Understanding the Unique Thermodynamic Behavior of MgTFSI₂/DME Solutions. Part 2: Thermodynamic Hypothesis, Raman Analyses, and Driving Forces

As described in part 1, a unique thermodynamic behavior was observed for solutions of magnesium bis(trifluoromethanesulfonimide) in 1,2-dimethoxyethane. These solutions display phase characteristics that are strongly dependent on temperature: between 287 and 373 K, the solutions separate into two immiscible phases (i.e., upper and lower), with different electrolyte concentrations, volumes, and densities. The driving forces for phase separation are explained by statistical thermodynamics on the basis of the conformers’ populations. By means of Raman spectroscopy analyses, we calculated the conformers’ distributions and individual conformers’ entropies, revealing a considerable increase in the entropy of the dilute upper phase with temperature. This increase is due to the combination of a decrease in salt concentration and an increase in the population of entropically favored TTT, TTG, TGG, and TGG′ conformers at the expense of TGT, which has the lowest inherent entropy. On the other hand, the denser lower phase is characterized by a higher molecular order due to high salt concentration and is richer in conformers that interact favorably in the polar medium. In the lower phase, the salt concentration increases with temperature, and the enthalpically favored TGT and TGG largely dominate. Above 333 K, the trends are reversed as the entropy of mixing becomes increasingly significant. This study may have broad implications for solution chemistry, the thermodynamics of mixtures, solution- and phase-transfer catalysis, and the nonaqueous electrochemistry of multivalent cations.