Adsorption kinetics of soluble surfactants and the phase transition model. 1. Theoretical simulation

Surfactants with strong cohesion can have purely diffusive adsorption and mixed re-equilibration relaxations. This idea is illustrated in this study by theoretical simulations of the relaxation in surface tension due to adsorption onto an initially clean planar interface and re-equilibration of an interface containing a monolayer which has been perturbed. A phase transition model is utilized to describe the adsorption kinetics of soluble surfactants with strong intermolecular attractions between the adsorbed molecules. The cohesion forces lead to the formation of a surface liquid phase, raise the energy barrier for desorption of the adsorbed molecule into the bulk sublayer and correspondingly lower the desorption rate. Novel regions are identified where the surface tension relaxation for adsorption onto a clean interface is diffusion controlled, and that for re-equilibration is mixed. The effects of the bulk concentration, the cohesion energy, and the compression ratio are also reported. Within these regions, adsorption experiments can be done to measure the diffusion coefficient, and re-equilibration experiments can be used to obtain the sorption rate constants.