The state and phase transitions of an adsorbate on a homogeneous solid surface. Part 1.—A model of a two-dimensional associated van der Waals gas and of a two-dimensional phase transition

Abstract

The state of an adsorbate on a mathematically homogeneous surface of a non-specific adsorbent over a wide range of temperature and surface coverage is considered. A model of a two-dimensional associated van der Waals gas is used. Besides the averaged long-distance intermolecular attraction considered in the usual van der Waals gas model, the new model also takes into account the formation of molecular associates at short distances (in a quasi-chemical approximation). These associates are formed not only as a result of the non-specific intermolecular interactions, but also additional specific intermolecular interactions, especially hydrogen bonds and complexing processes depending on the specific structure of the adsorbate molecule. Expressions have been derived for the thermodynamic characteristics of a two-dimensional associated van der Waals gas at temperatures above and below the two-dimensional critical temperature. A method is proposed for the determination of the critical parameters of the two-dimensional state, and the thermodynamic properties of a two-phase, two-dimensional system are considered.