Dynamics and phase transition in adsorbed monolayers of sodium dodecyl sulfate/dodecanol mixtures

The coadsorption of sodium dodecyl sulfate (SDS)/n-dodecanol mixtures from aqueous solution is theoretically and experimentally studied and compared with the adsorption of the single components. The results of the surface pressure adsorption kinetics are combined with imaging of the surface by Brewster angle microscopy (BAM). First-order phase transition with subsequent formation of condensed phase domains can occur in the adsorbed monolayers of the mixtures over a large range of usual measuring conditions. A simple theoretical model is developed for the description of the adsorption of the surfactant mixtures. The approach is based on the calculation of the surface pressure isotherm of the mixture from the corresponding dependencies of the single components, also including condensation (aggregation) of one component in the adsorbed monolayer. The approach can be used to calculate the dynamic surface pressure for the diffusion adsorption mechanism. The results obtained by the application of the theoretical model are in qualitative agreement with the experimental data. The theoretical results reproduce not only the experimental findings that the condensed phase consists only of dodecanol but also the shift of the phase transition point to essentially shorter adsorption times.