Composite Membranes for Water Desalination

Methods of forming coatings on the surface of a polyethylene terephthalate track-etched membrane by magnetron and electron-beam sputter deposition of polytetrafluoroethylene in vacuum are considered. It is established that the use of these modification methods leads to the formation of composite membranes consisting of two layers, one of which is the original track-etched membrane, characterized by an average level of hydrophilicity. The second layer has a hydrophobic nature. The water contact angle of this layer varies depending on its thickness and the modification method used. It is shown that deposition of coatings by magnetron sputtering of polytetrafluoroethylene leads to smoothing of structural inhomogeneities of the surface of the initial membrane. Deposition of coatings by electron-beam dispersion of polymer, on the contrary, causes an increase in surface roughness. The observed differences in the morphology of the composite membranes surface layer are related to the size of the deposited polymer nanostructures. Nanostructures formed on the surface of track-etched membranes, when polytetrafluoroethylene is dispersed under the action of electron beam, have substantially large sizes. A significant increase in roughness due to the increase in the size of nanostructures makes it possible to obtain coatings with high- and superhydrophobic properties. It is shown that composite membranes consisting of a hydrophilic base with a hydrophobic coating deposited on the surface provide higher separation selectivity during desalination of an aqueous solution of sodium chloride by membrane distillation compared with the initial polyethylene terephthalate track-etched membrane. In addition, the performance of two-layer composite membranes in the process of membrane distillation owing to low resistance to mass transfer is higher in comparison with a track-etched membrane made of polypropylene.