Enhancement of Carbon Dioxide Permselectivity of Immobilized Liquid Polyethylene Glycol Membrane by Addition of Metal Salts

Abstract

Immobilized liquid polyethylene glycol (PEG) membranes dissolving metal salts were investigated to clarify their abilities for permeation of gases (N₂, O₂, and CO₂), since the interaction of the gas molecules with the metal ions incorporated into PEG or nearly naked counter anions might cause enhanced transportation of the gases. The permeability for CO, was almost unchanged or slightly increased whereas those for, N, and O₂ were decreased by adding the salts, resulting in a considerable augmentation in the permselectivity for CO, (Table 1). This enhancement was found to arise chiefly from increase in CO₂ sorbability upon addition of the salts. The equilibrium isotherm for CO₂ in PEG containing the salt exhibited a typical dual mode type consisting of the Henry's law dissolution and the Langmuir type sorption (Fig.3). The parameters for the latter uptake showed good correlation with the pK, values of the conjugate acids of anions (Fig.5), suggesting this uptake to be caused by the acid base interaction between a CO₂ molecule and the naked anion induced through PEG coordination to the metal cation. The augmentation of CO₂ permselectivity was more significant for alkaline earth metal salts than alkali metal ones (Table 3). Such differences in the salt effect could be attributed to change of extents of anion activation in PEG between alkali and alkaline earth metal salts. The salt effect enhancing the CO₂ permselectivity was most remarkable for Na ⁺ and Ca²⁺ salts, respectively, among the alkali and alkaline earth metal salts employed here, demonstrating these effect to depend on the size of metal cations (Fig.7).