Protolytic equilibrium in lyophilic nanosized dispersions: Differentiating influence of the pseudophase and salt effects

Abstract

Abstract The so-called apparent ionization constants of various acids (mainly indicator dyes) in versatile organized solutions are analyzed. Aqueous micellar solutions of colloidal surfactants and related lyophilic colloidal systems display a strong differentiating influence on the acidic strength of indicators located in the dispersed pseudophase, i.e., non-uniform changes of p K a on going from water to the given system. This concept allows the influence of such media on acid-base properties of dissolved reagents to be rationalized. It is demonstrated that the differentiating phenomenon is the main reason for limitation of the common electrostatic model of acid-base interactions, and is the principal hindrance to exact evaluations of the interfacial electrical potentials of ionic micelles by means of acid-base indicators. Salt effects, i.e., the influence of supporting electrolytes on the apparent ionization constants of acid-base indicators in the Stern region of ionic micelles, are considered. These salt effects can be conventionally divided into two kinds, namely, general ( normal ) and special ( specific ) effects. While the first type adds up to screening of the surface charge, the second one consists in micellar transitions caused by hydrophobic counterions.