Interfacial Synthesis in the Preparation of Reverse Osmosis Membranes

Reverse osmosis membranes are currently finding increasing use in the production of potable water from salt-laden water sources, and in the purification and recycle of industrial process waters. Most reverse osmosis membranes are comprised either of polyamide hollow fibers or cellulose acetate sheet films. A new type of reverse osmosis membrane, made by the interfacial synthesis of ultrathin membranes directly on microporous support media, is being explored. Two types of interfacially formed membranes are specifically described. One is based on an ultrathin polyamide barrier layer made by interfacial reaction of polyethylenimine with isophthaloyl chloride. The barrier material is supported by a heat-polymerized intermediate zone of polyethylenimine, supported in turn by a microporous polysulfone substrate. The second membrane consists of a polyamide formed from piperazine and a mixed acyl halide reagent. This nonpolymeric amine leads to a different surface texture and the absence of an intermediate zone between the barrier film and the microporous polysulfone substrate. This latter membrane exhibits excellent water fluxes (as high as 4.0 m3/m2 -day at 100 atm) under seawater reverse osmosis test conditions, and shows very high rejection (99+%) of inorganic salts containing polyvalent anions. Differences in the morphology of the salt barrier layers for these two cases are described, and the resulting effects on membrane desalination properties are discussed.