Ionic association and conductance of [emim][BF4] and [bmim][BF4] in 1-butanol in a wide range of temperature

Abstract

The electrical conductances of dilute solutions of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4] and 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] in 1-butanol have been measured in the temperature range from T/K=(283.15 to 318.15) at 5K intervals. The ionic association constants, KA, the limiting molar conductances, Λo, and distance parameters, R, were obtained using the low concentration Chemical Model (lcCM). The examined ionic liquids are strongly associated in 1-butanol over the whole temperature range. From the temperature dependence of the limiting molar conductivities the Eyring’s activation enthalpy of charge transport was determined. The thermodynamic functions such as Gibbs energy, ΔGAo, entropy, ΔSAo, and enthalpy of the process of ion pair formation, ΔHAo, were calculated from the temperature dependence of the association constants. Negative values of ΔGAo indicate that the formation of ion pairs is a spontaneous process. The increase of temperature leads to more negative ΔGAo values, which means shifting the equilibrium toward the formation of ion pairs. The entropy values of association are positive, which means that as a result of association, the arrangement of the system decreases in comparison with the arrangement associated with the solvation of free ions.