On the description of interfacial properties using singular densities and currents at a dividing surface

On the basis of the conservation laws (or balance equations) for the continuously varying densities and currents describing a system with an interface between two bulk phases, interfacial densities and currents at the dividing surface are defined. It is shown that the dynamic behaviour of the interfacial densities is given in terms of the interfacial currents and the extrapolated bulk densities and currents alone. The resulting description is shown to be equivalent to one which uses the conservation laws (or balance equations) for densities and currents which are singular at the dividing surface. The freedom in the choice of the precise location of the dividing surface is discussed.