The Effect of Changing the Microstructure of a Microemulsion on

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The Effect of Changing the Microstructure of a Microemulsion on Chemical Reactivity C. Cabaleiro-Lago,‡ L. Garcıá-Rıó,*,† and P. Hervella† Departamento Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Santiago de Compostela, AVenida de las Ciencias s/n. 15782 Santiago de Compostela, Spain, and Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Vigo, Lagoas-Marcosende s/n, 36310 Vigo, Spain ReceiVed April 11, 2007. In Final Form: June 18, 2007 A kinetic study was carried out on various solvolytic reactions in water/ NH4OT /isooctane microemulsions. The NH4OT surfactant is a derivative of the sodium salt of bis(2-ethylhexyl) sulfosuccinate (NaOT or AOT), where the Na+ counterion has been replaced by NH4+. The counterion substitution effects the phase diagram of the system, and therefore, NH4OT-based microemulsions with high water content reaching values of W ) 350 (W ) [H2O]/[NH4OT]) can be obtained. The presence of high W values suggests a transition in the microemulsion microstructure from water-in-oil (w/o) to oil-in-water (o/w), as was confirmed by conductivity and 1H NMR self-diffusion measurements. The interpretation of the kinetic studies in terms of pseudophase formalism allows us to analyze the effect of the microemulsion on chemical reactivity, regardless of its microstructure. It has been confirmed that the values of the solvolytic rate constants at the interphase of oil-in-water microemulsions are similar to those obtained for aqueous SDS systems, showing that the hydration degree of the interphase of the oil-in-water microemulsions is independent of W. The influence of the surfactant counterion on the solvolytic rate constants was analyzed by comparing HOT-, NaOT-, and NH4OT-based microemulsions. An important influence on the rate constants caused by the changes in the structural properties of water has been observed as was confirmed by the water 1H NMR signals.

Introduction Association colloids are excellent systems for observing small changes in specific ions and other additive effects since the small changes in the interaction free energies of the individual amphiphiles are amplified by the enormous cooperativity of amphiphile aggregation and morphological transitions. Many aggregate properties, such as critical micelle concentration, Krafft temperature, phase transition concentrations, and sphere-to-rod transitions, are sensitive to changes in amphiphile headgroup structure, counterion type, and concentration, to the presence of other additives such as alcohols and urea, and to changes in amphiphile hydrophobicity.1-3 The microemulsions formed by the anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (NaOT or AOT) have been widely investigated4,5 as model systems since water-in-oil (w/o) microemulsions (L2 phase) are easily obtained without a cosurfactant.4,6 NaOT is known to form spherical, nanometer-size, molecular aggregates in a variety of nonpolar solvents.7-9 The hydrocarbon tails of the NaOT molecules are oriented toward the exterior of the aggregate, while the sulfonate headgroups * To whom correspondence should be addressed. Fax: +34 981 595012. E-mail: [email protected]. † Universidad de Santiago de Compostela. ‡ Universidad de Vigo. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley & Sons: New York, 2004. (2) Jonsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1998. (3) Romsted, L. S. Langmuir 2007, 23, 414. (4) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochem. Biophys. Acta 1988, 947, 209. (5) (a) Martinek, J. Eur. J. Biochem. 1986, 155, 453. (b) Huang, J. S. J. Surf. Sci. Technol. 1989, 5, 83. (6) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (7) (a) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Thorn-Leeson, D. J. Colloid Interface Sci. 1991, 36, 1. (b) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1994, 98, 12382. (c) Bardez, E.; Vy, N. C.; Zemb, T. Langmuir 1995, 11, 3374. (d) Berghenholtz, J.; Romagnoli, A.; Wagner, N. Langmuir 1995, 11, 1559. (8) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294. (9) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620.

with the sodium counterion are localized in the interior of the microemulsion. Particularly interesting is the ability of NaOT microaggregates to solubilize relatively large amounts of water in their polar core, forming the so-called water pool. The radius of these spherical aggregates has been shown to increase linearly with the molar ratio W ) [H2O]/[AOT].10 Previous studies of the effect of confining metallic cations in the polar core of microemulsions were carried out using surfactants derived from the widely used NaOT.11 These surfactants were prepared by exchanging the Na+ counterion of NaOT for monovalent, divalent, or trivalent ions. The water-in-oil microemulsions stabilized by such surfactants M(OT)n, where Mn+ represents the metal cation and OT- the amphiphilic anion bis(2-ethylhexyl)sulfosuccinate, were shown to exhibit outstanding properties which can be modulated as a function of the nature of the cation and the water content of the medium. The surfactant counterion is responsible for surface electric interactions at the micellar interphase. These interactions can have a strong influence both on the equilibrium shape and size of the micellar aggregates and on their phase diagrams. The ability to change the nature of the counterion in a NaOT microemulsion is a useful and flexible tool for understanding the structural properties of these aggregates. The electrical properties of the micellar interphase can also influence the hydration and intrinsic dynamical properties as has been shown for NaOT and Ca(OT)2.12 The specific identity of the NaOT counterion has an important effect on the structure of the aggregates formed in low-content water-in-oil microemulsions. The effect of replacing Na+ by two different symmetrical quaternary ammonium counterions, (10) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (11) (a) Zhu, X. X.; Bardez, E.; Dallery, L.; Larrey, B.; Valeur, B. New J. Chem. 1992, 16, 973. (b) Giordano, R.; Migliardo, P.; Wanderlingh, U.; Bardez, E.; Vasi, C. J. Mol. Struct. 1993, 296, 265. (12) (a) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Colloid Polym. Sci. 1995, 273, 899. (b) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Phys. ReV. E 1996, 54, 993. (c) Fioretto, D.; Freda, M.; Onori, G.; Santucci, A. J. Phys. Chem. B 1999, 103, 2631.

10.1021/la701051h CCC: $37.00 © 2007 American Chemical Society Published on Web 08/16/2007

Effects of Microstructure on Microemulsion ReactiVity

NH4+ and (C3H7)4N+, has been studied.13-15 The w/o systems with other bimetallic surfactants as Cu(OT)2, Co(OT)2, Cd(OT)2, Zn(OT)2, and Ni(OT)213,14,17-19 consist of spherical droplets at low water content and ellipsoidal or cylindrical objects at high water content, which is attributed14 to a weaker ability of these cations to screen the headgroups. The extensive use of microemulsions as microreactors for aqueous phase reactions10,20,21 requires precise knowledge of the states of the enclosed water. The mobility of water inside NaOT microemulsions is substantially reduced regardless of the counterion. Motion associated with solvation inside microemulsions is reduced both in amplitude and in time, compared to solvation dynamics measured in bulk water22,23 and in dilute or concentrated electrolyte solution. Various properties of the microemulsions impact the water structure and dynamics, including exchanging the normal Na+ counterion in NaOT with other counterions.24-26 Fioretto et al.27 studied Ca(OT)2 and Cu(OT)2 microemulsions using infrared and dielectric spectroscopy. These studies suggest that the water interacts more strongly with the surfactant in the case of doubly charged Ca2+ and Cu2+ counterions than in the case of Na+ counterions.12a,28 Although a variety of experimental methods such as NMR and light and neutron scattering are available for observing changes in bulk properties of organized solutions with changes in solution composition,2 direct observation of interactions between ions, molecules, and water in the interfacial regions of association colloids, biomembranes, and emulsions is very difficult. Many important ions, functional groups, and molecules that associate with interphases in association colloids, in particular water, have weak chromophores or are in low concentration and their signals are difficult to observe. This difficulty makes the utilization of chemical reactions a useful probe, especially for the investigation of interfacial properties. The aim of the present work is to study thoroughly the influence exerted by the surfactant nature and the counterion on the microemulsion properties, focusing especially on those properties that affect chemical reactivity. In this work we study the behavior of the water/ (13) Eastoe, J.; Towey, T. F.; Robinson, B. H.; Williams, J.; Heenan, R. K. J. Phys. Chem. 1993, 97, 1459. (14) Eastoe, J.; Robinson, B. H.; Heenan, R. K. Langmuir 1993, 9, 2820. (15) Eastoe, J.; Robinson, B. H.; Fragneto, G.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1990, 86, 2883. (16) Eastoe, J.; Chatfield, S.; Heenan, R. Langmuir 1994, 10, 1650. (17) (a) Petit, C.; Lixon, P.; Pileni, M. P. Prog. Colloid Polym. Sci. 1992, 89, 328. (b) Eastoe, J.; Fragneto G.; Robinson, B. H.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1992, 88, 461. (18) Eastoe, J.; Steytler, D. C.; Robinson, B. H.; Heenan, R. K.; North, A. N.; Dore, J. C. J. Chem. Soc., Faraday Trans. 1994, 90, 2479. (19) (a) Lisiecki, I.; Andre, P.; Filankembo, A.; Petit, C.; Tanori, J.; GulikKrzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 9168. (b) Lisiecki, I.; Andre, P.; Filankembo, A.; Petit, C.; Tanori, J.; Gulik-Krzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 9176. (20) Casado, J.; Izquierdo, C.; Fuentes, S.; Moya´, M. L. J. Phys. Chem. 1994, 71, 446. (21) Holmberg, K. Curr. Opin. Colloid Interface Sci. 2003, 8, 187. (22) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (23) (a) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature (London) 1994, 369, 471. (b) Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Kahlow, M. A.; Barbara, P. F. J. Phys. Chem. 1988, 92, 7039. (c) Vajda, S.; Jimenez, R.; Rosenthal, S. J.; Fidler, V.; Fleming, G. R.; Castner, E. W. J. Chem. Soc., Faraday Trans. 1995, 91, 867. (24) (a) Pant, D.; Riter, R. E.; Levinger, N. E. J. Chem. Phys. 1998, 109, 9995. (b) Faeder, J.; Albert, M. V.; Ladanyi, B. M. Langmuir 2003, 19, 2514. (25) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062. (26) Harpham, M. R.; Ladanyi, B. M.; Levinger, N. E. J. Phys. Chem. B 2005, 109, 16891. (27) (a) Fioretto, D.; Freda, M.; Mannaioli, S.; Onori, G.; Santucci, A. J. Phys. Chem. B 1999, 103, 2631. (b) Fioretto, D.; Freda, M.; Onori, G.; Santucci, A. J. Phys. Chem. B 1999, 103, 8216. (28) (a) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (b) MacDonald, H.; Bedwell, B.; Gulari, E. Langmuir 1986, 2, 704. (c) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409.

Langmuir, Vol. 23, No. 19, 2007 9587 Scheme 1

NH4OT/isooctane as a medium of reaction. The results may be analyzed in comparison with the previously studied behavior in NaOT-based microemulsions,29 and in HOT-based microemulsions by analyzing the influence of the counterion change (see Scheme 1). For the study of interfacial water properties we used a chemical reaction as a probe. To this end, we chose a chemical reaction where water is a reactant (i.e., a solvolysis reaction). Specifically, we studied the kinetic behavior of substituted benzoyl chlorides. Because the reactions in question exhibit no acid catalysis, any changes in reactivity must be directly related to changes in the physical properties of the water. Moreover, we studied the solvolysis of substituted phenyl chloroformates because the ratelimiting step in this reaction is water addition to the carbonyl group. Experimental Section Materials. The benzoyl chlorides were supplied in their highest commercially available purity by Aldrich and used as received. All were dissolved in isooctane from Aldrich. HOT was prepared from NaOT by ion exchange through Amberlite IR 120 (plus) resin. HOT surfactant was previously obtained as an intermediate in replacing the Na+ counterion.30 The extent of Na+/H+ exchange was assessed by using two methods, namely: (a) atomic absorption spectroscopy, which confirmed the absence of Na+ from the HOT samplesthe residual Na+ content was consistent with a degree of Na+/H+ exchange greater than 99%sand (b) acid-base titration, the results of which were also consistent with an extent of exchange exceeding 99%. NH4OT was prepared by neutralization of the acid with ammonium hydroxide.31 HOT was dissolved in ethanol (Aldrich), and aqueous ammonium hydroxide was then added to the stirred solution until the acid was completely neutralized (pH > 8). Excess ammonia and solvent were then removed using a rotary evaporator at 60 °C for 2 h to leave NH4OT. The product was initially dried in a vacuum oven at room temperature for 48 h and further dried by storage over P2O5 in a vacuum desiccator until required. Microemulsions were prepared by mixing isooctane, water, and 1.0 M NH4OTP/isooctane solution in appropriate proportions. Determination of Phase Diagram. Water-in-oil microemulsions were prepared by adding water to a solution of NH4OT in isooctane in glass vials sealed with screw caps and placed in a water bath at 25.0 °C. Phase equilibria were determined by visual observation, and microemulsion regions were identified as transparent and isotropic solutions. Proton NMR Spectra. 1H NMR spectra of H2O/NH4OT/isooctane microemulsions were recorded with the aid of a coaxial tube filled with DMSO-d6 (Aldrich, 99.9%) to lock onto the deuterium signal. The signals of tetramethylsilane (dissolved in the coaxial tube) were used as 1H NMR references. All spectra were recorded on a Bruker AM 500 MHz spectrometer. Heavy water has been used for selfdiffusion NMR measurements (see below), but H2O has been used (29) (a) Garcıá-Rıó, L.; Leis, J. R.; Moreira, J. A. J. Am. Chem. Soc. 2000, 122, 10325. (b) Garcıá-Rıó, L.; Leis, J. R.; Mejuto, J. C. Langmuir 2003, 19, 3190. (c) Garcıá-Rıó, L.; Hervella, P.; Leis, J. R. Langmuir 2005, 21, 7672. (30) Sheu, E. Y.; Lo Nostro, P.; Capuzzi, G.; Baglioni, P. Langmuir 1999, 15, 6671. (31) Steytler, D. C.; Jenta, T.; Robinson, B. H.; Eastoe, J.; Heenan, R. K. Langmuir 1996, 12, 1483.

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Cabaleiro-Lago et al.

Figure 1. Typical phase diagram of ternary NH4OT/isooctane/water systems under isothermal conditions (25.0 °C). The points correspond to experimental mixtures that are homogeneous on a macroscopic scale. for 1H NMR and kinetic experiments. The accuracy in determining chemical shifts is larger than 0.001 ppm. NMR Diffusion Studies. Pulse field gradient (PFG) 1H NMR experiments were performed on a 200 MHz Bruker DMX spectrometer equipped with a Bruker DIFF-25 gradient probe driven by a Bruker BAFPA-40 unit. All the experiments were carried out at a controlled probe temperature (25.0 ( 0.5)°C in 5 mm NMT test tubes. The self-diffusion coefficients of water, Dw, surfactant, Ds, and isooctane, Do, were obtained using pulse-gradient spin-echo proton NMR (1H PGSE) and followed the recommendations from previous works.32,33 The gradient strengths (G) were changed from 4.82 to 9.63 T m-1, and the gradient pulse duration (δ) was kept constant at 0.5 ms. The diffusion time (the time between leading edges of the field gradient pulses; ∆) was typically 20-100 ms. For molecules undergoing unhindered random motion and for a single species, the attenuation of the signal intensity is given by

[

(

I ) I0 exp -γ2G2δ2 ∆ -

δ D 3

)]

(1)

In eq 1, I denotes the observed intensity, I0 is the intensity in the absence of gradient pulses, γ is the magnetogyric ratio of proton, and the rest of the quantities are defined previously. A typical decay of the intensity versus k (k ) γ2G2δ2(∆ - δ/3)) from a self-diffusion experiment is given in the Supporting Information. In the present sample the main methylene peaks for surfactant and oil overlapped in the proton spectrum (see Figure S-1 in Supporting Information) so the decay of the peak amplitude has to be fit to a biexponential function. Data were fitted to a monoexponential or biexponential decay equation, depending on the number of compounds that contribute to the NMR signal. Water, surfactant, and oil self-diffusion coefficients were calculated using the IKFIT analysis, with at least 10 exponential data points and with an echo decay intensity variation higher than 1 order of magnitude.34 The uncertainty in the fit of eq 1 to the data is, in general, much smaller than 5%. (32) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445. (33) Antelek, B. Concepts Magn. Reson. 2002, 14, 225. (34) Nilsson, M.; Cabaleiro-Lago, C.; Valente, A. J. M.; So¨derman, O. Langmuir 2006, 22, 8663.

The samples were prepared by weighing the proper amounts of isooctane, NH4OT, and a mixture of 95:5 D2O/H2O in NMR tubes, sealed with Teflon caps and left it for equilibration in a water bath at 25 °C overnight. Conductivity Measurements. Conductance (K) measurements (with (0.1% accuracy) were taken at a frequency of 3.8 kHz using a Crison microCM 2202 conductimeter equipped with a cell supplied by Crison (cell constant of 0.997 cm-1). The conductimenter was calibrated using two standard conductivity solutions supplied by Crison ([KCl] ) 0.0100 M, K ) 1413 µS cm-1 at 25.0 °C, and [KCl] ) 0.100 M, K ) 12.88 µS cm-1 at 25.0 °C). The microemulsions were introduced in a receptacle of 50 mL of volume which could be stirred using a Teflon bar with a magnetic nucleus, and with a hermetically sealed lid with two openings through which the electrode was inserted to determine the specific conductivity and a thermometer to determine the temperature at which the sample was found. Water was added in order to vary the W-parameter of microemulsion composition. The water used for the preparation of the microemulsions was distilled and deionized with a conductivity of K ) 0.100.50 µS cm-1. While measurements were being made, the temperature was kept constant by means of a cryostat thermostat Teche TE-8D RB-5 with a precision of (0.1 °C. Kinetic Studies. Solvolysis reactions were followed by monitoring the UV absorbance of substrate solutions, concentration range (1.02.0) × 10-4 M, using a Cary 500 scan UV-vis-NIR spectrophotometer fitted with thermostated cell holders or an stopped flow spectrophotometer with unequal mixing. When the kinetics were carried out in a stopped flow spectrophotometer, the substrate dissolved in isooctane was placed in the smaller syringe (0.1 mL), and microemulsion was placed in the larger syringe (2.5 mL). The wavelengths used for the kinetic studies fell in the range of 250300 nm. The integrated first-order rate expression was fitted to the absorbance-time data by linear regression (r > 0.999) in all cases. The rate constants, kobs, could be reproduced with an error margin of 5%. All the experiments were carried out at (25.0 ( 0.1)°C.

Results Phase Diagram. The phase diagram for the ternary system NH4OT/isooctane/water was determined at 25.0 °C and is given


Figure 2. Electrical conductivity as a function of water content of the microemulsion, W ) [H2O]/[NH4OT], at 25.0 °C. (O) [NH4OT] ) 0.80 M, and (b) [NH4OT] ) 0.60 M.

in Figure 1. As may be seen from the diagram, the monophasic area extends from the binary mixtures NH4OT in isooctane to ternary mixtures with more than 90% (w/w) of water. This area represents a system which is homogeneous on a macroscopic scale only, and it may well contain a diversity of microstructures. Microemulsions consist of an aqueous microenvironment, a lipophilic microenvironment, and a surfactant. The simplest representation of a dilute microemulsion is the “droplet model”. The formulations consist of a low percentage of oil or water in the internal phase, solubilized by a surfactant film. When larger quantities of oil or water are present, a bicontinuous structure is formed in which water and oil should be separated by an interfacial layer. The transition from w/o microemulsion into o/w happens gradually and continuously without any phase separation. Conductivity Measurements. The conductivity of a microemulsion reflects the microstructure of the solution.35 An o/w microemulsion has conductivity in the same range as that for the aqueous phase. The conductivity in the presence of bicontinuous microemulsion is comparable to that of electrolyte solutions and decreases linearly with the decrease of the water volume fraction. When the water content is lowered, the conductivity drops sharply due to percolative phenomena.36 This steep decrease is governed by power laws that are characterized by critical exponents.37 Below the percolation transition (at low water content and in the presence of closed microdomains) the conductivity of a waterin-oil microemulsion decreases with the decrease of the surfactant concentration.38 Figure 2 shows the specific conductivity. For very low W values, W < 1, the conductivity κ ≈ 20 µS cm-1 is much larger than the value corresponding to a w/o microemulsion (0.001-0.1 µS cm-1) and indicates that the percolative phenomena has been reached. On increasing W, the conductivity increases, reaching a maximum value for W ≈ 40 ([NH4OT] ) 0.80 M) and W ≈ 60 ([NH4OT] ) 0.60 M). The presence of a maximum value suggests that the microemulsion microstructure changes with the water content. In the low water content region, a steep rise in conductivity in a narrow zone of increasing the water concentration evidences a special mechanism of conductance of the microdispersions of water in an oil(35) Jonstromer, M.; Jonsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (36) Garcıá-Rıó, L.; Leis, J. R.; Mejuto, J. C.; Penã, M. E. Langmuir 1994, 10, 1676. (37) (a) Bisal, S.; Bhattacharya, P. K.; Moulik, S. P. J. Phys. Chem. 1990, 94, 350. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (38) Eicke, H. F.; Shepherd, J. C. W. HelV. Chim. Acta 1974, 57, 1951.

Langmuir, Vol. 23, No. 19, 2007 9589

Figure 3. Relative self-diffusion coefficients of (O) water and (b) oil as a function of water contents.

continuous medium, which is in agreement with a reverse microemulsion structure. The solutions with high conductivites in the region of the maximum may be bicontinuous microstructures like the one found in the middle phase in Winsor microemulsions. In the high water content region, the decrease in the conductivity with water concentration agrees with the formation of o/w microemulsions where the high percentage of the superficial charge is neutralized. Self-Diffusion Study. Molecular self-diffusion is sensitive to friction, obstruction, and solvation, but in particular, any confinement into closed domains dramatically reduces the longrange molecular displacement, which has been studied in these experiments. The study of solvents’ long-range molecular selfdiffusion is perhaps the most general and straightforward approach to microemulsion microstructure, in particular, for the distinction between droplet and bicontinuous structures. This approach has recently become widely used and has been reviewed and examined in relation to alternative structural approaches, which are mainly scattering techniques and different types of electron microscopy. As we noted in the experimental section for the self-diffusion experiments in NH4OT-based microemulsions we replace H2O by D2O with a 5% of H2O, expecting that only small changes in the phase diagram will result. Experiments were carried out starting with a mixture of 40% isooctane and 60% NH4OT and increasing the water content. The interpretation of the diffusion data in terms of the microemulsion microstructure is generally straightforward.39 The extent to which a solvent forms domains that extend over macroscopic distances is deduced from the relation between a solvent’s self-diffusion coefficient (D) in the microemulsion and that of the neat solvent (D0). For a o/w microemulsion, the relative self-diffusion coefficient for water Dw/D0w is close to unity, and the relative self-diffusion coefficient for isooctane Do/D0o is much less than 1 (see Figure 3), and for a w/o structure, the opposite is true. For bicontinuous structures, high D/D0 values are obtained for both water and isooctane. For high water content, the data suggest an oil-in-water structure since Dw/D0w is close to unity and both surfactant and octane show very low self-diffusion coefficients of similar value (see Table 1). On the contrary, for low water content water diffusion is slow and indicates the formation of a water-in-oil microemulsion. The considerable difference between the self-diffusion coefficient of the water and the surfactant can be explained if we consider a partial distribution of the water in the oil phase.39 (39) Jian, X.; Ganzuo, L.; Zhiquiang, Z.; Guowei, Z.; Kejian, J. Colloids Surf., A 2001, 191, 269.

9590 Langmuir, Vol. 23, No. 19, 2007

Figure 4. Chemical shift of the water signal, in 1H NMR spectroscopy, with W in microemulsions of (b)water/HOT/isooctane, (9)water/NH4OT/isooctane, and (O) and water/NaOT/isooctane.

For the intermediate region the much lower self-diffusion value for the surfactant and the relative high values for water and isooctane suggest a bicontinuous structure where the diffusion of oil and water is not especially hindered. Our results shown in Figure 3 and Table 1 suggest that for water content higher than 70% the microemulsion microstructure should be o/w. This water percentage would correspond to a value of W > 70. Proton 1H NMR Spectra. In this work, we performed a kinetic study on NH4OT microemulsions aiming: (i) to investigate the effect that the microemulsion transition from w/o to o/w might have on chemical reactivity and (ii) to compare the observed behavior in HOT, NaOT, and NH4OT microemulsions in order to analyze the influence of the counterion on the physical properties of water and its effect on chemical reactivity. Changes in the physical properties of water are shown by means of analyzing the resonance signals of the hydrogen atoms of water molecules. 1H NMR spectra were recorded at different W values for HOT,40 NaOT,41 and NH4OT water/surfactant/isooctane microemulsions (Figure 4). 1H NMR spectroscopy is a very useful technique for characterizing the state of water in microemulsions.41,42 The observed 1H NMR chemical shifts result from the weighted average of different water forms since exchange between the different types of water is fast relative to the typical time scale of NMR spectroscopy. As is well-known, we can consider the existence of four different types of water in the microemulsions and that the shift of the water proton signal in the 1H NMR spectrum with W can be helpful in determining the contribution of each type of water to the total signal. Previous studies40 with NaOT and HOT water/surfactant/ isooctane microemulsions have shown that the cation-bound water is predominant in the HOT system, whereas anion-bound water is predominant in the NaOT system. Most of the water present in NaOT microemulsions at low W values interacts with the SO3- groups of the surfactant. Hydration of the anionic headgroups in NaOT molecules increases the electron density on the hydrogen atoms in water molecules and shifts the water signal upfield as a result. As the water content increases, the system contains gradually larger amounts of free water in the microemulsion droplets that are available for hydrogen bonding. This decreases the electron density on the hydrogen atoms and shifts the signal for free water downfield (i.e., to higher δ values) (40) Fernańdez, E.; Garcıá-Rıó, L.; Parajo´, M.; Rodriguez-Dafonte, P. J. Phys. Chem. B 2007, 111, 5193. (41) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (42) Novaki, L. P.; Pires, P. A. R.; El Seoud, O. A. Colloid Polym. Sci. 2000, 278, 143.


Figure 5. Influence of the surfactant concentration on the solvolysis rate constant of 4-MeO in microemulsions of water/NH4OT/isooctane at 25.0 °C. (O) W ) 4, (b) W ) 5.8, (0) W ) 15, (9) W ) 20.8, and (×) W ) 40. Table 1. Self-Diffusion Coefficients for NH4OT/Isooctane/Water Microemulsions at 25.0 °Ca self-diffusion coefficient (D) 1010 m2 s-1 % water

W

Dw

Do

DS

0 10 19 32 46 55 64 68 73 100

0 3.7 7.7 15.5 28.0 40.2 58.5 69.9 89.0

1.28 1.95 4.65 6.02 7.98 10.7 12.7 14.5 19.2

2.2 2.12 2.51 2.56 2.52 2.32 1.74 1.23 0.451 -

0.281 0.489 0.581 0.622 0.674 0.683 0.578 0.3 -

a

Experiments were carried out starting with a 60% isooctane/40% NH4OT mixture and adding different amounts of water.

relative to bound water. As a result, the chemical shift for water changes from 3.9 ppm at low W values to levels close to that for bulk water (4.8 ppm) at high W values. In HOT microemulsions, the affinity of water molecules is stronger for H+ cations than for surfactant headgroups. As a result, the charge donation from the SO3- group of HOT to the interfacial water molecules is smaller than in the case of the NaOT. The charge donation from the water molecules to the H+ cations decreases the electron density on the hydrogen atoms in water molecules and shifts the water signal to higher δ values, this effect being more pronounced on decreasing W. In NH4OT-based microemulsions we observe that the chemical shift of the hydrogen atoms of water molecules is W independent. This behavior should be a consequence of the balance between anion-bound water (the chemical shift decreases on decreasing W) and the cation-bound water (δ increases on decreasing W). Comparison of the 1H NMR obtained for NaOT and NH4OT indicates that the percentage of counterion-bound water is larger with NH4+ than with Na+. As will be shown later, these differences are reflected on solvolytic reactions. Kinetic Study. The counterion change in the NH4+ microemulsion by Na+ has an important effect on the microstructure and physical properties of the interfacial water of the microemulsions. With the objective of investigating the effect of these property changes in chemical reactivity, we have chosen a chemical reaction where water is a reactant (i.e., a solvolysis reaction). Specifically, we studied the kinetic behavior of substituted benzoyl halides, diphenylmethyl chloride, and phenyl chloroformates. The changes in reactivity must be directly related to changes in the physical properties of the water. Two types of experiments were carried out in the kinetic study. (i) We varied


Langmuir, Vol. 23, No. 19, 2007 9591

Table 2. Values of the Distribution Constant for Different Substrates of the Microemulsion, Koi, for Different W Values

the concentration of the surfactant, maintaining the W parameter constant. Thus, water properties do not alter, and we can analyze the effect of the microemulsion composition on the distribution of the reactives and, therefore, on reactivity. (ii) In a second type of experiments we varied the W parameter, and therefore, we varied the properties of the water microemulsion, observing its effect on chemical reactivity. Figure 5 shows the influence of the microemulsion composition on the observed rate constant, kobs, in the solvolysis of 4-MeO (see Table 2 for abbreviations). As can be seen, kobs increases together with the surfactant concentration. This behavior is a consequence of the incorporation of 4-MeO into the microemulsion interphase where the solvolysis process takes place. Benzoyl chloride, chloroformates, and DPhMeCl present little solubility in water so that in microemulsions they will be distributed between the isooctane and the interphase. Upon an increase in the NH4OT concentration, the substrate concentration at the interphase also increases. Given that the solvolysis rate in isooctane is negligible in comparison with the one taking place in the interphase, the hydrolysis rate increases when increasing the percentage of substrate at the interphase. Figure 6 shows the influence of W on kobs in the solvolysis of 4-MeO, 4-Me, and DPhMeCl. These processes take place via a dissociative mechanism43 where the solvation of the outgoing Cl- is a determinant of the reaction rate. As can be observed in all three cases, kobs increases together with the water content. The increase of kobs with W is of approximately 1000 times for DPhMeCl (when W increases from W ) 1 to W ) 260), and of approximately 400 times for 4-MeO and 4-Me (when W increases from W ) 1.5 to W ) 300). As shown later, this behavior is a consequence of the changes produced in the interphase properties by the increase in the water content of the system. On increasing W, a higher degree of hydration causes an increase in the value of kobs. This behavior is typical for solvolytic processes that take place via a dissociative mechanism and is caused by the increase of the solvation capacity of the outgoing group. Nevertheless, the results shown in Figure 6 should be cautiously analyzed since the values of kobs do not correspond directly to the rate constant at the interphase but contain terms corresponding to parameters of the microemulsion composition and of the substrate distribution. When the solvolytic reaction takes place via an associative mechanism,44 the studied behavior differs from that shown in Figure 6. Figure 7 shows the variation of the rate constant observed with the water content of the microemulsion in the solvolysis of PhOCOCl and 4-Cl-PhOCOCl. As can be observed, kobs decreases when W increases until it reaches a minimum value of W ≈ 100 and subsequently increases again. This variation of the rate constant will basically be due to the changes in the nucleophilic character of the interfacial water when the hydration degree is varied. However, as in the case of the results shown in Figure (43) (a) Song, B. D.; Jencks, W. P. J. Am. Chem. Soc. 1989, 111, 8470. (b) Bentley, T. W.; Koo, I. S. J. Chem. Soc., Perkin Trans. 2 1989, 1385. (c) Bentley, T. W.; Carter, G. E.; Harris, H. C. J. Chem. Soc., Perkin Trans. 2 1985, 983. (44) Kyong, J. B.; Yoo, J. S.; Kevill, D. N. J. Org. Chem. 2003, 68, 3425.

6, the values of kobs should be cautiously interpreted since kobs includes parameters of microemulsion composition and distribution constants of the substrates. A similar behavior has been observed for 4-CF3 and 4-Cl.

Discussion The kinetic data for reactions in water-in-oil microemulsions can only be interpreted in terms of reactivity if the local reactant concentrations and the intrinsic rate constants for the different pseudophases in the microemulsion can be obtained from apparent rate constants. Our research group has developed a kinetic model based on the pseudophase formalism that can be used to obtain a quantitative interpretation of the influence of the microemulsion composition on chemical reactivity.45 The pseudophase formalism was appropriate for solvolytic reactions, assuming that the microemulsion consists of three different pseudophases: an aqueous phase (w), a medium essentially consisting of isooctane (o), and an interphase consisting primarily of surfactant (i). On the basis of the solubility of our substrates in water and organic solvents, we assume that they are distributed between the interphase and the continuous medium of the microemulsion (Scheme 2). There is a controversy in the scientific community about the microstructure of water-in-oil microemulsions, mainly at very low water concentrations. In fact, some authors have suggested that for low water concentrations the aggregate microstructure will deviate from the droplet model. Therefore, the pseudophase model that considers the existence of three pseudophases could be wrong. However, recent results from our laboratory studying the effect of AOT-based microemulsions on the reaction between crystal violet and sulfite ion represent irrefutable evidence of the existence of three well-differentiated microenvironments in the AOT-based microemulsions, even at very low water concentrations.46 For the application of the kinetic treatment, the partitioning reagent distribution along the microenvironments of the microemulsions must be faster than the solvolytic reaction rate. The kinetics of solubilisate exchange between water droplets of waterin-oil microemulsions have been widely studied by Robinson et al.,47 Fletcher et al.,48 and Pileni et al.49 Their approach involves an analysis of a reaction in a water-in-oil microemulsion involving reactant species totally confined within the dispersed water droplets, so that a necessary step prior to their chemical reaction is a transfer of reactants into the same droplet. When the chemical reaction is fast (close to diffusion controlled), the overall reaction rate is likely to be controlled by the rate of interdroplet transfer of reacting species. The interdroplet transfer rate was measured as a function of the droplet size, temperature, surfactant, and (45) Garcıá-Rıó, L.; Leis, J. R.; Mejuto, J. C. J. Phys. Chem. 1996, 100, 10981. (46) Fernańdez, E.; Garcıá-Rıó, L.; Mejuto, J. C.; Pe´rez-Lorenzo, M. Colloids Surf., A 2007, 295, 284. (47) (a) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985. (b) Robinson, B. H.; Steytler, D. C.; Tack, R. D. J. Chem. Soc., Faraday Trans. 1 1979, 75, 481. (48) Clark, S.; Fletcher, P. D. I.; Ye, X. Langmuir 1990, 6, 1301. (49) Jain, T. K.; Cassin, G.; Badlali, J. P.; Pileni, M. P. Langmuir 1996, 12, 2408.

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Figure 6. Influence of W on the solvolysis rate constant of (O) 4-MeO, (b) 4-Me, and (0) DPhMeCl in microemulsions of water/ NH4OT/isooctane at 25.0 °C.

Figure 8. Linealization of data from Figure 5 according to eq 3. (O) W ) 4, (b) W ) 5.8, (0) W ) 15, (9) W ) 20.8, and (4) W ) 40. Scheme 2

Equation 2 can be rewritten as

1 1 Z ) + kobs ki kiKoi Figure 7. Influence of W on the solvolysis constant rate of (O) PhOCOCl and (b) 4-Cl-PhOCOCl in microemulsions of water/ NH4OT/isooctane at 25.0 °C.

continuous medium. Exchange rates were determined using very fast chemical reactions as indicators for exchange. Three types of reactions were investigated: proton transfer, metal-ligand complexation, and electron transfer. Similar exchange rates were found for all three reactions. For AOT as dispersant, exchange occurs with a second-order rate constant of 106-108 M-1 s-1, depending on the droplet size and temperature. The exchange rate constants are 2-4 orders of magnitude slower than the droplet encounter rate, as predicted from simple diffusion theory. Kinetic Model. On the basis of this kinetic diagram (Scheme 2), we can obtain the following expression for the pseudo-firstorder rate constant, kobs, according to the composition of the microemulsion:

kobs )

kiKoi Koi + Z

(2)

where ki is the rate constant of solvolysis at the interphase of the microemulsion, Koi is the distribution constant of the substrate between the continuous medium and the interphase, Koi ) ([substrate]iZ)/[substrate]o, and Z is the composition parameter of the microemulsion, which is defined as Z ) [isooctane]/ [surfactant] by analogy with the parameter W. The subscripts o and i refer to the continuous medium and the interphase respectively. The concentrations refer to the total volume of the microemulsion.

(3)

This equation predicts the existence of a linear dependency between 1/kobs and the Z parameter of composition of the microemulsion. Figure 8 shows the realization of eq 3 for the solvolysis of 4-MeO (data shown in Figure 6) in NH4OT-based microemulsions for different W values. For PhOCOCl similar plots are shown in the Supporting Information. On the basis of the ordinates and gradients of Figure 8 we can obtain the values Koi and ki. Table 2 shows the mean values of Koi obtained for various substrates. Values of Koi at different W values for the different substrates are reported in the Supporting Information. For each substrate Koi values are independent of the water content of the system, which is consistent with the fact that the reaction takes place only in the interphase of the microemulsion and the substrates are distributed between the continuous medium and the interphase. The mean values of Koi shown in Table 2 are compatible with those that were previously obtained for microemulsions of water/NaOT/isooctane.29a Equation 4, obtained from eq 2, allows us to calculate the real rate constants at the interphase of the microemulsion, ki. We will employ the mean values of Koi shown in Table 2 and values of the observed rate constants (kobs), such as the ones shown in Figures 6 and 7. Thus, the values of ki gathered in Tables 2 and 3 of the Supporting Information are obtained.

ki )

kobs(Koi + Z) Koi

(4)

An important aspect that must be taken into account is that, in some cases, the rate constants at the interphase present a very significant variation with W. This behavior has been studied


previously in our laboratory and is based on the changes that take place in the water structural properties of the microemulsions when W varies. Another outstanding aspect is that the distribution constant, Koi, does not alter with W, as happens with the rate constant, ki, which may present significant variations (see Tables 2 and 3 in Supporting Information). As exposed by various techniques,22,28a,c,41,50 altering W changes the microviscosity, polarity, and other properties of the system; these changes, however, seem not to affect Koi. The origin of this disparate sensitivity51 must be the differences in the enthalpies of solvent transfer between the reactants and transition states. The smaller degree of sensitivity of the enthalpy of transfer of benzoyl chlorides, chloroformates, or DPhMeCl in comparison with the transition state is the reason why the distribution constants, Koi, are not modified when the reactivity is altered. By analyzing the variation of the rate constants and the microemulsion composition three different behaviors can be identified. The solvolysis rates of DPhMeCl, 4-MeO, and 4-Me decrease together with the water content of the system. The solvolysis of chloroformates, PhOCOCl and 4-Cl-PhOCOCl, just as for 4-CF3, increases when the W parameter of the microemulsion composition decreases. The observed behavior for 4-Cl is intermediate: for low values of the rate constant at the interphase, ki, W decreases on increasing the water content until it reaches the minimum value of W ) 25. Subsequently, ki increases together with W. This difference in behavior lies in the different mechanisms through which the studied solvolytic reactions take place. The mechanism of solvolysis of benzoyl chlorides in water and various solvents is well-known.43 The acyl group is transferred via a dissociative, associative, or concerted displacement mechanism. Benzoyl chlorides with electron-releasing groups react via a dissociative mechanism giving an acylium intermediate; by contrast, electron-withdrawing groups favor an associative mechanism involving a tetrahedral intermediate. 4-MeO and 4-Me can be used as examples of benzoyl chlorides where the solvolytic reaction takes place via a dissociative mechanism preferentially.43 On the other hand, 4-CF3 is solvolyzed preferentially via an associative mechanism. As has been established,52 the analysis of the solvolysis rate constants of diphenylmethyl chloride in a wide range of solvents shows a negligible sensitivity to changes in solvent nucleophilicity but a high sensitivity to the solvent ionizing power. These results are a clear indication that this solvolytic reaction also takes place via a dissociative pathway. On the other hand, the solvolysis of chloroformates has been widely studied in an aqueous medium and in different solvent mixtures, and two mechanisms for the reaction have been proposed: an addition-elimination reaction, where the water addition step determines the reaction rate, and an ionization mechanism.44 The solvolysis mechanism of substituted phenyl chloroformates is well established over a wide range of hydroxylic solvents by an addition-elimination mechanism, with the addition step being rate determining. Thus, we can assimilate the two behaviors into the mechanism through which the reaction takes place. For reactions that are transferred via a dissociative mechanism, the rate-limiting step (50) (a) Venables, D. S.; Huang, K.; Schmuttenmaer, C. A. J. Phys. Chem. B 2001, 105, 9132. (b) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869. (c) D’Aprano, A.; Lizzio, A.; Liveri, V. T.; Aliotta, F.; Vasi, C.; Migliardo, P. J. Phys. Chem. 1988, 92, 4436. (d) Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 2000, 104, 11075. (e) Zhou, G.-W.; Li, G.-Z.; Chen, W.-J. Langmuir 2002, 18, 4566. (51) Garcıá-Rıó, L.; Mejuto, J. C.; Pe´rez-Lorenzo, M. New J. Chem. 2004, 28, 988. (52) (a) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17, 121. (b) Kevill, D. N. AdV. Quant. Struct.-Prop. Relat. 1995, 1, 79.

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of the reaction is the ionization of the C-Cl bond, the solvation of the headgroup, Cl-, being the main factor able to alter the reaction rate. The hydration of the anionic headgroups of the surfactants increases the electron density on the hydrogen atoms in the water molecule, with the consequent breakage of the hydrogen bonds of the normal water. This type of interaction causes a reduction in the electrophilic character of the water and, consequently, an increase in its nucleophilic character.29c The decrease of the electrophilic character of water on decreasing W causes a decrease in its capacity of solvating the negative charge developed on the outgoing Cl-, and therefore, the reaction rate decreases. The behavior studied in the solvolysis of 4-CF3 and chloroformates is very different and is due to the fact that the rate-limiting step of the reaction is the water addition to the carbonyl group. In this case, the variation of the nucleophilic character of water will be the determining factor for explaining the observed kinetic behavior. As noted before, the interaction of water molecules with the anionic headgroups of the surfactant increases its nucleophilic character and, therefore, increases the reaction rate on decreasing W. The Effect of the Structural Inversion. Many microemulsions have homogeneous regions of their phase diagrams that include aggregate structural changes as the bulk compositions of the microemulsions change.53 The main thermodynamic driving force for making oil and water mix into a homogeneous phase comes from the preference for the surfactant to form a monolayer. However, given a certain polar/apolar interfacial area, other more subtle effects determine the preference for one structure relative to another. In our current understanding, the three most important free energy contributions are due to the curvature of the surfactant monolayers, entropies on the aggregate level, and surface forces or aggregate interactions.54 Values for the solvolysis rate constant at the interphase of the system have been obtained for W values comprising between W ) 1 and W ) 350. It is evident that a microemulsion of W ) 350 should have an oil-in-water microstrucure, whereas for low values of W (W < 50), the microstructure corresponds to a water-in-oil system. This inversion of the system microstructure has been proved by determining the diffusion coefficients of water and isooctane and by studying their variation with W. As can be seen from Figure 3, the microstructure corresponds to a water-in-oil system for values of W < 30, to a bilayer for 30 < W < 70, and finally, to a oil-in-water microstructure for values of W > 70. In spite of this microstructure change, the values of the rate constants at the interphase present no discontinuity on varying W that could indicate the above-mentioned transitions. We only observed that for values of W > 100 the rate constant at the interphase, ki, practically does not alter with W. This result is consistent with the fact that the polarity at the interphase of the oil-in-water microemulsions is completely hydrated and a W increase does not cause a major solvation capacity of the outgoing Cl-. It is important to emphasize that the rate constant at the interphase for the solvolysis of 4-MeO presents a high sensitivity to polarity. Thus, ki for the solvolysis of 4-MeO increases 260 times on increasing W from W ) 1.5 to W ) 350. However, for values of W > 100 the rate constant ki scarcely increases from ki ) 0.298 s-1 (for W ) 105) up to ki ) 0.349 s-1 (for W ) 356). Figure 9 represents an example of the influence of W on ki for the solvolysis of 4-MeO and PhOCOCl in NH4OT-based (53) (a) Sjo¨blom, J.; Lindberg, R.; Friberg, S. E. AdV. Colloid Interface Sci. 1996, 65, 125. (b) Binks, B. P.; Espert, A.; Fletcher, P. D. I.; Soubiran, L. Colloids Surf., A 2003, 212, 135. (54) Olsson, U. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: Chichester, 2002; Vol. 2, p 333.

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Figure 9. Influence of W on the values of the solvolysis rate constant at the interphase, ki, of microemulsions of water/NH4OT/isooctane at 25.0 °C. (O) 4-MeO, (0) PhOCOCl. Filled symbols correspond to the rate constant in aqueous micelles of SDS.

microemulsions and aqueous micelles of SDS.55 It is important to emphasize the fact that in the solvolysis of 4-MeO the value of the rate constant obtained at the interphase of NH4OT-based microemulsions for W > 100 is approximately equal to the one obtained at the interphase of aqueous micelles of SDS. This behavior could be considered evidence that the interphase of the oil-in-water microemulsions is completely hydrated once these microemulsions are formed. The behavior studied in the solvolysis of PhOCOCl differs from the one of 4-MeO. For the dissociative mechanism the solvolysis rate constant in aqueous micelles of SDS is approximately equal to the value obtained in water-in-oil microemulsions with lower W values. The solvolysis of PhOCOCl takes place via a step of water addition to the carbonyl group. The reaction rate will be affected by the nucleophilic character of water and by the medium capacity of solvating the negative charge that disperses into the carbonylic oxygen. The positive charge in the nucleophilic water molecule is dispersed into other solvent molecules by hydrogen bonding. The balance between the solvation capacity of the negative charge on the carbonyl oxygen atom and the nucleophilic water attack will be responsible for the observed behavior. It has been discovered that in ethanolwater solvent mixtures the solvolysis rate increases on increasing the ethanol percentage, reaching a maximum value of 60-70% EtOH, and subsequently starts decreasing. On increasing the ethanol percentage, the solvent nucleophilicity, NOTs,52a increases from NOTs ) -0.44 (for 100% water) up to NOTs ) 0.06 (for 100% EtOH). Therefore, if the reaction rate were affected only by the solvent nucleophilicity, it would have to increase gradually on increasing the percentage of EtOH. On the other hand, the solvent ionizing power, YOTs, decreases from 4.1 (for 100% water) to YOTs ) -1.96 (for 100% EtOH), so the rate would gradually decrease on increasing the percentage of EtOH if the solvation of the negative charge on the carbonylic oxygen atom were the only responsible factor. The solvolysis rate constant of PhOCOCl in bulk water, kH2O, has a value of kH2O ) 1.30 × 10-2 s-1, while in aqueous micelles55 of SDS kSDS ) 2.30 × 10-3 s-1. This decrease, a consequence of the incorporation of PhOCOCl into the micelles of SDS, demonstrates that the solvation capacity of the negative charge that disperses into the carbonylic oxygen has a slight effect on (55) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654.


Figure 10. Influence of W on the values of the solvolysis rate constant of 4-MeO at the interphase, ki, of water/surfactant/isooctane microemulsions at 25.0 °C. (O) HOT, (b) NH4OT, and (4) NaOT.

the rate of this reaction. In w/o microemulsions, decreasing W increases the nucleophilic character of water and decreases the water capacity of solvating the negative charge that disperses into the carbonylic oxygen. The dominating effect is the increase of the nucleophilic character of water, and thus, the reaction rate increases on decreasing W (kW ) 1.5 ) 2.4 × 10-3 s-1 and kW ) 100 ) 7.6 × 10-4 s-1). For high W values, two factors cause the decrease of the rate constant at the interphase of the microemulsion relative to the value of water: a lower capacity of solvation of the negative charge on the carbonylic oxygen and a lower nucleophilic character of water. When passing from a w/o microemulsion to an o/w there is a high excess of water capable of solvating the negative charge that disperses into the carbonylic oxygen, which causes a slight increase in the reaction rate. Influence of the Counterion on Chemical Reactivity. The results obtained in this work, as well as other results previously obtained in our laboratory,32c,43,56 allow us to analyze the influence of the nature of the counterion on solvolysis reactions. Figure 10 shows the influence of the microemulsion composition on the solvolysis rate constant of 4-MeO in water/surfactant/isooctane microemulsions of HOT, NaOT, and NH4OT. As can be observed, in all cases ki increases on increasing the water content of the system as a consequence of the greater capacity of solvation of the outgoing Cl-. However, it is well-known that, for low W values, the rate constant is higher in HOT than in NH4OT and NaOT. From W ≈ 10, the rate constant is practically independent of the counterion nature. Moreover, on increasing the water content of the system, the water properties of the microemulsion are almost similar to those of normal water. Therefore, on increasing W, the water properties in the microemulsion become independent of W, and accordingly, the rate constant should be insensitive to the counterion nature for high W values. The strong enhancement of ki with W demonstrates that the electrophilic solvation of the outgoing group, Cl-, is mainly responsible for the energy barrier for this reaction. Thus, the differences of reactivity depending on the surfactant counterion must be related to its capacity of altering the electrophilic character of the interfacial water. The 1H NMR chemical shifts of water hydrogen atoms (Figure 4) indicate that the interaction H+‚‚‚ OH2 is stronger than NH4+‚‚‚OH2 and Na+‚‚‚OH2, causing the electrophilic character of water to follow the sequence HOT > NH4OT > NaOT. This gradation of the electrophilic character fits the sequence of reactivity observed for low W values. Also (56) Fernandez, E.; Garcıá-Rıó, L. ChemPhysChem. 2006, 7, 1888.


Langmuir, Vol. 23, No. 19, 2007 9595

earlier, these water-counterion interactions dictate the electrophilicity of the medium (see Scheme 3, right). HOT microemulsions provide a more electrophilic medium than NaOT. As a result, solvolysis reactions taking place via an associative mechanism are invariably slower in HOT than in NaOT or NH4OT, and the rate constant decreases on decreasing W.

Conclusions

Figure 11. Influence of W on the value of the solvolysis rate constant of 4-CF3 at the interphase, Ki, of water/surfactant/isooctane microemulsions at 25.0 °C. (O) HOT, (b) NH4OT, and (4) NaOT. Scheme 3

worth considering here is the fact that in the case of NH4OT a second route of reaction is possible where the solvation of the outgoing group is assisted directly by the counterion, as it has been previously confirmed in microemulsions of NH4DEHP.57 Figure 11 shows the behavior observed when comparing the three studied microemulsions using the solvolysis of 4-CF3 as a probe. For NaOT- and NH4OT-based microemulsions the rate constant at the interphase increases on decreasing the water content of the system due to the enhancement of its nucleophilic character. In both cases, the predominant interaction, as confirmed by the results of 1H NMR (Figure 4), takes place between the headgroup of the surfactant and the water molecules. This interaction (Scheme 3, left) causes an increase of the electronic density on the water molecule and, consequently, an enhancement of its nucleophilic character. Due to the decrease of the water molecules available for solvation, the electronic density will increase on decreasing W and, therefore, will increase the nucleophilicity of water on decreasing the size of the microemulsion droplet. In water/HOT/isooctane microemulsions, the strong interaction between water and H+ ions prevails at any drop size. As noted (57) Garcıá-Rıó, L.; Hervella, P.; Rodriguez-Dafonte, P. Langmuir 2006, 22, 7499.

The use of a surfactant derived from aerosol OT (AOT or NaOT), where the counterion Na+ has been replaced by NH4+, has allowed us to widen the range of compositions of microemulsion stability, since a continuous transition from water-inoil microstructure to oil-in-water is observed. The reactivity of varied solvolytic processes in NH4OT-based microemulsions that take place via associative and dissociative mechanisms has been studied. The obtained results allow us to confirm the validity of the pseudophase formalism independently of the system microstructure, since no discontinuity has been observed when changing the microstructure from w/o to o/w. The comparison of the kinetic results for the microemulsions prepared with derived surfactants of aerosol OT, where the counterions are H+, Na+, and NH4+, indicates that the electrophilic character of water follows the sequence HOT > NH4OT > NaOT as a consequence of the decrease of the strength of the interaction M+‚‚‚OH2. The use of NH4OT-based microemulsions has allowed us to investigate the influence of water content in the microemulsion on the solvolytic processes when passing from a w/o microemulsion to an o/w one. The results demonstrate that the degree of solvation at the interphase of an o/w microemulsion does not practically alter with the molar ratio, W, being very similar to the hydration presented by the aqueous micelle systems. When studying the solvolysis of 4-MeO, it is observed that the rate constant at the interphase, ki, is practically independent of W for W > 100 and is similar to the value observed in aqueous micelles of SDS. The solvolysis of PhOCOCl shows a different behavior, and ki increases on increasing the water content of the system for values of W > 100 tending to reach the value obtained for micelles of SDS. This behavior of PhOCOCl is a consequence of the balance between the different factors responsible for the energy barrier for the solvolysis process, such as the nucleophilic and electrophilic character of water. Acknowledgment. Financial support from Ministerio de Ciencia y Tecnologıá (Project CTQ2005-04779) and Xunta de Galicia (PGIDT06-PXIC209033PN and PGIDIT04TMT209003PR) is gratefully acknowledged. Supporting Information Available: 1H NMR spectrum of a NH4OT/isooctane/water microemulsion (Figure S-1). Echo decays (Figure S-2). Surfactant concentration vs solvolysis rate constant (Figure S-3). Linearized version of Figure S-3. Tables of distribution constants and W and ki values. This material is available free of charge via the Internet at http://pubs.acs.org. LA701051H

The Effect of Changing the Microstructure of a Microemulsion on

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