Solvatomer dynamics of aluminium sulfate in dimethyl sulfoxide/water mixtures

The ultrasonic absorption spectra between about 200 kHz and 2 GHz have been measured for 0.1 mol l−1 solutions of Al2(SO4)3 in several mixtures of dimethyl sulfoxide (DMSO) and water at 25°C. A suitable description of the spectra is obtained with a sum of two Debye-type spectral terms and a term reflecting correlated non-critical fluctuations in ion concentration. The outer–outer-sphere/outer-sphere ion complex equilibrium of the electrolyte seems to be reflected by the low-frequency Debye term, the formation/dissociation of outer–outer-sphere complexes by the concentration fluctuation term. DMSO exchange from solvatomers appears to be the mechanism behind the high-frequency Debye term.