Significant Structure Theory of Surface Tension

The approximation is made that the dividing surface between a liquid and its vapor phase is a monomolecular layer, in which a molecule has a free volume larger than for an interior molecule and a potential energy less than for the latter. By introducing the approximation into the significant structure theory of liquids, a partition function fN is derived which involves the terms belonging to the surface as well as to the bulk liquid. The surface tension is calculated by γ = (∂A/∂Ω)N,V,T, where A is the Helmholtz free energy which equals —kTlnfN, Ω is the surface area, N the total number of molecules in the system, V the volume of the liquid, and T the absolute temperature. The surface tensions of nonpolar substances at various temperatures are calculated with good results. Our theory is compared with other theories, and it is found that the present theory most satisfactorily predicts the surface tensions at various temperatures.