Ion association and hydration in 3:2 electrolyte solutions by dielectric spectroscopy: aluminum sulfate

Abstract

Broadband dielec. measurements utilizing state-of-the-art coaxial reflectometry and traveling-wave interferometry have been made on aq. solns. of the 3:2 electrolyte aluminum sulfate at 25° over the frequency range 0.2 ≤ ν (GHz) ≤ 89 and at total solute concns. 0.012 ≤ c (M) ≤ 0.65. Detailed anal. of the solute contribution to the dielec. spectra revealed the simultaneous presence of double solvent-sepd. (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs. Concns. of the various ion-pair types and the equil. consts. for their formation were detd. using calcd. dipole moments and other relevant quantities. The 2SIPs and SIPs were found to persist down to low concns. in this notionally strong electrolyte. Good agreement was found with earlier Raman and NMR studies for CIP concns. and with thermodn. detns. of the overall ion assocn. const. In contrast to divalent metal sulfate solns., both SIPs and, to a lesser extent, 2SIPs remain present at quite high Al2(SO4)3(aq) concns. The persistence of SIPs and 2SIPs was consistent with the effective hydration nos. obtained from an anal. of the cooperative H-bond relaxation mode of bulk water in the solns. This anal. indicated that the hydration of Al3+ ions is extremely strong, with significant effects on the dielec. relaxation timescale even beyond the second hydration shell, at least in dil. solns.