Association of ionic liquids in solution: a combined dielectric and conductivity study of bmimCl in water and in acetonitrile

Ion association of the ionic liquid bmimCl 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, Λ, of dilute solutions (c ≤ 0.006 M) in the temperature range 273.15 ≲ T/K ≤ 313.15. Whilst acetonitrile solutions of bmimCl exhibit moderate ion pairing, with an association constant of K°(A) ≈ 60 M(-1) and increasing with temperature, bmimCl is only weakly associated in water (K°(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.