Synthesis of multilayered, polyelectrolyte membranes begins with immersion of a porous, charged substrate into a solution containing a polyelectrolyte, as shown above. A second immersion in a solution containing an oppositely charged polyelectrolyte results in an additional layer on the surface, and repetition of the process produces multilayer films. This procedure, initially demonstrated by Decher and coworkers, rapidly yields a thin film because immersion times need not be longer than a few minutes. The synthesis is extremely versatile because the only restriction on constituents is that they contain sufficient charge. Thus, many traditional polyelectrolytes as well as proteins, DNA, and inorganic sheets can be used in the formation of multilayered polyelectrolyte films. See Decher, G. Science, 277, 1232-1237 (1997) for a discussion of these films.
Our work with these films involves developing selective, ultrathin membranes for use in gas, liquid, and ion separations. The figure below illustrates the concept of a composite membrane. Because the film is ultrathin, it still allows reasonable flux while providing selectivity. The support gives mechanical strength to the system but presents little resistance to mass transport. SEM images clearly show that multilayered polyelectrolyte films can cover an underlying substrate without filling pores. Nanofiltration studies show that the films allow selective ion transport, with selectivity depending largely on ion charge. Cl-/SO42- selectivities can reach values of 30, and a recent paper summarizes our work in this area (Journal of Membrane Science 283 (2006) 366–372). Recent work focused on removal of reactive dyes from waste streams and F- from potable water. Work with ion-separation membranes is supported by the Department of Energy Office of Basic Energy Sciences.
