Pervaporation studies with polyaniline membranes and blends

Permeabilities (P), diffusion coefficients (D) and time lags (Θ) for water, alcohols and carboxylic acids passed through undoped and doped polyaniline membranes are reported. Permeabilities through undoped polyaniline are stable and linear, but both permeabilities and diffusion coefficients tend to be low. As an example, for water, P=0.11±0.05 g mm/m2 h, with a diffusion coefficient of D=5.0×10−10 cm2/s. Methanol has the highest permeability through undoped polyaniline (P=0.84±0.02 g mm/m2 h), despite its small diffusion coefficient (D=1.5×10−10 cm2/s). Fully HCl-doped polyaniline membranes have higher permeabilities (up to 5–6 g mm/m2 h) with correspondingly larger diffusion coefficients (e.g. Dwater>1.0×10−8 cm2/s) and shorter time lags. Larger alcohols (e.g. 2-propanol) and carboxylic acids (e.g. acetic acid) show no permeation through doped polyaniline. Therefore, fully HCl-doped polyaniline membranes can be used to effectively separate water from acetic acid (αH2O≈200). Unfortunately, when HCl-doped polyaniline is used as a pervaporation membrane, the permeation rates of the feed decline over time. For example, the permeability of water declines from a high of ∼2.5 g mm/m2 h to a much lower average steady-state value of 0.3 g mm/m2 h over 48 h. This effect has been traced to the leaching of HCl dopants out of the polyaniline membrane and into the feed solution. The problem of dopant leaching can be eliminated by using a polymer dopant in a blend with polyaniline. The permeation rates of ethanol/water feeds through blends of 10 wt.% polyamic acid/90 wt.% polyaniline and 10 wt.% polyacrylic acid/90 wt.% polyaniline are linear and stable. This demonstrates that polymeric dopants eliminate the problem of dopant leaching. Both blends possess permeabilities and selectivities in-between undoped and doped polyaniline.