The complex dielectric permittivity of concentrated aqueous alkaline aluminate solutions at total concentrations 1 ≲ [Al]T/mol dm-3 ≲ 6 with either [Na]T = 8.33 mol dm-3 or [Al]T/[Na]T = 0.750 has been determined at 25 °C in the frequency range 0.5 ≲ ν/GHz ≤ 20. All solution spectra could be represented as a superposition of a Cole-Cole relaxation-time distribution for the solvent relaxation with an additional low-frequency Debye dispersion assigned to the solute. The variation of the effective hydration number, deduced from the water dispersion amplitude, shows that the aluminate ion, Al(OH)4-, is less strongly solvated than the hydroxide ion, OH-. Additionally, a "melting" of the hydration shells is observed at high concentration, probably due to co-sphere overlap and/or packing effects. The features of the solute relaxation are consistent with solvent-shared ion pairs of Na+ with a previously proposed dimeric anion, [(HO)3Al-O-Al(OH)3]2-. However, the data are not sufficient for an unequivocal assignment.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry