Cation Hydration and Ion Pairing in Aqueous Solutions of MgCl and CaCl.

Broadband dielectric relaxation spectroscopy (DRS) has been used to investigate aqueous solutions of MgCl and CaCl up to concentrations of about 1.8 mol L 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 Mg and Ca 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 Ca(aq) was found to be slightly more labile compared with Mg(aq). Two or three low-intensity solute-related modes were observed at frequencies ≲10 GHz for MgCl(aq) and CaCl, 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 CaCl 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.