Features of Ion Hydration and Association
in Aqueous Rubidium Fluoride Solutions at Ambient Conditions,
M. V. Fedotova, S. E. Kruchinin, H. M. A. Rahman, and R. Buchner,
J. Mol. Liq. 159 (2011) 9-17. DOI: 10.1016/j.molliq.2010.
04.009
Abstract
Aqueous RbF solutions have been investigated at ambient conditions by
dielectric relaxation spectroscopy (DRS) over the concentration range of
0≤ c/M ≤ 5 and by statistical mechanics (RISM integral equation
theory) in the concentration range of 2.15-9.18 M. The behavior of the
dielectric spectra and their characteristic parameters (dispersion amplitudes
and relaxation times) is discussed, as well as the pair correlation functions
and partial coordination numbers derived from statistical mechanics. The paper
focuses on the influence of salt concentration on the features of ion hydration
and association. The observed changes in the dielectric properties indicate a
decrease of the effective hydration number from ~15-18 at infinite dilution to
~3.5 at 5 M and possibly weak association involving stable ion pairs at
c ≤ 0.5 M. A major finding is that both ions, Rb+ and
F-, slow down surrounding water molecules, leading to the emergence
of a separate "slow water" relaxation in the spectra. According to
the analysis of RISM equilibrium properties, an average number of ~6 water
molecules coordinates both Rb+ and F- at c=2.15 M. This
number decreases by ~19 % for the cation and by ~10% for the anion as c
growths up to 9.18 M. The RISM-data indicate a significant increase in the
number of contact and solvent-separated ion pairs with increasing c. The
possibility of ion clustering is discussed for concentrated RbF(aq) solutions.
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