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

A systematic study of the dielectric relaxation spectra of aqueous solutions of LiCl and Li2SO4 has been made at solute concentrations of 0.05 <= c/ M <= 1.0 and 2.0, respectively, and over a wide range of frequencies (0.2 <= v/GHz <= 89) at 25 degrees C. 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-separated (2SIP) and solvent-shared (SIP) ion pairs, respectively. Consistent with spectroscopic studies, no contact ion pairs were detected over the studied concentration range. The overall ion association constants K A obtained were in good agreement with literature data for both salts. Detailed analysis of the solvent relaxations indicated that Li+ has a significant second solvation sheath although there were differences between the effective hydration numbers obtained from LiCl and Li2SO4, which might arise from competition for the solvent from the anions.