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.
Aqueous solutions of five lanthanide salts: LaCl3, La(NO3)3, La2(SO4)3, Eu(NO3)3 and Eu2(SO4)3 have been studied at 25 °C by dielectric relaxation spectroscopy over the frequency range 0.05 ≤ ν/GHz ≤ 89. Detailed analysis of the solvent-related modes located at higher frequencies showed that both La3+ and Eu3+ are strongly hydrated, even including partial formation of a third hydration shell similar to that of Al3+(aq). Up to two solute-related modes could be detected at lower frequencies, due to the formation of various types of 1 : 1 ion pairs (IPs). All five salts showed modest levels of association in the order Cl- < NO3- ≪ SO42-, mostly in the form of double-solvent-separated IPs with small amounts of solvent-shared IPs. Overall association constants, , calculated from the stepwise IP formation constants were consistent with literature values.
Alkyltrimethylammonium alkanecarboxylates, [CxMe3N]+[Cy]−, with medium to long alkyl chains on both cation and anion (x,y = 6–10), were investigated in aqueous solution by density and zeta potential measurements, ITC, and DRS.
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