Pervaporation separation of water/ethanol mixtures through polysaccharide membranes. I. The effects of salts on the permselectivity of cellulose membrane in pervaporation

In the separation of water/alcohol mixtures through cellulose membranes, the addition of trace amounts of specific salts to the feed mixture (10^?5?5 × 10^?3 mol/kg) caused the marked increase of the separation factor and only the salts composed of a multivalent metal ion and a multivalent counteranion were effective for the increase of the selectivity. For example, when 1.5 × 10^?3 mol/kg CoSO₄ was added to water/ethanol mixture (35/65 wt/wt), the separation factor was 194 and the permeation rate was 5.14 kg/m² h. But, in the absence of the salt, the separation factor was only 6.7 and the permeation rate was 5.63 kg/m² h. We presumed that the increase of the selectivity by the addition of specific salts would be attributed to the contraction of the “holes” produced by the thermal motion of polymer chains and this contraction would be correlated with the conformation change of cellulose molecule.