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Langmuir 1997, 13, 3084-3089
Articles Micellar, Microemulsion, and Salt Kinetic Effects upon the Reaction Fe(CN)2(bpy)2 + S2O82F. Sa´nchez, M. L. Moya´, A. Rodrı´guez, R. Jime´nez, C. Go´mez-Herrera, C. Ya´nes, and P. Lo´pez-Cornejo* Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Sevilla, c/Profesor Garcı´a Gonza´ lez s/n, 41012 Sevilla, Spain Received October 15, 1996. In Final Form: March 26, 1997X The reaction of oxidation of Fe(CN)2(bpy)2 with S2O82- has been studied in micellar solution of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in AOT/decane/water microemulsions and in NaNO3 solutions. It is shown that the Bro¨nsted equation is an alternative to the pseudophase model to rationalize the observed kinetic effects in all the media. According to our results, in the range of composition used in this work for the microdisperse systems, microemulsions behave as concentrated salt solutions and micellar systems as dilute electrolyte solutions.
Introduction The study of chemical reactivity at interfaces occupies an important place in chemistry.1 Electron transfer, ion transfer, and proton transfer at the interfaces between two immiscible phases are fundamentally important in understanding processes such as liquid chromatography, phase transfer catalysis, drug delivery problems in pharmacology, and different phenomena in membrane biophysics. The uptake of pollutants by water clouds, an important atmospheric phenomenon, involves reactions such as ionization at the water liquid/vapor interfaces. Microdisperse systems such as micelles and microemulsions are of interest due to their application in a large number of fields2-5 such as cosmetics, solar energy conversion, production of semiconductor microcolloids, biological systems, etc. Besides, these systems have an additional interest because they affect the kinetics of reactions,6-11 especially when the reactions take place at the interface of these media. Up to now, the pseudophase model has generally been * Author to whom the correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Benjamin, L. Chem Rev. 1996, 96, 1449. (2) Fendler, J. H. Membrane Mimetic Chemistry; Academic Press: New York, 1982. (3) (a) Fletcher, P. D. I.; Robinson, B. H. J. Chem. Soc., Faraday Trans. I 1985, 81, 2667. (b) Bergsto¨m, K.; Holmberg, K. Colloids Surf. 1992, 63, 273. (4) (a) Hatton, T. A. ACS Symp. Ser. 1987, 342, 170. (b) Genkin, M. V.; Davydov, K.; Blovolova, L. H.; Yuschishina, A. N.; Krylov, O. V. J. Mol. Catal. 1989, 56, 249. (5) Lo´pez-Quintela, M.; Rivas, J. In Structure, Dynamics and Equilibrium Properties of Colloidal Systems, Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Pub.: Dordrecht, 1990. (6) Bunton, C. A. In Kinetics and Catalysis in Microheterogeneous Systems, Gratzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker, New York, 1991; vol. 38, chapter 2. (7) Bravo, C.; Herve´s, P.; Leis, J. R. J. Phys. Chem. 1990, 94, 8816. (8) Chaimovich, H.; Aleixo, F. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, F. H. In Solution Behaviour of Surfactants: Theoretical and Applied Aspects, Mittal, K. L., Fendler, E. J., Eds.; Plenum Press, New York, 1982; vol. 2, p 949. (9) Pelizzetti, E.; Fisicaro, E.; Minero, C.; Sassi, A.; Hidaka, H. J. Phys. Chem. 1991, 95, 761. (10) (a) Iglesias, E.; Leis, J. R.; Pen˜a, M. E. Langmuir 1994, 10, 662. (b) Iglesias, E.; Montenegro, L. J. Chem. Soc., Faraday Trans. 1995, 91, 1349. (11) (a) Garcı´a-Rı´o, L.; Leis, J. R.; Iglesias, E. J. Phys. Chem. 1995, 99, 12318. (b) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko´, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673.
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used to explain reactivity in microdisperse systems. This model, as will be shown later, is the counterpart for micellar systems of the Olson-Simonson model, used in the interpretation of kinetic salt effects. Both the pseudophase and the Olson-Simonson models emphasize the association of the reactants with the micelles and the counterions of the supporting electrolyte, respectively. However, as was shown by Scatchard,12 the OlsonSimonson effects can be described taking as a starting point the more general Bro¨nsted equation, which can be derived directly from the transition state theory, once the activity coefficients are adequately calculated (see below). So, given the similarities between the pseudophase and Olson-Simonson models, it seems logical to suppose that the Bro¨nsted equation could also be useful in the interpretation of effects observed in microdisperse systems. Our goal is to show that, as well as the pseudophase model, a treatment, starting from the Bro¨nsted equation, also permits the explanation of kinetic trends observed in both micellar and microemulsion systems. In fact, as will be seen below, taking this equation as a basis, it is shown that microemulsions and micelles formed by ionic surfactants act, from a kinetic point of view, in a similar way as electrolytes do. In this paper, the results corresponding to the oxidation of Fe(CN)2(bpy)2 (dicyanobis(2,2′-bipyridine)iron(II)) by S2O82- (peroxodisulfate ion) in aqueous micellar solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and AOT/ decane/water microemulsions are presented. This reaction was previously studied in concentrated salt solutions.13 Experimental Section Materials. Sodium bis(2-ethylhexyl) sulfosuccinate (Merck, purity > 99%) was stored in a vacuum desiccator over P2O5 for several days before use. Sodium peroxodisulfate was from Carlo Erba (P.A.). Dicyanobis(2,2′-bipyridine)iron(II) trihydrate was prepared by the method from the literature14 and the purity checked by HCN analysis. Decane (Merck), used as the oil phase was dried and stored over molecular sieves (4 Å), which had been activated by heating at 200 °C under reduced pressure for 24 h (12) Scatchard, G. Natl. Bur. Stand. Circ. (U.S.) 1953, No. 524-185. (13) Mu´n˜oz, E.; Graciani, M. M.; Jime´nez, R.; Rodrı´guez, A.; Moya´, M. L.; Sa´nchez, F. Int.J. Chem. Kinet. 1994, 26, 299. (14) Schilt, A. A. J. Am. Chem. Soc. 1960, 82, 3000.
© 1997 American Chemical Society
Micellar, Microemulsion, and Salt Kinetic Effects and then cooled in a vacuum over a silica gel. NaNO3 used as background electrolyte was from Merck (P.A.). Tris(2,2′-bipyridine)ruthenium(II) chloride from ICN Biomedicals Inc. was used without further purification. 9-Methylanthracene from Merck was purified by sublimation. Preparation of Sample Solutions. AOT/decane/water microemulsions were prepared by dissolving AOT in the corresponding organic solvent and a further addition of different amounts of aqueous solution of the iron complex and of the peroxodisulfate ions. Then, these mixtures were shaken to obtain perfectly clear water/oil microemulsions. All the samples were prepared by weight. AOT micelles were prepared by dissolving AOT in water and then adding the quantity of solid iron complex and Na2S2O8 in order to get the desired concentrations. Determination of Critical Micellar Concentrations (cmc). In order to obtain the cmc value of AOT in micellar solution, three different methods were used: (a) Surface tensions were measured in the presence and absence of the reactants using a platinum plate attached to a KRU ¨ SS Digital Tensiometer K10 (Kru¨ss Gmbh, Hamburg, Germany). Prior to each measurement, the plate was heated briefly until glowing by holding it above a Bunsen burner. The vessel was cleaned using chromic sulfuric acid, boiled for a prolonged period in distilled water, and then flamed with a Bunsen burner before use. The precision in the measurement was (0.1 dyn cm-1. (b) Densities of the solutions were measured with an Anton Paar vibrating-tube densimeter (622 DMA). The densimeter was calibrated with water (F ) 0.997 045 g cm-3) and n-nonane (F ) 0.713 850 g cm-3). The uncertainty in density values was (0.0001 g cm-3. The temperature (298.15 K) was regulated through a cascade water bath apparatus (Heto) with a stability within (0.01 K and checked by a digital thermometer (Anton Paar DT 10020). Density values have an uncertainty of 6 × 10-6 g cm-3. (c) Refractive indexes were measured using an Abbe refractometer (Atago) connected to a water flow thermostat which was maintained at constant temperature (298.1 ( 0.1 K). The uncertainty in the refractive indexes was (10-4. Fluorescence Quenching Measurements. Aggregation numbers of AOT micelles in water were measured from changes in the fluorescence intensity of Ru(bpy)32+ by addition of 9-methylanthracene (MA) as the quencher, at excitation and emission wavelengths of 450 and 630 nm, respectively, using a Perkin Elmer 650-40 spectrofluorometer. The concentrations of Ru(bpy)32+ and MA were 1 × 10-5 and (1.3-33) × 10-5 mol dm-3, respectively. Kinetic Measurements. All the kinetic runs were carried out in a Hitachi 150-20 UV-visible spectrophotometer. The reaction studied was monitored by following the changes in absorbance of the Fe(CN)2(bpy)2: at 520 nm in micelle and aqueous salt solution and at 530 nm in AOT/decane/water microemulsions. In all the experiments, temperature was maintained at 298.1 ( 0.1 K. The reaction in the micellar systems showed the same rate law as in the aqueous solution,13 first-order with respect to each of the reactants (see Figure 1). The process was studied under pseudo-first-order conditions, using an excess of peroxodisulfate ions in all runs. The secondorder rate constants in microemulsions were always obtained dividing kobs by the aqueous peroxodisulfate concentration. For these systems, the second-order rate constants must be used in the discussion (but see below) since solubility problems of the reactants in these media15 do not permit to use the same reactant concentrations in all the experiments. Since these problems do not appear in the salt and micellar solutions, it is possible to use the pseudo-first-order rate constant directly. All kinetics were repeated at least three times. The uncertainty in the rate constant was estimated to be less than 2% for micellar and salt solutions and less than 5% for microemulsions. In microemulsions, the iron(II) complex and S2O82- concentrations used were within the range (5-10) × 10-5 and (1-3) × 10-2 mol dm-3, repectively. (15) Blandamer, M. J.; Burgess, J.; McGowan, J. C. J. Chem. Soc., Dalton Trans. 1980, 616.
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Figure 1. Plot of log(ko/s-1) values versus log([S2O82-]/mol dm-3) in AOT micelles at [surfactant] ) 5 × 10-3 mol dm-3 (O) and AOT/decane/water microemulsions at Wo ) 10 and [surfactant] ) 0.3 mol dm-3 (0). The concentrations of the reactants in micellar and salt solutions were [Fe(CN)2(bpy)2] ) 1 × 10-4 mol dm-3 and [S2O82-] ) 3.5 × 10-3 mol dm-3. All these concentrations refer to the aqueous phase. Calorimetric Measurements. In order to have some information on the micelles-reactants interactions, calorimetric measurements were carried out. For this purpose a Thermometric (TAM 2277) flow microcalorimeter was used. The work temperature was 298.15 ( 0.01 K. To calculate the enthalpy corresponding to the interaction of the Fe(CN)2(bpy)2 with the micelles, the enthalpy of dilution of AOT in micellar solutions with water and with a solution containing the iron complex were measured. For the injection of the solution, a peristaltic pump with two channels (Gilson, Minipuls 3) connected to the microcalorimeter was used. The flow rate of each pump was controlled by weight (∼0.004 cm3 s-1). The molar enthalpies of dilution and mixing (J mol-1) were obtained by the following expression:
f ∆H ) (w - wo) m fF
(1)
where w and wo are the thermal effects, expressed as power in watts, corresponding to the experimental measurement and the baseline produced by two identical liquids, respectively. f is a correction factor for lost heat, mf the final concentration in moles per kilogram of the solute and F the flow rate expressed in kilograms per second at the outlet of the microcalorimeter. The calorimeter was calibrated electrically for each enthalpy measurement. The uncertainty of the molar enthalpy was
