Hydrophilic membranes for pervaporation: An analytical review

An analytical review has been attempted on the issues encountered in selecting polymers for hydrophilic pervaporation (PV) membranes. It is well known that permselectivity is determined by selective sorption and by selective diffusion. Selective sorption is governed by the presence of the active centers in the polymer which are capable of specific interactions with water. The analysis of the influences of the type of interactions of water-active centers of the polymer on the membrane performances are presented. Selective diffusion is governed by the rigidity and the regularity of the polymer structure and also by the constructure of the polymer's interspace. There is a net of bonds and crosslinks in the polymer due to existence of inter- and intramolecular interactions in it. This net is responsible for the stability of the mass transfer properties of the polymer to the feed water. The influences of the degree of swelling and frequencies of the crosslinks in the polymer are analysed. The main role of the selective diffusion in the selective permeation is demonstrated. The possibility of the existence of two different channels (hydrophilic and hydrophobic) for the permeation of water and organics is discussed, and it has been concluded that there are two different channels for friable polymers which have fragments with not so strong inter- and intra-molecular interactions. But in the polymers with strong inter- and intra-molecular interactions and a narrow net of these bonds (e.g., polyelectrolyte complexes), the hydrophobic channels are strongly collapsed. They can be opened only by water at its high feed concentrations. It was concluded that one of the most prospective ways to create highly permeative and highly selective materials for dehydration of organics by PV is using polyelectrolyte complexes (especially for the separation of water with organic molecules, which have more than three carbon atoms in total).