Investigation of Thermodynamic Properties of Dimethyl Phosphate-Based ILs for Use as Working Fluids in Absorption Refrigeration Technology

Abstract

In the current research, the binary solution containing ionic liquid (IL), 1-ethyl-1-methylmorpholinium dimethyl phosphate ([C1C2MOR][DMP]), 1-ethyl-1-methylpiperidinium dimethyl phosphate ([C1C2PIP][DMP]), or N,N,N-triethyl-N-methylammonium dimethyl phosphate ([N1,2,2,2][DMP]) with ethanol are investigated as new working fluids for absorption refrigeration technology. The IL was mixed with ethanol, which was considered as a refrigerant. Experimental (vapor + liquid) phase equilibria (VLE) of these binary systems were 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 were correlated using non-random two-liquid (NRTL) within the maximum average relative deviation of 0.45%, which confirms the effectiveness of using such a model for calculations. Each of the proposed binary systems exhibit a negative deviation from Raoult’s law, which is a very important characteristic for working pairs used in absorption heat pumps or absorption refrigerators. From a technological point of view, measurements of physicochemical properties play an important role. In this research, liquid density and dynamic viscosity were determined at temperatures from T = (293.15 to 338.15) K at ambient pressure over the whole concentration range. These properties were correlated using empirical equations. From experimental density data, the excess molar volumes were determined and correlated using the Redlich–Kister type equation. Ionic liquid: [C1C2MOR][DMP] and [C1C2PIP][DMP] were synthesized and characterized using NMR analysis. The thermophysical characterization of pure ILs, including glass transition temperature (Tg) and heat capacity at the glass transition temperature (ΔgCp), was determined using the differential scanning calorimetry technique (DSC) at atmospheric pressure. In this work, the combination of basic studies on the effect of the cation structure of an ionic liquid on the properties of their solutions with ethanol and the possibility of future application of the tested systems in a viable refrigeration system are presented.

Physicochemical and Thermodynamic Investigation of Ethanolic Solution of Phosphonium-Based Ionic Liquids─Measurements, Correlations, and Application to Absorption Cycles

“…Figure 5 shows that the ethanolic system with the ammonium-based ionic liquid shows the lowest density. These data, supplemented by the promising vapor pressure values presented earlier, 23 Temperature dependence of the dynamic viscosity for the binary systems under study was described using the following Andrade-type equation…”

“…It ought to be noted that all ionic liquids except [C 1 Py]­[DMP] exhibited a glass transition temperature and heat capacity at the glass transition temperature. The following series shows how these numbers rise: [C 1 C 2 PYR]­[DMP] ( T g = 189.7 K, Δ g C p = 119.6 J mol –1 K –1 ) < [N 1,2,2,2 ]­[DMP] ( T g = 194.4 K, Δ g C p = 154.4 J mol –1 K –1 ) ∼ [C 1 C 2OH PYR]­[DMP] ( T g = 194.9 K, Δ g C p = 148.1 J mol –1 K –1 ) < [C 1 C 2 IM]­[DMP] (this work) ( T g = 203.8 K, Δ g C p = 144.1 J mol –1 K –1 ) < [C 1 C 1 IM]­[DMP] (this work) ( T g = 207.4 K, Δ g C p = 128.9 J mol –1 K –1 ) < [C 1 C 2 PIP]­[DMP] ( T g = 211.0 K, Δ g C p = 103.9 J mol –1 K –1 ) < [C 1 C 2 MOR]­[DMP] ( T g = 222.2 K, Δ g C p = 132.7 J mol –1 K –1 ). Only a glass transition occurred for the ionic liquids [C 1 C 1 IM] [DMP], [C 1 C 2 MOR]­[DMP], and [C 1 C 2OH PYR]­[DMP] in the temperature range investigated.…”

“…Ionic liquids’ melting points (and associated heat effects) increase in the order listed below: T m = 285.2 K (0.727 kJ mol –1 ) for [C 1 C 2 PYR]­[DMP] < 294.9 K (13.97 kJ mol –1 ) for [N 1,2,2,2 ]­[DMP] ∼ 295.2 K (2.026 kJ mol –1 ) for [C 1 C 2 PIP]­[DMP] < 311.7 (16.85 kJ mol –1 ) for [C 1 C 2 IM]­[DMP] (this work) < 318.5 K (10.10 kJ mol –1 ) for [C 1 Py]­[DMP] (this work). Additionally, the (solid–solid) polymorphic transition was found to occur at 274.5 K with an enthalpy of 0.862 kJ mol –1 for the ionic liquid [C 1 C 2 PYR]­[DMP] .…”

“…10,44,45 Systems of this type of ionic liquid with ethanol have less information accessible. The information that is currently accessible relates to the densities, viscosities, or heat capacities of binary IL systems with the following cations: 1,3dimethylimidazolium, 10,24,29 1-ethyl-3-methylimidazolium, 11,38 1-butyl-3-methylimidazolium, 38 1-ethyl-1-methylpyrrolidinium, 20 1-hydroxyethyl-1-methyl-pyrrolidinium, 20 1-ethyl-1methylmorpholinium, 23 1-ethyl-1-methyl-piperidinium, 23 and N,N,N-triethyl-N-methylammonium. 23 calculated, and they have been compared with those for {ionic liquid + water} and a lithium bromide aqueous solution, thus demonstrating the potential for the use of {IL + ethanol} systems in a real refrigeration unit.…”

“…Numerous articles in the literature show the thermodynamic and physicochemical characteristics of pure ILs based on dimethyl phosphate, including liquid density, − viscosity, ,,,,, or heat capacity. ,, Systems of this type of ionic liquid with ethanol have less information accessible. The information that is currently accessible relates to the densities, viscosities, or heat capacities of binary IL systems with the following cations: 1,3-dimethylimidazolium, ,, 1-ethyl-3-methylimidazolium, , 1-butyl-3-methylimidazolium, 1-ethyl-1-methylpyrrolidinium, 1-hydroxyethyl-1-methyl-pyrrolidinium, 1-ethyl-1-methylmorpholinium, 1-ethyl-1-methyl-piperidinium, and N,N,N -triethyl- N -methylammonium Table lists the experimental and literature VLE values for the {IL + ethanol} binary system.…”