Mechanism of Pore Formation in Reverse Osmosis Membranes During the Casting Process


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 μm) skin of high density and very small pores which is chiefly responsible for desalination. The gel forms the backing (∼100 to 250 μm) and contains large pores.

It is shown that low evaporation rates accompanied by shrinkage during evaporation 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 polymer‐solvent 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.


Chemical and Biochemical Engineering

Keywords and Phrases

Membrane Casting; Membranes; Osmosis; Polymers; Reverse

Document Type

Article - Journal

Document Version


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© 1983 Wiley-Blackwell, All rights reserved.

Publication Date

01 May 1983