Preferential Sorption-Capillary Flow:

Preferential sorption-capillary flow mechanism was the fundamental approach for the practical development of reverse osmosis procedure for converting sea water within potable water. If the surface of the membrane in contact along with the solution is of such a chemical nature which it has preferential sorption for water or preferential repulsion for solutes then a multimolecular layer of preferentially sorbed pure water could exist at the membrane surface. Continuous removal of this interfacial water could then be effected through letting it flow under pressure by membrane capillaries. This concept also defines a critical pore diameter needed on the membrane surface for maximum rate of withdrawal of pure water. This is obviously double the thickness, t, of the interfacial pure water layer. If the pore diameter is higher, after that the rate of withdrawal of water will be higher but the solute separation will be lower because not only the adsorbed pure water but also bulk of the solution will also flow by the pores. If the pore diameter is smaller, a solute separation could be maximum but the rate of permeation will be decreased. The connecting pores in the interior bulk of the membrane from the surface could be and should be bigger. These requirements are necessary from a practical point of view since, the total resistance to the permeating water will be low.