THE EFFECT OF BULK CONCENTRATION ON SURFACTANT ADSORPTION PROCESSES -THE SHIFT FROM DIFFUSION-CONTROL TO MIXED KINETIC DIFFUSION CONTROL WITH BULK CONCENTRATION

The surface tension relaxation at interfaces between surfactant soluti ons and immiscible fluids is calculated bg the rate that surfactant ad sorbs in a two-step process: surfactant in the sublayer adsorbs, which , in turn, establishes a diffusive flux from tile bulk. Therefore, bot h the adsorption and diffusion rates determine the behavior. These rat es are commonly determined by minimizing the difference between experi mental surface tension vs. time profiles and mass transfer model predi ctions. Usually, only a limited range of bulk concentrations C-0 is st udied, and the profiles are found to he in agreement with a diffusion- control model, It is argued here that such apparent agreement is insuf ficient evidence to disregard the role of tile adsorption/desorption k inetics in the adsorption process. In this paper, the concept of a shi ft in controlling mechanism from diffusion control at dilute concentra tion to mixed diffusion-kinetic control at more elevated bulk concentr ation is explored. This idea is illustrated theoretically for clean in terface adsorption on a pendant bubble or drop using the Langmuir adso rption model in dimensionless form. It is further developed using the model constants for C(12)E(8).