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.

(c) 2011 Berger Georg