Thermophysical characterization of N-methyl-2-hydroxyethylammonium carboxilate ionic liquids

Abstract

The thermophysical properties including density, heat capacity, thermal stability and phase behaviour of protic ionic liquids based on the N-methyl-2-hydroxyethylammonium cation, [C2OHC1NH2]+, with the carboxylate anions (propionate, [C2COO]−, butyrate, [C3COO]−, and pentanoate, [C4COO]−) are reported and used to evaluate structure-property relationships. The density was measured over the temperature and pressure ranges, T = (298.15 to 358.15) K and p = (0.1 to 25) MPa, respectively, with an estimated uncertainty of ±0.5 kg · m−3. The pressure dependency of the density for these ionic liquids (ILs) is here presented for the first time and was correlated using the Goharshadi–Morsali–Abbaspour (GMA) equation of state, from which the isothermal compressibility, thermal expansivity, thermal pressure, and internal pressure were calculated. The experimental PVT data of the protic ILs were predicted by the methods of Gardas and Coutinho (GC), and Paduszyńki and Domańska (PD). The thermal stability was assessed by high resolution modulated thermogravimetric analysis within the range T = (303 to 873) K. The heat capacity was measured in the temperature range T = (286.15 to 335.15) K by modulated differential scanning calorimetry with an uncertainty in the range (1 to 5) J · K−1 · mol−1. The Joback method for the prediction of ideal gas heat capacities was extended to the ILs and the corresponding states principle was employed to the subsequent calculation of liquid heat capacity based on critical properties predicted using the modified Lydersen–Joback–Reid method. The Valderrama’s mass connectivity index method was also used for liquid heat capacity predictions. This series of N-methyl-2-hydroxyethylammonium was used to establish the effect of the anion alkyl chain length on the ionic liquid properties.