Thermophysical properties of n‑alkyl‑ammonium nitrate ionic liquids (n=2,3,4) pure and water saturated for energy applications

Abstract

Phase transition temperatures of three ionic liquids (ILs) with ammonium cations of different alkyl chain length and nitrate common anion were determined in this paper by differential scanning calorimetry. Thermal stability of these ILs in air and nitrogen atmosphere and the maximum operation temperatures of the compounds were also studied using a thermogravimetric analyser. Furthermore, thermophysical properties as density, viscosity, thermal conductivity, and heat capacity against temperature of these pure ionic liquids were determined and compared with the corresponding values for water saturated samples. For this purpose, different techniques were employed: for density and viscosity a rotational automated viscodensimeter; for thermal conductivity measurements a thermal conductimeter that follows the transient hot-wire method; and for heat capacity measurements a microdifferential scanning calorimeter. Different behaviours were observed depending on the analysed thermophysical properties. In case of dried samples, density and thermal conductivity decrease with alkyl chain; for saturated samples, same tendency is observed for density but is the opposite in case of thermal conductivity. Isobaric specific heat and viscosity increase with alkyl chain length in case of dried samples; for saturated samples same tendency is observed for the viscosity, whereas the tendency is the contrary in case of isobaric specific heat