Cation Hydration and Ion Pairing in Aqueous Solutions of MgCl₂ and CaCl₂

Broadband dielectric relaxation spectroscopy (DRS) has been used to investigate aqueous solutions of MgCl₂ and CaCl₂ up to concentrations of about 1.8 mol L^-1 at 25 °C over the frequency range 0.07 ≤ ν/GHz ≤ 89. Detailed analysis of the dominant solvent mode centered at ∼20 GHz showed that both Mg²⁺ and Ca²⁺ are strongly solvated, each immobilizing ∼20 water molecules on the DRS timescale. This is consistent with the formation of two well-defined hydration layers around both cations. The hydration shell of Ca²⁺(aq) was found to be slightly more labile compared with Mg²⁺(aq). Two or three low-intensity solute-related modes were observed at frequencies ≲10 GHz for MgCl₂(aq) and CaCl₂, respectively. Two of these modes were attributed to the formation of double-solvent-separated and solvent-shared 1:1 ion pairs. The third mode (observed at very low frequencies and only for some CaCl₂ solutions) was thought to be due to an ion-cloud relaxation. No evidence was found for "slow" water or, consistent with the strong cation hydration, for contact ion pairs. The overall association constants for MgCl⁺(aq) and CaCl⁺(aq) calculated from the ion-pairing constants were very small but in good agreement with literature values obtained from other techniques.