Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

Ion association of the ionic liquid [bmim][Cl] in acetonitrile and in water was studied by dielectric spectroscopy for salt concentrations c <= 1.3 M at 298.15 K and by measurement of molar electrical conductivities, L, 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 K-A(o) approximate to 60 M-1 and increasing with temperature, [bmim][Cl] is only weakly associated in water (K-A(o) approximate to 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,lambda(infinity) ([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.