Pervaporation separation of water/ethanol mixtures through polysaccharide membranes. IV. The relationships between the permselectivity of alginic acid membrane and its solid state structure

Membranes composed of alginic acid, which is one of the anionic polysaccharides, were investigated as pervaporation membranes for the separation of water/alcohol mixtures. The selectivity of the alginic acid membrane increased markedly by more than 10,000 when the carboxyl group of alginic acid was neutralized by bases. The counter cation (Li+, Na+, K+, Rb+, and Cs+) influenced the permeation rate for the alginate membranes, which varied from 0.1 kg/m2h for the Li version to 1.0 kg/m2h for the Cs version, but the counterion had hardly any effect on the selectivity. The behavior could not be explained simply by the affinity of the alginate membranes toward water, but could be explained well by the conformation change and mobility of alginate molecules investigated by CP/MAS 13C‐NMR, water mobility within the alginate membrane, and crystallinity of the membrane.