Reactions in microemulsions. 5. Effect of surface charge and reaction products on a chlorin-sensitized photoreaction

Abstract

The kinetics of the chlorin-(chlorophyll a and pheophytin a) sensitized photoreduction of methyl red by ascorbate has been examined in oil-in-water microemulsions of three surfactant charge types: anionic, neutral, and cationic. In all cases, the reaction remains zero order in oxidant and exhibits saturation (limiting quantum yield) at high reductant concentration. When a pH 7 buffer is employed as the aqueous phase, the limiting quantum yield decreases in the order anionic > neutral > cationic. However, the reverse order obtains for the initial increase of quantum yield with ascorbate concentration. In both cases, the results are ascribed primarily to the effect of surface charge on local concentrations of ionic species (hydrogen ion and ascorbate monoanion), although there does appear to be some contribution from nonelectrostatic effects. One of the products of the chemical reduction of methyl red, N,N-dimethyl-p-phenylenediamine, has been found to increase quantum yield two- to threefold.