A Study on Surfactant Adsorption Kinetics: Effect of Bulk Concentration on the Limiting Adsorption Rate Constant

Abstract

A limiting adsorption rate constant βl was defined as the value at which the mixed diffusive-kinetic controlled surface tension profile is indistinguishable from the diffusion-controlled one (Hsu et al. J. Chem. Eng. Jpn. 1996, 29, 634). The value of βl depends on surfactant, bulk concentration, the shape of fluid interface, and the process (adsorption or desorption). In this work, the dependence of βl on surfactant and bulk concentration (C) was examined. First, the dependence of βl vs C of eight well-known surfactants was investigated. In the second part, the effects of equilibrium constant (surfactant activity), molecular interaction, and maximum surface concentration were systematically studied. The difference of βl for a planar and a spherical interface with 0.1 cm radius of curvature was also examined. A universal relationship in terms of dimensionless variables was obtained. Using this universal relationship, one needs only the model constants of adsorption isotherm of surfactant to evaluate the value of βl at any concentration and to know the dependence of βl as a function of surfactant concentration.