The study of excess enthalpy for {ionic liquid plus ethanol} binary systems as new working fluids in absorption refrigeration technology

Binary systems composed of phosphonium - based ionic liquid and water, or short-chain alcohols exhibit properties which render them suitable as candidates for working fluids in absorption refrigeration technology. This work presents the study of the excess enthalpy (H-E) of mixing in ethanolic solutions of the following ionic liquids: 1-ethyl-1-methylpiperidinium dimethyl phosphate, [C1C2PIP][DMP], 1-ethyl-1-methyl-pyrrolidinium dimethyl phosphate, [C1C2PYR][DMP], N,N,N-triethyl-N-methylammonium dimethyl phosphate, [N-1,N-2,N-2,N-2] [DMP],1-hydroxyethyl-1-methylpyrrolidinium dimethyl phosphate, [C1C2OHPYR][DMP], 1-ehtyl-1-methylmorpholinium diethyl phosphate, [C1C2MOR][DEP], 1-ethyl-1-methylpyrrolidinium diethyl phosphate, [C1C2PYR] [DEP], and 1-ethyl-3-methylimidazolium diethyl phosphate, [C1C2IM][DEP] is presented. The experiment was performed at temperature T = 298.15 K and under ambient pressure using isothermal titration calorimetry technique. This work presents the effect of the core structure of the cation and the effect of the carbon chain length in the anion of the ionic liquid on the H-E value of the systems studied. Experimental H-E values were correlated using Redlich - Kister type equation. The standard deviations for the correlation vary from sigma = 9 J.mol (-1) for {[C1C2OHPYR][DMP] + ethanol} to 28 J.mol (-1) for {[C1C2IM][DMP] + ethanol} showing significant agreement between experimental and calculated values. The negative excess enthalpy of mixing over the entire composition range was found in each system which indicate stronger (IL - ethanol) interactions associated with hydrogen bond formation in this system, compared to (IL - IL) and (ethanol - ethanol) interactions. For each system, the minimum of the excess enthalpy of mixing was determined, which increases in the following series: [N-1,N-2,N-2,N-2][DMP] (H-E = 2645.9 J.mol (-1)) < [C1C2PYR] [DMP] < [C1C2IM][DEP] <[C1C2PYR][DEP] < [C1C2PIP][DMP] < [C1C2MOR][DMP] < [C1C2OHPYR][DMP] (H-E = 881.5 J.mol (-1)). This characteristic is desirable from the point of view of the possibility of using these systems in absorption refrigeration technology.