Models for biological ion exchangers. II. Solvent selectivity in DMSO-water-Dowex 50W.
Dimethylsulfoxide-water-Dowex 50W-X8 systems are characterized by measurements of solvent selectively and proton magnetic resonance spectra. The H+-, Li+-, NH4+-, NHe4+-, NBu4+-, Mg2+-, Zn2+-, and La3+- forms are studied over a wide range of binary-solvent mole fractions. The relative selectivities for water by the ion exchanger, based on an integrated Kipling parameter, are Zn2+-form (reference) --1.00, Mg2+ --0.43, La3+ --0.38, Li+ +0.17, NMe4+ 0.20, NH4+ 0.31, NBU4+ 0.33, and H+ 0.68, the polyvalent counterions preferring DMSO. All of the ionic forms except the NH4+-form exhibit over much of the mole fraction range separations between the external water and the internal water peaks exceeding 50 Hz, the magnitude of the separation varying with the counter-ion. Comparison of results is facilitated by maintaining a constant ratio between the total number of moles of solvent and the number of equivalents of ions exchanger.