Dynamic surface elasticity of adsorption layers in the presence of a surface phase transition from monomers to large aggregates

Reorganization processes in the adsorbed layer, like the phase transition of the adsorbed molecules from gaseous to liquid state, can play an important role in determining the equilibrium and dynamic behavior of surfactant systems. In fact, a model [Vollhardt D. et al. J. Pkys. Chem. B 2000, 104, 5744] allowing for two possible adsorption states of the molecules, in the two different phases, has been successfully applied to describe the equilibrium properties and the aging of liquid interfaces. The problem of the rheological behavior of an adsorption layer in the presence of phase transition processes is here addressed, which represents a rather important need to increase the insight into the properties of these surfactant systems and to support the experimental investigations. Thus, the main goal of the paper is the calculation of the frequency dependence of the dynamic surface elasticity (or dilational viscoelasticity), within the framework of the phase transition model. The dynamic surface elasticity of the insoluble monolayers, is also derived as a particular case of the general approach given here. According to the results, significant differences exist with respect to the predictions of the classical models. The specific features of the modulus and phase of the surface elasticity allow information about the kinetics of the aggregation process to be derived by experimental data.