 1H NMR, to investigate the interactions of solubilized ED and ED-W with AOT reverse aggregates. Both techniques indicate that these solubilizates interact similarly with AOT, namely by solvating the surfactant headgroup, and that ED-AOT and ED-W-AOT interactions are stronger than their water-AOT counterparts. Although curve fitting of the νOD band requires the use of two peaks, the relationship between individual peak areas and ED/S and/or ED-W/S for only one of them agrees with chemistry, Figure 3. The other much smaller peak is needed because the νOD band is asymmetric, i.e., it does not arise from solubilizate molecules present in a layer of different structure. Therefore IR results indicate that treatment of experimental data in terms of the coexistence of structurally different solubilizate layers within the pool is an oversimplification. The changes of slopes of graphs of certain physical properties as a function of increasing [solubilizate]/[AOT], e.g., Figures 2 and 4 may well reflect the expected decrease in the solubilizate-surfactant interactions after completion of the first solvation shell of the headgroup. Our conclusions agree with those previously reached on water solubilization by reverse aggregates.14,16b,26-28 Acknowledgment. L. P. Novaki thanks FAPESP for a postdoctoral fellowship, O. A. El Seoud thanks FAPESP for financial support, and CNPq for a research productivity fellowship. N. M. Correa thanks FOMEC and CONICET for travel funds and a postdoctoral fellowship. We thank Prof. P. R. Olivato for making the FTIR spectrophotometer available to us. LA991503P