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/NH 3 (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, CO 2, 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) ≥ HI(aq) which parallels the size of the solvated halogen acids.