The Solvent Properties of Water in Desalination Membranes

The solvent properties of water in regular copolyoxamide and cellulose acetate membranes have been investigated. Partition coefficients of chlorides between the water in dense cellulose acetate membranes and a bulk aqueous phase decreased in the order: Cs+ > K+ > Na+ > Li+ > Ca2+ > Mg2+. In copolyoxamide membranes, partition coefficients of both chlorides and nitrates showed similar trends. Partition coefficients in methanol appeared to be unity. KCl and CsCl were accumulated into the surface water from extremely low concentrations (0.1 mM) but not from moderate or high concentrations. Partition coefficients of the chlorides of Na+, Li+, Ca2+, and Mg2+ were all less than unity and decreased with increasing concentration. In explanation of these findings, it is proposed that, when the hydrophobic pores of the membranes are wet by prolonged soaking in water, the high surface energy resulting from unfavorable surface-water interactions leads to an expansion of water out of the pores and a decrease in its macroscopic density. This “stretched” water has anomalously low entropy and high hydrogen-bond energy, which together profoundly change its solvent properties. By generating osmotic pressure gradients of opposite sign, an accumulated solute opposes expansion and stretching of pore water, while an excluded solute reinforces it. Infrared spectra supported this hypothesis: spectral characteristics of water in dense cellulose acetate membranes were accentuated by MgCl2 and abolished by KCl.