Layer-by-layer Self-assembly Membranes for Solvent Dehydration by Pervaporation
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In this study, polyelectrolyte membranes were prepared by layer-by-layer self-assembly on top of an interfacially polymerized polyamide substrate, and these thin-film-composite membranes were studied for pervaporative dehydration of ethylene glycol, ethanol and isopropanol. The performance of composite membranes based on polyethylenimine/poly(acrylic acid) (PEI/PAA) multilayers on a polyamide substrate showed good selectivity and stability for ethylene glycol dehydration. In order to understand the formation process of the polyelectrolyte multilayers, the growth of polyelectrolyte multilayers fabricated on the inner surface of cuvette was investiagted. The membrane surface became increasingly hydrophilic with an increase in the number of polyelectrolyte double layers, which favored water permeation for pervaporative dehydration of organic solvents. Water contact angle on the membrane surface decreased from 68° to 20° when 7 polyelectrolyte bilayers were deposited on the polyamide substrate. Although the (PEI/PAA) based polyelectrolyte membranes showed good performance for dehydration of ethylene glycol, these membranes did not perform well for the dehydration of ethanol and isopropanol at relatively high feed alcohol concentrations. This was found to be caused by insufficient stability of PEI/PAA bilayers and the polyamide substrate in the ethanol and isopropanol. To improve the performance of the composite membranes for dehydration of ethanol and isopropanol, the outermost surface layer was deposited with PEI, followed by crosslinking. A further improvement in the membrane selectivity was accomplished by substituting the PEI with partially protonated chitosan in the last few polyelectrolyte bilayers during membrane fabrication. It was demonstrated that using interfacially polymerized polyamide membrane as a substrate, polyelectrolyte membranes with less than 8 bilayers could be fabricated for the dehydration of alcohol and diol. This represents a siginificant advancement as a large number of polyelectrolyte bilayers (as many as 60) are often needed. Glutaraldehyde crosslinked polyelectrolyte self-assembled membranes comprising of chitosan and PAA were also prepared for isopropanol/water separation. The resulting membrane showed stable performance with good permeation flux and separation factor. The effects of crosslinking conditions (e.g., concentration and temperature of crosslinking agent, and crosslinking time) on the membrane performance were studied. Alternatively, using PEI as polycation, when anionic PAA was substituted with alginate in the last few polyelectrolyte bilayers during membrane fabrication, stable membranes with a good performance were obtained without the need of chemical crosslinking. The polyethylenimine/alginate self-assembly membranes showed good selectivity and stability for dehydration of ethanol. For instance, a permeation flux of 0.24 kg/(㎡ h) and a separation factor of 206 were obtained at room temperature at 10 wt% feed water concentration with a membrane comprising of 10 double layers of polyelectrolytes.