Structure and phase transitions into ionic adsorption layers on liquid interfaces

Abstract

The structure of ionic adsorption layers is studied via a proper thermodynamic treatment of the electrostatic and non-electrostatic interactions between the surfactant ions as well as of the effect of thermodynamic non-locality. The analysis is also applied to phase transitions into the ionic adsorption layer, which interfere further with the oscillatory-diffusive structure of the electric double layer and hydrodynamic stability of squeezing waves in thin liquid films. Adsorption is an important phenomenon, which is responsible for many applications in food and cosmetic industry, flotation, etc. Usually, the adsorption of soluble surfactants on the air/water interface is described via the Langmuir-Blodgett monolayer concept originating from the two-dimensional physics of insoluble surfactants. The complexity of the Gibbs excess quantities leads to additional confusion.1 The Gibbs adsorption is an integral over the concentration in the solution and thus the real distribution of the surfactant could be far away from the monolayer idealization. In the literature, there are many attempts to describe the adsorption of soluble surfactants as a result of specific attractive and repulsive forces between the dissolved molecules and the air/water surface.2 They originate mainly from the interaction with the bulk water and the most important forces are dispersion, electrostatic and image ones. Since the dispersion forces decreases strongly by distance, the calculated adsorption layers are very thin, which corresponds well to the picture of insoluble surfactants. Recently, an excess interaction in thin liquid films was successfully attributed to the so-called adsorption disjoining pressure,3,4 originating from the overlap of two adsorption layers. The latter could not be explained from the classical theory, where