Association of Ionic Liquids in Solution:
A Combined Dielectric and Conductivity Study of [bmim][Cl] in Water and in
Acetonitrile
M. Bešter-Rogač, A. Stoppa, J. Hunger, G. Hefter and R. Buchner,
Phys. Chem. Chem. Phys. 13 (2011) 17588-17598. DOI:
10.1039/c1cp21371g
Abstract
Ion association of the ionic liquid [bmim][Cl] in acetonitrile and in water was
studied by dielectric spectroscopy for salt concentrations c≤ 1.3M at
298.15K and by measurement of molar electrical conductivities, Λ of
dilute solutions (c≤ 0.006 M) in the temperature range
273.15≤T/K≤313.15. Whilst acetonitrile solutions of [bmim][Cl]
exhibit moderate ion pairing, with an association constant of
Κ°A≈ 60 M-1and increasing with
temperature, [bmim][Cl] is only weakly associated in water (
Κ°A≈ 6 M-1) and ion pairing
decreases with rising temperature. Only contact ion pairs were detected in both
solvents. Standard-state enthalpy, entropy and heat capacity changes of ion
association were derived, as well as the activation enthalpy of charge
transport and the limiting conductivity of the cation,
λ∞([bmim]+). These data, in conjunction with
effective solvation numbers obtained from the dielectric spectra, suggest that
the solvation of [bmim]+ is much weaker in water than in
acetonitrile. |
