Thermodynamic Properties of 1-Butyl-3-methylimidazolium Hexafluorophosphate in the Condensed State

Abstract

Thermodynamic functions for 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) are reported in a range of temperatures from (5 to 550) K, based on new measurements by calorimetry. Heat capacities of the crystal, glass, and liquid phases for [C4mim][PF6] were measured with a pair of calorimeters. A vacuum-jacketed adiabatic calorimeter was used at temperatures between (5 and 310) K, and a heat bridge-scanning calorimeter was used from (300 to 550) K. With the adiabatic calorimeter, the fusion Tfus = 283.51 K, = 19.60 kJ·mol-1, and the glass transition Tg = 190.6 K were observed. The [C4mim][PF6] test sample was determined to have a mole fraction purity of 0.9956 by a fractional melting analysis. Densities of the liquid were measured in a range of temperatures from (298 to 353) K with a pycnometer equipped with a capillary neck. An unexpected endothermal transition, with a very small enthalpy change of 0.25 J·g-1 (0.071 kJ·mol-1), was observed in a range of temperatures from (394 to 412) K. Heat capacity jumps were determined at the glass transition Cs = 81.6 J·K-1·mol-1 and fusion Cs = 44.8 J·K-1·mol-1, and the observed entropy change at fusion is S (283.51 K) = 69.23 J·K-1·mol-1.