Morphology of the rejecting surface of aromatic polyamide membranes for desalination

Abstract

In highly productive reverse osmosis membranes heterogeneous structures of high specific volume are developed in the rejecting layer by two quite different methods, both of which appear to involve fractal processes. In the asymmetric system the membrane specific volume is increased by adding to the polyamide solution a molecularly compatible component that is readily hydrolyzed and extracted from the concentrated surface layer, itself probably quenched without phase inversion, to develop a volume in substantial excess over the percolation limit. Available information about the factors and functionality of the condensation reaction used to form the thin-film composite structures indicates a non-equilibrium disordered aggregation of dense, but functional, polyamide particles to create a thin layer where every colloidal particle is surrounded by passages for transport. While percolation theory suggests that the asymmetric membrane is formed by the disordered aggregation of open spaces, the thin-film composite seems formed by a similar random and non-equilibrium aggregation of dense, hyperbranched polyamide particles.