Ion association and hydration in aqueous solutions of LiCl and Li2SO4 by dielectric spectroscopy

Abstract

A systematic study of the dielec. relaxation spectra of aq. solns. of LiCl and Li2SO4 has been made at solute concns. of 0.05 ≤ c/M ≤ 1.0 and 2.0, resp., and over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) at 25°. The spectra were best described by a superposition of four Debye processes, consisting of the two well-known water relaxations at ca. 8 and 0.5 ps and two ion-pair contributions at ca. 200 and 20 ps, corresponding to the presence of double-solvent-sepd. (2SIP) and solvent-shared (SIP) ion pairs, resp. Consistent with spectroscopic studies, no contact ion pairs were detected over the studied concn. range. The overall ion assocn. consts. K°A obtained were in good agreement with literature data for both salts. Detailed anal. of the solvent relaxations indicated that Li+ has a significant second solvation sheath although there were differences between the effective hydration nos. obtained from LiCl and Li2SO4, which might arise from competition for the solvent from the anions.