Do Ionic Liquids Slow Down in Stages?

High impact recent articles have reported on the existence of a liquid-liquid (L-L) phase transition as a function of both pressure and temperature in ionic liquids (ILs) containing the popular trihexyltetradecylphosphonium cation (P_666,14⁺), sometimes referred to as the "universal liquifier". The work presented here reports on the structural-dynamic pathway from liquid to glass of the most well-studied IL comprising the P_666,14⁺ cation. We present experimental and computational evidence that, on cooling, the path from the room-temperature liquid to the glass state is one of separate structural-dynamic changes. The first stage involves the slowdown of the charge network, while the apolar subcomponent is fully mobile. A second, separate stage entails the slowdown of the apolar domain. Whereas it is possible that these processes may be related to the liquid-liquid and glass transitions, more research is needed to establish this conclusively.