Two Liquids in One: Liquid-Liquid Transition in Ionic Liquids*

A liquid-liquid transition is a transformation from one liquid structure to another through a first-order phase transition. This type of phase transition has been reported in various systems, including water, silicon, phosphorous, and triphenyl phosphite. Recently, a liquid-liquid transition has been identified in ionic liquids bearing the trihexyltetradecylphosphonium cation. Understanding this transition is vital to our understanding of the liquid state in general. In this study, a homologous series of ionic liquids with various anions has been investigated using X-ray scattering techniques, broadband dielectric spectroscopy, Raman spectroscopy, and differential scanning calorimetry to characterize the nature of the liquid-liquid transition and identify molecular parameters that influence the phase behavior in these materials. The results suggests a spinodal decomposition mechanism of the liquid-liquid transition and the type of anion plays a key role in determining the phase behavior of the material.