Abstract
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 degrees C over the frequency range 0.07 <= nu/GHz <= 89. Detailed analysis of the dominant solvent mode centered at similar to 20 GHz showed that both Mg2+ and Ca2+ are strongly solvated, each immobilizing similar to 20 water ...
Abstract
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 degrees C over the frequency range 0.07 <= nu/GHz <= 89. Detailed analysis of the dominant solvent mode centered at similar to 20 GHz showed that both Mg2+ and Ca2+ are strongly solvated, each immobilizing similar to 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 soluterelated modes were observed at frequencies less than or similar to 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 MgC1+(aq) and CaC1+(aq) calculated from the ion-pairing constants were very small but in good agreement with literature values obtained from other techniques.
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