J. Phys. Chem. 1995,99, 12318-12326
12318
Influence of Water Structure on Solvolysis in Water-in-Oil Microemulsions L. Garcia-Rio and J. R. Leis* Departamento de Quimica Fisica, Facultad de Quimica, Universidad de Santiago, 15706 Santiago de Compostela, Spain
E. Iglesias Departamento de Quimica Fundamental e Industrial, Facultad de Ciencias, Universidad de La Coruiia, La Coruiia, Spain Received: May 22, 1995@
The kinetics of solvolysis of diphenylmethyl chloride, 4-nitrophenyl chloroformate, benzoyl chloride, p-anisoyl chloride, and bis(4-nitropheny1)carbonate in water/AOT/isooctane microemulsions with various water/surfactant mole ratios W (AOT = sodium bis(2-ethylhexyl)sulfosuccinate) were interpreted by using a pseudophase model in which the substrates are assumed to be distributed between the isooctane and interface phases. The W-dependence of the intrinsic rate constants k for solvolysis at the interface depends on the solvolysis mechanism: for S N reactions, ~ k decreased with W, which is attributed to decreasing polarity of the interface; contrariwise, S Nreactions ~ are accelerated by decreasing W, which is attributed to increasing nucleophilicity of interfacial water.
Introduction Microemulsions are pseudohomogeneous mixtures of waterinsoluble organic compounds, water, and a surfactantkosurfactant mixture. They are often good solvents for both inorganic salts and nonpolar organic molecules, and therefore are excellent media for reactions between compounds with widely differing solubility characteristics. In anhydrous media, surfactants form small polydisperse aggregate^.^ Added water is solubilized in the polar cores of the micelles, which grow accordingly. In dilute systems, the structure of microemulsions is accounted for by the “droplet m ~ d e l ” which ,~ considers the disperse phase to be formed by monodisperse spherical droplets, each one surrounded by a layer of surfactant molecule^.^-^ Droplet radius depends linearly on the water content of the microemulsion, as confirmed by several experimental techniques8 The relationship for the AOT (sodium bis(2-ethylhexyl)sulfosuccinate)/water/isooctane system, which forms stable microemulsions without a cosurfactant, is R, = 1.3W A.9-1 The interest in the structure of water in microemulsions is due to its possible effects on the reactivity of disolved moleculesI2and the possible application of these simple systems as models of complex biological structures containing trapped water.’h-CThe studies on the structure of water in microemulsions have used numerous techniques: electron solvation measurement^,'^ NMR and ESR spectro~copy,’~ UV-vis spectr~photometry,’~ small-angle neutron, X-ray, and light scattering,’Sl6 fluorescence decay,” microcalorimetry,I8 and IR and Raman spectro~copy,’~ and two or even three water types have been identified. Water added to a nonaqueous solution (W
