Formation of asymmetric membrane by deposition of a hybrid sol-gel sublayer on top of a Langmuir film skin

We report the assembly of an asymmetric membrane consisting of octadecyltrimethoxysilane (ODTMS) Langmuir film covalently attached to a sol-gel based sublayer, supported by a mesoporous membrane. The ODTMS Langmuir film was compressed at the interface of a subphase containing tetramethoxysilane (TMOS), formamide and polyethylenimine. Exposure of the Langmuir film to NH₃ vapours increased the pH at the interface (pH_interface) and catalysed the condensation of TMOS. The assembly of the layers was of a self-healing nature, due to the π-pressure set-up of the Langmuir film and the faster diffusion of the NH₃ through the defects. The resulting asymmetric film was transferred onto polysulfone ultrafiltration membranes to form a thin film composite (TFC) structure composed of a Langmuir organic skin, the sol-gel blend sublayer and the support. The asymmetry of the film was confirmed by various methods and a gradual transition from mesoporous support to dense ceramic-polymeric film was seen. A preliminary study demonstrated that the modification of the polysulfone membrane improved its selectivity from ultra to nanofiltration range. The obtained fluxes were significantly higher as compared to commercial membranes of similar selectivity. This generic approach is applicable for assembling large-area selective TFC membranes with ultrathin skin layers and high fluxes of effluents.