Molecular orientation as a controlling process in adsorption dynamics

The formation of a surfactant adsorption layer at the liquid/fluid interface is basically described by a diffusion controlled adsorption. When adsorbed molecules undergo changes in their molecular interfacial orientation, the adsorption rate can also change simulating another adsorption mechanism. A generalised model is presented which takes into consideration both the transport of surfactant in the bulk phase by diffusion and rate equations describing the different steps of molecular arrangement at the interface: adsorption/desorption fluxes and the process of reorientation. The most general case results when the characteristic times of all three processes are of the same order of magnitude. Model calculations are used to estimate the effect of the three characteristic times: diffusion relaxation time, transfer rate constant, and reorientation rate constant. Moreover, the influence of the involved physical parameter on the dynamic surface pressure is analysed.