Surface segregation and surface tension of liquid mixtures

A model has been developed in which surfaces are treated as separate phases with a thickness corresponding to a monolayer. It is argued that the surface tension of liquids is a measure of the excess surface chemical potential of the surface atoms relative to the bulk atoms. Equations for the calculation of the surface composition and surface tension of liquid mixtures are developed. Using only the surface tension and molar volume data of the pure components, excellent correspondence between calculated and experimental surface tension values was obtained. The method was also tested on liquid systems showing immiscibility. The surface coverage calculated from the present model is compared with that calculated using Gibbs adsorption equation. The surface coverage of the solute species increases with increasing solute concentration. However, depending on the surface properties of the system, the excess surface coverage may pass through a maximum value and then decrease with increasing solute concentration.