Molecular theory of transport in fluid microstructures: Diffusion in interfaces and thin films

Gradient theory of equilibrium fluid microstructures is extended to nonequilibrium isothermal processes. The theory is applied to steady diffusion across liquid‐liquid interfaces. It is shown that local equilibrium across a fluid‐fluid interface is maintained even in the presence of very high diffusion fluxes and that the concept of an interface acting as an applied potential energy for a diffusing species is of limited utility. The theory is suited for practical, semiempirical calculations of structure, stress, and isothermal transport in systems containing interfaces.