Heat capacities of ionic liquids and their heats of solution in molecular liquids

Abstract

Heat capacities of ionic liquids (IL): 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF 4), 1-ethyl-3-methyl imidazolium bis((trifluoromethyl)sulfonyl) imide (EMImN(CF 3SO 2) 2), 1-bytyl-3-methylimidazolium tetrafluoroborate (BMImBF 4) and N, N-methyl, propyl pyrrolidinium bis((trifluoromethyl)sulfonyl) imide (MPPyN(CF 3SO 2) 2) were measured from 283.15 to 358.15 K. Room temperature heat capacities have also been estimated by an additive group contribution method, based on the assumption that the heat capacity of a molecular compound equals the sum of individual atomic-group contributions. The C p 293.15 K estimated values are about 12% higher than experimental values. The estimates suggest that heat capacities of ionic liquids do not differ considerably from those typical for molecular liquids (ML). The heats of solution, Δ sol H, of ionic liquids EMImBF 4, BMImBF 4 EMImN(CF 3SO 2) 2 and MPPyN(CF 3SO 2) 2 were measured in water, acetonitrile (AN) and methanol, as a function of ionic liquid concentration c m. The measured Δ sol H values decrease with decreasing c m. Enthalpies of ionic liquid transfer, Δ t H°, from water to methanol and acetonitrile were calculated from measured solution enthalpies. Values of Δ t H° for the transfer from water to methanol are positive and those for the transfer to acetonitrile are negative.