Gas, liquid and enantiomeric separations using polyaniline

Conjugated polymers are excellent candidates for membrane separations since their porosity on the molecular level can be controlled through chemical doping. Polyaniline is especially attractive since its simple acid/base doping/dedoping chemistry enables a controllable level of doping to be readily achieved using dopants of different size, shape or even chirality. Adding dopants to polyaniline leads to a decrease in permeability to gases, while removing these dopants leads to enhanced permeability. By partially redoping polyaniline, a membrane with one of the highest selectivities for the permeation of oxygen over nitrogen has been created. Liquids can be selectively permeated in a process known as pervaporation. As-cast polyaniline shows little selectivity for water over the organic acids formic, acetic and propionic. This is due to the hydrophobic nature of the emeraldine base form of polyaniline leading to a solubility selectivity that favors organic acids over water, while diffusivity selectivity favors the smaller water molecules over the larger organic molecules. High selectivities for water over organic acids can be achieved by using fully hydrochloric acid-doped polyaniline membranes which become hydrophilic and thus favor water over the organic acids due to both solubility and diffusivity. Chiral polyaniline films can be created by doping with a strong chiral acid such as S(+)-camphorsulfonic acid during film formation. Removal of the dopants with base creates a new form of chiral polyaniline that can distinguish between the amino acids L-phenylalanine and its enantiomer D-phenylalanine. (C) 2001 Elsevier Science B.V. All rights reserved.