Anhydrous halogen acid interaction with polyaniline membranes: A gas permeability study

Anhydrous halogen acids react with polyaniline films in a very different manner than aqueous halogen acids. While aqueous acid-doped films can be readily undoped in aqueous base leaving little residual halide behind, some anhydrous acid-doped films, undoped with anhydrous ammonia followed by sublimation of the ammonium halide, leave a significant amount of residual halide behind. This varies from approximately 2.8% Cl for HCl(g), to 1.3% Br for HBr(g), to 0.4% I for HI(g) to <0.02% F for HF(g) doped/NH3(g) undoped films. Reflectance Fourier transform infrared (FT-IR) spectroscopy confirms the presence of C-Cl and C-Br bonds, while no C-F bonds are observed. Gas permeability data are consistent with the residual halide content observed by elemental analysis and FT-IR, in that HF and HI doped/undoped polyaniline films show increased permeability to all gases (He, H-2, CO2, Ar, O-2, and N-2) tested, while HCl and HBr doped/undoped films show lower permeabilities. The decreased permeabilities for HCl and HBr doped/undoped films indicate that the residual halide reduces free volume in the film. This effect can be contrasted with aqueous acid doping/undoping in which free-volume and permeability increase in the order HF(aq) > HCl(aq) > HBr(aq) greater than or equal to HI(aq) which parallels the size of the solvated halogen acids.