Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces

TL;DR

This paper develops a free-energy based theory for surfactant adsorption kinetics at fluid-fluid interfaces, explaining diffusive and electrostatic effects, and aligning with experimental observations for both ionic and non-ionic surfactants.

Contribution

It introduces a comprehensive model that unifies diffusive and electrostatic effects in surfactant adsorption kinetics, extending previous models and explaining recent experimental results.

Findings

Diffusion-limited adsorption for non-ionic surfactants.

Electrostatic interactions cause kinetically limited adsorption in ionic surfactants.

Salt addition screens electrostatic effects, restoring diffusion-limited behavior.

Abstract

We present a theory for the kinetics of surfactant adsorption at the interface between an aqueous solution and another fluid (air, oil) phase. The model relies on a free-energy formulation. It describes both the diffusive transport of surfactant molecules from the bulk solution to the interface, and the kinetics taking place at the interface itself. When applied to non-ionic surfactant systems, the theory recovers results of previous models, justify their assumptions and predicts a diffusion-limited adsorption, in accord with experiments. For salt-free ionic surfactant solutions, electrostatic interactions are shown to drastically affect the kinetics. The adsorption in this case is predicted to be kinetically limited, and the theory accounts for unusual experimental results obtained recently for the dynamic surface tension of such systems. Addition of salt to an ionic surfactant solution leads to screening of the electrostatic interactions and to a diffusion-limited adsorption. In addition, the free-energy formulation offers a general method for relating the dynamic surface tension to surface coverage without relying on equilibrium relations.