Temperature effects on ion association and hydration in MgSO4 by dielectric spectroscopy

A detailed investigation of aqueous solutions of magnesium,sulfate has been made by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 <= v/GHz <= 89), concentrations (0.02 <= m/mol kg(-1) <= 2.3) and temperatures (5 <= 5 upsilon/degrees C <= 65). Consistent with literature data from other techniques, the spectra show clear evidence at all temperatures for the simultaneous existence of double solvent-separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs and a triple ion M), Mg2SO42+ (aq). Increased formation of Clips and especially Tls occurs with increasing temperature. The overall standard ion-association constant, KA degrees, corresponding to the equilibrium: Mg2+ (aq) + SO42- (aq) reversible arrow MgSO40 (aq) is in good agreement with literature data at lower temperatures but is overestimated at higher temperatures due to processing difficulties. Despite the limited precision of the spectra, analysis of the individual steps in the ion-association process is possible for the first time. The 2SIPs ore formed with little disturbance to their hydration shells, the (partial) destruction of which appears to occur mostly during the formation of SIPs. Effective hydration numbers derived from the DRS spectra indicate that both Mg2+ and SO42- influence solvent water molecules beyond their first hydration spheres but that MgSO4(aq) is less strongly hydrated than the previously studied CuSO4(aq).