Dynamic Surface Tension of Aqueous Solutions of Ionic Surfactants: Role of Electrostatics

TL;DR

This study investigates how electrostatic interactions influence the dynamic surface tension of ionic surfactants at the air-water interface, revealing a kinetically controlled adsorption process with an electrostatic barrier.

Contribution

It introduces a detailed analysis of electrostatic effects on surfactant adsorption kinetics, highlighting limitations of existing Poisson-Boltzmann models.

Findings

Adsorption is diffusion-limited at short times.

Longer times show a kinetically controlled process with an adsorption barrier.

Poisson-Boltzmann theory does not fully explain the electrostatic barrier.

Abstract

The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully account for the observed potential barrier. The possibility of a surface phase transition is expected from the fitted isotherms but has not been observed by Brewster angle microscopy.