Thermodynamic Properties of {Diethyl Phosphate-Based Ionic Liquid (1) + Ethanol (2)} Systems, Experimental Data and Correlation

Abstract

In this work, three solutions consisting of ionic liquid and ethanol were considered for forward-looking use in absorption refrigeration technology. It is proposed to use the following ionic liquids as absorbents: 1-ethyl-3-methylimidazolium diethyl phosphate (abbreviated as [EMIM][DEP]), 4-ethyl-4-methylmorpholinium diethyl phosphate ([EMMOR][DEP]), and 1-ethyl-1-methyl-pyrrolidinium diethyl phosphate ([EMPYR][DEP]). Since the thermodynamic and physicochemical properties of working fluids determine the efficiency of the refrigerator while looking for alternative operating fluids, it is crucial to perform their extensive characteristics. Therefore, in this work, (vapor + liquid) phase equilibria (VLE), liquid density (ρ), and dynamic viscosity (η) were determined for the ethanolic solution of the three mentioned diethyl phosphate-based ionic liquids. 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. Non-random two-liquid (NRTL) equation with temperature-dependence parameters was used to correlate experimental data. Physicochemical properties including liquid density and dynamic viscosity were determined at temperatures from (293.15 to 338.15) K at ambient pressure over the whole concentration range. These properties were correlated using empirical equations. From experimental density and viscosity data, the excess molar volumes and the deviations from additivity were determined and correlated using the Redlich–Kister-type equation. Two of the tested ILs, [EMMOR][DEP] and [EMPYR][DEP], were synthesized and characterized using NMR analysis. The thermophysical characterizations of pure compounds, including glass transition temperature (Tg) and heat capacity at the glass transition temperature (ΔgCp), have been determined using a differential scanning calorimetry technique (DSC) at atmospheric pressure. The work presented is a combination of basic studies on the effect of the cation structure of an ionic liquid on the properties of their solutions with ethanol, with the possibility of future application of the tested systems in a viable refrigeration system.