Abstract
Broadband dielec. relaxation spectroscopy (DRS) has been used to investigate aq. solns. of MgCl2 and CaCl2 up to concns. of about 1.8 mol L-1 at 25 °C over the frequency range 0.07 ≤ ν/GHz ≤ 89. Detailed anal. of the dominant solvent mode centered at ∼20 GHz showed that both Mg2+ and Ca2+ are strongly solvated, each immobilizing ∼20 water mols. on the DRS timescale. This is consistent with ...
Abstract
Broadband dielec. relaxation spectroscopy (DRS) has been used to investigate aq. solns. of MgCl2 and CaCl2 up to concns. of about 1.8 mol L-1 at 25 °C over the frequency range 0.07 ≤ ν/GHz ≤ 89. Detailed anal. of the dominant solvent mode centered at ∼20 GHz showed that both Mg2+ and Ca2+ are strongly solvated, each immobilizing ∼20 water mols. 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 obsd. at frequencies ⪅10 GHz for MgCl2(aq) and CaCl2, resp. Two of these modes were attributed to the formation of double-solvent-sepd. and solvent-shared 1:1 ion pairs. The third mode (obsd. at very low frequencies and only for some CaCl2 solns.) 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 assocn. consts. for MgCl+(aq) and CaCl+(aq) calcd. from the ion-pairing consts. were very small but in good agreement with literature values obtained from other techniques.
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