The solvent shells of cluster ions produced by direct electric field extraction from glycerol/water mixtures

Electric field extraction of gaseous negative ions directly from water/glycerol solutions by use of a track membrane technique was investigated. The distributions of numbers of solvent molecules in the extracted cluster ions for different compounds were obtained. It is shown that the extraction mechanism is a direct field-stimulated evaporation of cluster ions from liquid, with a subsequent loss of several solvent molecules in the vacuum. For relatively simple ions a good correspondence of results was obtained with a continuous medium model. It was found that the number of solvent molecules in a cluster shell, for more complicated ions such as amino acids, is significantly greater than that for halide ions or ions of simple organic acids. An increase in the number of solvent molecules in the case of amino acid negative ions is rationalized in terms of the existence of several charged groups, each of which gives an additional contribution to the cluster shell.