Mechanism of pore formation in reverse osmosis membranes during the casting process

Abstract

The process of pore formation during the casting of reverse osmosis membranes is analyzed. The process consists of two steps. The first step is the evaporation step, where the cast polymer solution is allowed to dry for 1 -100 s. The second step is the gel formation step, where the cast is soaked in water leaving behind the membrane in form of a gel. The evaporation step gives rise to a thin (-0.1 gm)skin of high density and very small pores which is chiefly responsible for desalination. The gel forms the backing (-100 to 250 pm) and contains large pores.It is shown that low evaporation rates accompanied by shrinkage during evap oration gives rise to an instability leading to the formation of the skin region. The evaporation effect is fast, is confined to the skin region, and gives rise to very small pores. The gel formation is shown to be a very slow process which cannot interfere with the skin formation due to the vast differences in their rates of formation. It also gives rise to larger pores. All key features of the above experimental observations are explained.The kinetics of the process depend on the diffusion coefficients D and Dp of the solvent and the polymer. However, the main factor is the solution chemistry of the polymersolvent system which controls both the effectiveness of the skin and the gel formed. For the first time, the relevant thermodynamic parameters which determine the extent and sizes of pore formation have been obtained. P. NEOGI Department of Chemical EngineeringUniversity of Missouri Rolla, MO 65401 SCOPESeparation with membranes is not new, but it still holds promise for wider use in the future. Only asymmetric polymeric membranes are discussed here. Reverse osmosis membranes are made of ionizable polymers like cellulose acetate. In presence of water, the walls of the membrane pores electrify, setting up an electrical field inside the pores. Under these conditions, it is possible to hinder the transport of ions through these pores, and at the same time allow the passage of water. Desalination is thus effected, the success of which depends on the ability of the polymer to electrify as well as on the sizes of the pores. The latter also plays a crucial role in ultrafiltration since the pores do not permit the passage of particles larger than their diameters. Filtration or separation of macromolecules, biological cells, and colloidal particles are thus possible. Consequently, membranes find important application in separation of proteins and enzymes, particularly as these materials are heat-sensitive and the process requires neither heating nor cooling. They also find application in biomedical engineering like in fabrication of artificial kidneys. It is noteworthy that these membranes often have very high separation or rejection efficiencies.It is evident that the separation takes place due to the existence of pores of desirable radii. Although extensive research has been done on the membrane (and pore) forming properties of a number of polymers, no reliable design criteria have emerged, ...

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