Surfactant adsorption kinetics and exchange of matter for surfactant molecules with changing orientation within the adsorption layer

Recently developed thermodynamic adsorption isotherms are used to derive an improved diffusion controlled adsorption kinetics model for surfactants at liquid interfaces. The isotherms consider molecular reorientation processes within the adsorption layer, controlled by the actual surface pressure. It will be shown that the consideration of interfacial reorientation explains apparently too high diffusion coefficients. Moreover, a simple model for the matter exchange for sinusoidal surface area changes is presented which considers the molecular reorientations as the only relaxation process. As shown previously, the adsorption kinetics of alkyl dimethyl phosphine oxides is influenced by interfacial reorientation. While the lower homologs follow the classical diffusion model, the higher homologs (C-13-C-15) yield diffusion coefficients several times larger than the physically expected values. Assuming two different adsorption states, again reasonable diffusion coefficients result. New adsorption kinetics data for oxyethylated alcohols C10EO8 in terms of dynamic and equilibrium surface (interfacial) tensions at the water-air and water-hexane interfaces are analyzed with the reorientation model. The data agree well with the model derived for molecules, which can exist in two (or more) orientation states.