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Abstract
The volume change resulting from the interaction of two phases is investigated on a thermodynamic basis. This volume change is separated into two parts: one part originates from the inhomogeneity of the density vs. distance, whereas, the other part arises from the inhomogeneity of the mole fraction vs. distance function in the interfacial range of the phases. The latter volume change is analyzed within the framework of Hansen's convention obtaining the Gibbs‐Duhem equation with conditional relative surface excess (CRSE) quantities. The CRSE quantities are introduced as the differences in the corresponding surface excesses between an investigated and a reference system. The shift of the solid/liquid or liquid/liquid interfacial tensions caused by the adsorption of a component is related to the CRSE molar volume and the surface excess concentration of the adsorbing component. The CRSE volume density of the adsorbing component is proposed as a definition for the thermodynamic thickness of an adsorption layer. It is shown that the CRSE molar volume is equivalent to the apparent molar volume that belongs to a component in the adsorbed state. This apparent molar volume can be derived from density measurements on suspensions for adsorption at solid/liquid interfaces. The resulting CRSE molar volume can be used for the calculation of the pressure coefficient of solid/liquid interfacial tension when surface active material is present. The method proposed can be used to estimate the efficiency of surfactants in enhanced oil recovery at high pressure from data at atmospheric pressure.
Acknowledgment
Janet Elliott holds a Canada Research Chair in Interfacial Thermodynamics. Géza Horváth‐Szabó acknowledges the support of Professor Miklós Nagy.