Dynamics of Adsorption from Micellar Surfactant Solutions at Expanding Fluid Interfaces in Relation to the Emulsification Process

A detailed theoretical model of micellar kinetics is developed and applied to the case of surfactant adsorption at fluid interfaces. The adsorption gives rise to diffusion of surfactant monomers and micelles, and to release of monomers by the micelles. The numerical solution of the problem reveals the existence of four distinct kinetic regimes. At the greatest expansion rates (lowest surface age), the surfactant adsorption is affected by the fast micellar process (regime AB). At smaller expansion rates, the fast process equilibrates and the adsorption occurs under diffusion control (regime BC). With the further decrease of the expansion rate, the surfactant adsorption is affected by the slow micellar process (regime CD). Finally, at the lowest expansion rates, both the fast and slow micellar processes are equilibrated, and the adsorption again occurs under diffusion control (regime DE). For each separate kinetic regime, convenient analytical expressions for the dynamic interfacial tension and adsorption are derived. At low micelle concentrations, “rudimentary” kinetic diagrams are observed, which are characterized by merging or disappearance of the regimes BC and CD. Usually, only one of the kinetic regimes is experimentally detected. The developed theoretical model enables one to identify which of the four regimes is observed in a given experiment, and to interpret properly the obtained data. We applied the model to process available and new data obtained by means of various experimental methods for dynamic interfacial tension. Very good agreement between theory and experiment is achieved. A quantitative criterion is developed, which shows whether a given emulsifier is “fast” or “slow”.