Molar Enthalpy of Mixing for Choline Chloride/Urea Deep Eutectic Solvent + Water System

The molar enthalpies of mixing for binary systems of choline chloride (chcl)/urea deep eutectic solvents (mole ratios of 1:1.5, 1:2, and 1:2.5) with water were measured at 308.15 and 318.15 K under atmospheric pressure with an isothermal calorimeter. The binary mixture of (chcl/urea (1:2.5) + water) showed endothermic behavior over the entire range of compositions, while the binary mixtures of (chcl/urea (1:1.5) + water) and (chcl/urea (1:2) + water) showed endothermic behavior first and then was changed to be exothermic with increasing content of deep eutectic solvents. The Redlich–Kister (RK) equation and the nonrandom two-liquid (NRTL) model were used to fit experimental molar enthalpies of mixing. The NRTL model with the fitted parameters was further used to predict the vapor pressure for the three systems and was compared with the experimental data from literature. For the binary mixtures of (chcl/urea (1:2) + water), the predicted vapor pressure agreed well with the experimental data only when the temperature was lower than 333.15 K and the mole fraction of chcl/urea (1:2) was lower than 0.1. Otherwise, the deviation increased greatly with an increase of the amount of chcl/urea (1:2).