  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.
