Comments on modelling the diffusion-controlled adsorption of surfactants

In this study, dynamic interfacial tension data are obtained with a drop volume tensiometer at the mineral oil-water interface for Triton X-100 dissolved in the water phase, and for Span 80 dissolved in the oil phase. Surfactant adsorption is modelled under diffusion-controlled conditions using either long or short diffusion time approximations. A dimensionless parameter relating adsorption time, t(a), to a characteristic diffusion time of each surfactant, tau(D), is introduced in order to determine the general applicability of the diffusion approximations. In the case of Triton X-100, the long time approximation is valid, and the ratio of t(a)/tau(D) is found to be much greater than one. In the case of Span 80, the short time approximation provides a good fit to the experimental data, and the ratio t(a)/tau(D) is found to be on the order of unity or less. A consideration of experimental data found in the literature for other surfactants leads to the general conclusion that the ratio t(a)/tau(D) is useful in characterising diffusion-controlled adsorption of surfactants.