ABSTRACT

Semi-permeable membranes are quasi-two-dimensional barriers which, given a fluid mixture of two or more species of particles on one side, allow the passage either way through the membrane of one or more of the molecular species, in either direction. This chapter considers only binary mixtures, but generalization to multi-component mixtures on one or both sides of the membrane is straightforward. Semi-permeable membranes are used for a wide range of applications. Those based on pressure and/or concentration difference across the membrane include microfiltration, ultrafiltration, and reverse osmosis separation. The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. Most theoretical studies of osmosis and reverse osmosis have been carried out using macroscopic continuum hydrodynamics. The models used include those that treat the wall as either nonporous or porous.