Thermodynamic characterization of {1-ethyl-1-methyl-pyrrolidinium dimethylphosphate, [C1C2PYR][DMP], or 1-hydroxyethyl-1-methylpyrrolidinium dimethylphosphate, [C1C2OHPYR][DMP] (1) + ethanol (2)} binary systems

Abstract

In this work, {1-ethyl-1-methylpyrrolidinium dimethylphosphate, [C1C2PYR][DMP], or 1-hydroxyethyl-1-methylpyrrolidinium dimethylphosphate, [C1C2OHPYR][DMP] (1) + ethanol (2)} binary systems were studied as an alternative working pair for the absorption heat pump cycle. It is well known that the knowledge of the thermodynamic properties of the working fluid used plays a key role in the coefficients of performance values, and thus in the efficiency of the apparatus. Therefore, in this work thermodynamic and physicochemical characteristics of ethanol solutions of two dimethylphosphate-based ionic liquids are presented. The isothermal VLE was measured by an ebulliometric method within a temperature range from T = (328.15 to 348.15) K with an increment of 10 K and pressures up to 90 kPa. Experimental VLE data have been successfully correlated using the NRTL equation using temperature-dependence parameters. Additionally, experimental VLE data were tested for thermodynamic consistency using the Van Ness test. The liquid density and dynamic viscosity were determined as a function of IL's mole fraction over a wide composition range at temperature from T = (293.15 to 338.15) K with an increment of 5 K at ambient pressure. For the correlation of physicochemical properties, empirical equations were applied. From experimental density data, the excess molar volumes were determined and correlated using the Redlich – Kister type equation. Additionally, thermophysical properties of pure ILs including glass transition temperature, heat capacity at glass transition, temperature and enthalpy of (solid + solid) phase transition as well as temperature and enthalpy of melting were determined using differential scanning calorimetry (DSC) technique. Data presented in this work, compared to the available literature data for ethanolic solutions of other dimethylphosphate-based ILs, allows discussing the impact of the IL's cation structure on the presented physicochemical and thermodynamic properties. In addition, the data presented in this work were compared to data for a water solution of lithium bromide, commercially used in absorption refrigeration technology.