 The component that needs to be transported must first be dissolved in the membrane.The general approach of the solution-diffusion model is to assume that the chemical potential of the feed and permeate fluids are in equilibrium with the adjacent membrane surfaces such that appropriate expressions for the chemical potential in the fluid and membrane phases can be equated at the solution-membrane interface.This principle is more important for "dense" membranes without natural pores such as those used for reverse osmosis and in fuel cells.