Alkali cation selectivity of sephadex g-25 in water and aqueous mixtures of methanol, ethanol and tert.-Butanol

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The column chromatographic partitioning of some alkali salts in Sephadex G-25 and the divinyl sulphone cross-linked dextran gel DVS-9 in water and in aqueous methanol, ethanol and tert.-butanol in G-25, is described. In water the order of elution is Li +<Na +<Cs +<Rb +<K + in G-25 and Li +<Na +<Rb +<Cs +<K + in DVS-9; the selectivity in G-25 is poor and somewhat less than in DVS-9. Comparison of the two gels suggests that they have an affinity for the alkali salts. The cation distribution coefficients (G-25) are anion-dependent, and affinity in this quasi-electroneutral gel presumably involves ion pairs. In all the binary solvent mixtures studied, the alkali cation selectivity was much enhanced and, at least in methanol, the complete aqueous order was preserved. In both methanol and ethanol but not in tert.-butanol the distribution coefficient of Li + alone of the cations was affected. In methanol separation of all five lower cations is possible on a column of only moderate length. Preliminary studies also indicated that alkaline earth cation selectivity was considerably enhanced in water-methanol mixtures. Using data on the composition of the imbibed solvent it was shown that for the mixed solvent system partitioning due to the different solubilities of the alkali salts in the solvent compartments inside and outside the gel cannot account for the observed values. In analogy with other systems, the elution order suggests, as concluded for the pure aqueous systems, that affinity for the gel is also involved at least in the aqueous methanol system. Aqueous tert.-butanol seems to be a special case as this alcohol was almost completely excluded from the gel, and it is suggested that at least in this mixture a decreased swelling pressure of the gel may contribute in part to the higher alkali cation affinity.