Layer-by-Layer Self-Assembled Polyelectrolyte Membranes for Solvent Dehydration by Pervaporation

In this study, polyelectrolyte membranes were fabricated by layer-by-layer self-assembly on an interfacially polymerized polyamide substrate for pervaporative dehydration of ethanol, isopropanol and ethylene glycol. Poly(acrylic acid) (PAA) was used as the anionic polymer, while polyethyleneimine (PEI) and chitosan (CS) acetate were used as the polycations. The polyamide membrane (in the category of loose RO) was chosen as the substrate because of (i) its charged surface for easy self-assembly of the polyelectrolytes under electrostatic forces and (ii) tighter surface structure that required fewer polyelectrolyte layers than what would be needed with commonly used UF membrane substrate to achieve a desired permselectivity. The polyelectrolyte/polyamide composite membranes were shown to work very well for the separation of water from ethylene glycol; however, the membrane performance was compromised significantly for the separation of water from ethanol and isopropanol. This was found to be primarily due to excessive swelling of polyamide substrate by ethanol and isopropanol, which was in agreement the solubility parameters of the substances involved. When the polyelectrolyte/polyamide composite membranes were in contact with aqueous solutions of ethanol and isopropanol, the thin polyelectrolyte skin layer on the surface of the polyamide substrate were not selective enough to prevent penetration of ethanol and isopropanol through the polyelectrolyte layer to reach the polyamide substrate, making the composite membrane instable in the aqueous ethanol and isopropanol solutions. To overcome this problem, chemical cross-linking of the outer layer of the polyelectrolyte was used, which was shown to improve the membrane selectivity significantly for ethanol/water and isopropanol/water separations while maintaining a high permeation flux. The effects of feed composition and temperature on the separation performance were also investigated.