Dielectric Relaxation of Concentrated Alkaline Aluminate Solutions

The complex dielec. permittivity of concd. aq. alkali aluminate solns. at total concns. 1 .ltorsim. [Al]T/mol dm-3 .ltorsim. 6 with either [Na]T = 8.33 mol dm-3 or [Al]T/[Na]T = 0.750 has been detd. at 25 °C in the frequency range 0.5 .ltorsim. ν/GHz ≤ 20. All soln. spectra could be represented as a superposition of a Cole-Cole relaxation-time distribution for the solvent relaxation with an addnl. low-frequency Debye dispersion assigned to the solute. The variation of the effective hydration no., deduced from the water dispersion amplitude, shows that the aluminate ion, Al(OH)4-, is less strongly solvated than the hydroxide ion, OH-. Addnl., a "melting" of the hydration shells is obsd. at high concn., 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