Low temperature infrared spectroscopic study of the solvation of ions in water

Abstract

In general, when aqueous electrolyte solutions are frozen, the infrared spectrum in the O–H stretching region reveals narrow bands assignable to ice and to salt hydrates. Using as solvent dilute solutions of HOD in D₂O, we have detected several novel hydrate bands using this procedure. However, in some cases, glassification occurred, and in others it could be achieved using certain dilute additives, such as t-butyl alcohol. These glasses gave bands which were appreciably narrower than those for the solutions at 0°C, and consequently, two or more features could sometimes be resolved. This has enabled us to assign bands to HOD molecules hydrogen bonded to the halide ions and to various oxyions. These bands were all on the high frequency side of the main water band and were generally narrower than this band. The shift is discussed in terms of the number of primary and secondary solvent molecules associated with the anions, and the narrowing in terms of the precision of anion hydrogen bonding.

In contrast, most cations simply induce a small shift in the broad water band, which may be to high or low frequencies. This shows that the spectral properties of water molecules bound to the cations are dominated by the other water molecules to which each is hydrogen bonded.

At high concentrations of salt, new bands were obtained, shifted from the position of the anion solvate band by the cations. These are assigned to solvent shared ion-pairs.