EFFECT OF COHESIVE ENERGIES BETWEEN ADSORBED MOLECULES ON SURFACTANT EXCHANGE PROCESSES - SHIFTING FROM DIFFUSION CONTROL FOR ADSORPTION TO KINETIC-DIFFUSIVE CONTROL FOR RE-EQUILIBRATION

Cohesive forces between surfactant molecules adsorbed onto a fluid interface raise the energy barrier for desorption of the adsorbed molecule into the bulk sublayer and correspondingly lower the desorption rate. For adsorption onto an initially clean interface, the reduction in the desorption rate affects the tail-end of the process because it is only at these extended times that the surface concentration becomes high enough for the cohesive energy effect to become important. However, when a concentrated equilibrium monolayer is perturbed, the subsequent re-equilibration is affected by the lower desorption rate during the entire period of the process. In this study these ideas are illustrated 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. For these two relaxation processes, comparative studies are made of the relationship between the relaxation limited only by diffusion and that controlled by both diffusion and sorption kinetics. 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. It is shown that 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.