Ultrathin Membranes

The first useful ultrathin membranes were cellulose acetate reverse osmosis membranes produced by Loeb and Sourirajan, two researchers at the University of California at Los Angeles. The development of these thin, and hence high flux, membranes led to the reverse osmosis industry in the 1960s. In the Loeb-Sourirajan technique, a solution containing ap proximately 20% polymer is cast as a thin film on a nonwoven fabric web and is then precipitated by immersion in a bath of water. The water very rapidly precipitates the top surface of the cast film, forming the selective skin. This skin then slows down the entry of water into the underlying polymer solution, which precipitates much more slowly, forming a more porous substructure. A scanning electron micrograph showing the porous substructure and the selective skin of a Loeb-Sourirajan membrane is shown in Figure 1. The selective layer thickness is typically less than 0.2 |im.

About one-third of the reverse osmosis and almost all ultrafiltration membranes currently produced are made by the Loeb-Sourirajan technique. This type of membrane is also widely used in gas separation processes.

In recent years, new approaches have been developed to produce anisotropic membranes with even thinner selective layers than those made by the Loeb-Sourirajan method. Selective layers only a few tens of nanometers in thickness, and effectively free of imperfections, have been claimed for these so-called thin-film composite membranes. Thin-film composite membranes can be made by a number of methods, of which two are particularly important: coating with a dilute polymer solution and interfacial polymerization. In the coating method, which was developed first, a very dilute solution of the polymer is prepared in a volatile solvent, such as hexane. A thin film of this polymer solution is deposited on the microporous support surface by immersing and then slowly withdrawing the support from the solution. As the solvent evaporates, an extremely thin polymer film is left behind. This technique is used to manufacture ultrathin membranes for gas separation and pervaporation.

The second important method for preparing composite membranes is interfacial polymerization. In this method, an aqueous solution of a reactive monomer, such as a diamine, is deposited in the pores of a microporous support membrane. The membrane is then immersed in a water-immiscible solvent solution containing a multivalent reactant, such as a triacid chloride in hexane, which causes the monomer to polymerize and cross-link. Polymerization is confined to the interface of the two immiscible solutions, so a thin, highly selective layer is formed. The procedure is illustrated in Figure 2. The interfacial polymerization technique is used to produce most of today's reverse osmosis membranes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment