Driving Forces of UCST and LCST Liquid-Liquid Phase Separations

A liquid mixture of two or more components can phase separate into two liquid phases by changing the temperature. Ordinary chemical perception would attribute this transition to a competition between the favorable enthalpy of the biphasic state and the favorable entropy of the (mixed) monophasic state, rationalizing the stability of these states, respectively, at low and high temperatures. This is indeed the case for a UCST liquid-liquid phase separation. However, in contrast, in LCST behavior, the monophasic mixed state is observed at low temperatures, whereas the biphasic state is observed at high temperatures. This perplexing behavior has triggered different hypotheses about mechanisms capable of explaining it. In this paper, we study, by molecular dynamics simulations, two ionic liquids (ILs), [Hbet]-[Tf₂N] and [P₄₄₄₄]-[TMBS], that when mixed with water exhibit, respectively, a UCST and an LCST liquid-liquid phase separation. The computational phase diagrams of these two IL/water mixtures were shown to be in very close agreement with those obtained experimentally. Here, we investigate the effective interactions between the ions in aqueous solutions at a very large dilution. We find that for an LCST IL, an increase in temperature induces substantial strengthening of the effective cation-anion and cation-cation attractions (whereas the anion-anion interactions are not significantly altered). Conversely, a UCST IL displays marginal changes in the effective interactions between the ions with a change in temperature. Calculations of thermodynamic parameters indicate that for an LCST IL, the driving force for forming cation-anion contact pairs is predominantly entropic. This positive entropy change for association, which we also attribute to the formation of the biphasic state, does not include the mixing entropy and is ascribed to changes in the properties of surrounding water molecules. On the other hand, for a UCST IL, the driving force for cation-anion association is purely enthalpic. These findings are supplemented by demonstrating that the reorientations and translations of water molecules in the first solvation shell around the LCST ions (in particular, around the cation) are slower than those around the corresponding UCST ions. Given the anomalous behavior of strengthening effective interactions upon heating, we conclude that the transition of a mixed homogeneous state to a liquid-liquid phase-separated state upon a temperature increase (LCST behavior) is driven primarily by hydrophobic interactions.