Sieving:

The simplest category of membrane is one in which functions merely as a sieve. If such a membrane has to be effectual, a solvent molecule must be much smaller than the molecules or ions of the solutes and the pore size of the membranes must be intermediate among the two. This condition is simply attainable in the case of solutions whereas the solute is a macromolecule or a polymer while considerable size difference exists among solute and the solvent molecules. Within ultrafiltration and microfiltration, sieving is one of the predominant mechanisms considered. Therefore, no sharp distinction exists among water molecules and small inorganic ions and non-electrolytes and obviously sieving is not the mechanism through that a membrane could retain small inorganic ions and small non-electrolytes and permit permeation of water.