Effect of Chemical Composition on Enthalpy of Evaporation and Equilibrium Vapor Pressure

TL;DR

This paper derives a relation based on the Clausius-Clapeyron equation linking the enthalpy of evaporation, component concentrations, and vapor pressure in solutions, highlighting surface layer effects and differences between ideal and non-ideal solutions.

Contribution

It introduces a model that considers the enthalpy of evaporation as a sum of energetic changes across three stages, emphasizing surface layer processes and adsorption effects.

Findings

D(DH) increases with concentration of the second component.

Surface layer processes are the main contributors to enthalpy of evaporation.

Adsorption influences the shape of D(DH) versus concentration curves.

Abstract

Proceeding from the Clausius-Clapeyron equation, the relation is derived that establishes a correlation between the partial enthalpy of evaporation from binary solutions, concentrations of components, and equilibrium vapor pressures. The difference between enthalpies of evaporation of components from solutions and those from the pure liquids, D(DH), depends on the chemical nature and concentrations, X, of solutions. The effect of concentrations on D(DH) makes different appearances in ideal and non-ideal solutions, although, as a whole, D(DH) increases with the growth of concentration of the second component. A model is introduced, which considers D(DH) as the sum of energetic changes of three sequential stages: passage of molecules from the bulk liquid into the surface layer, exit of the molecules on the outer side of the interface, and the following desorption into the gas phase. In the framework of the model, the main contribution to enthalpy of evaporation comes from the processes in the surface layer. It is suggested that adsorption from solutions, which changes the chemical composition of the surface layer with respect to that of the bulk solution, determines, to great extent, the difference in the forms of the curves D(DH)=f(X) for ideal and non-ideal solutions.