Adsorption kinetics from concentrated micellar solutions of ionic surfactants at the water—air interface

Abstract

The dynamic surface tension of concentrated ionic surfactant solutions was studied both theoretically and experimentally. Formulae (in agreement with the experimental data) were derived and these establish connections between surface-active monomer concentrations and total surfactant concentration. The problem of the kinetics of adsorption from concentrated micellar ionic surfactant solutions on a freshly-formed surface was formulated and numerically solved. The possibility of an abnormal (increasing) dependence of dynamic surface tension on total surfactant concentration is demonstrated. Also, approximate analytical expressions for dynamic surface tension in the region of long and short surface lifetimes have been derived. Experimental studies on the dynamic surface tension of sodium tetradecylsulphate and hexadecylsulphate in the concentration range up to 200 times the CMC were performed. The maximum bubble pressure method has been improved and a fully automated and computerized instrument for measurements of surface tension and surface lifetimes developed. It has been established that at surfactant concentrations 5–10 times (or more) greater than the CMC, dynamic surface tension does not depend on the total surfactant concentration. Within the scope of existing theoretical knowledge this result agrees with the adsorption mechanism, which is controlled by slow dissociation of micelles and the surface barrier at long surface lifetimes, and also by monomer diffusion and fast micelle dissociation in the region of short lifetimes. Rate constant values for the fast and slow dissociation of micelles, obtained from dynamic surface tension measurements, show good agreement with results of relaxation studies.