Study on enthalpy and molar heat capacity of solution for ionic liquid [C3mim][OAc](1-propyl-3-methylimidazolium acetate)

Abstract

Using the solution–reaction isoperibol calorimeter, molar enthalpies of solution, ΔsolHm, for ionic liquid [C3mim][OAc] with different molalities were measured in the temperature range from (288.15 to 308.15±0.01)K with an interval of 5K. In terms of Archer’s method, the standard molar enthalpies of solution, ΔsolHmθ, for [C3mim][OAc] were obtained in the temperature range of (288.15 to 308.15±0.01)K. According to Glasser’s theory of lattice energy the hydration enthalpy of cation and anion in infinite dilution aqueous [C3mim][OAc], (ΔH++ΔH−)=−514kJ·mol−1, at 298.15K. Plotting the experimental values of ΔsolHmθ against ΔT, a good straight line was obtained and the slope of the line is the standard molar heat capacity of solution. ΔCp,mθ=200J·K−1·mol−1, for [C3mim][OAc] and the specific heat capacity of solution, ΔCpθ=1.09J·g−1·K−1, was also obtained. The contribution per methylene to the molar solution enthalpy ΔsolHmθ (−CH2−)=0.76kJ·mol−1 and the contribution from fitted values calculated for the temperature, ΔsolHmθ (−CH2−)=0.90kJ·mol−1, at 298.15K, they are approximately equal. The values of the standard molar heat capacity of solution, ΔCp,mθ, and the specific heat capacity of solution, ΔCpθ, decrease with the increase of chain of methylene (–CH2–) of [Cnmim][OAc] (n=2,3).