What Determines the Eutectic Temperature for Ionic and Nonionic Acetamide Deep Eutectic Solvents? Entropy–Enthalpy Tug‐of‐War

We report here a theoretical formalism for predicting eutectic temperature for several experimentally known deep eutectic solvents (DESs) (ionic and nonionic) by separating the enthalpic and entropic contributions to the change in free energy of mixing. By incorporating both attractive and repulsive interactions into the calculation of enthalpic and entropic changes, this theory provides a thermodynamic explanation for the negative deviations from ideal behaviour in DESs. We have examined five acetamide-based DESs where the other components are, respectively, urea, lithium bromide, lithium nitrate, calcium nitrate, and strontium nitrate. Enthalpic contribution has been obtained from the simulated trajectories, while the entropic component is calculated from the two-phase thermodynamics method. The predicted eutectic temperatures are better than those from the ideal solution model when compared against experiments. Interestingly, the nature of interparticle interaction (ionic and nonionic) has been found to decide the dominance of either enthalpy or entropy, thereby setting up a tug-of-war between these two thermodynamic forces for stabilizing the liquid phase of these two-component systems at a temperature much lower than the melting temperatures of the neat components. This theory can be suitably expanded to predict the melting temperature versus composition curve for binary DESs.