Percolation Phase Transition from Ionic Liquids to Ionic Liquid Crystals

TL;DR

This study uses molecular dynamics simulations to reveal that ionic liquids undergo a critical percolation phase transition from nanoscale segregated liquids to ionic liquid crystals as side-chain length increases.

Contribution

It uncovers the microscopic mechanism of phase transition in ionic liquids, demonstrating a percolation transition as side chains lengthen, which was previously unclear.

Findings

Formation of non-polar domains before transition

Abrupt percolation phase transition observed

Insights into phase behavior of ionic liquids

Abstract

As typical complex liquids, ionic liquids (ILs) exhibit phases beyond the description of simple liquid theories. In particular, with an intermediate cationic side-chain length, ILs can form the nanoscale segregated liquid (NSL) phase, which will eventually transform into the ionic liquid crystal (ILC) phase when the side chains are adequately long. However, the microscopic mechanism of this transformation is still unclear. In this work, by means of coarse-grained molecular dynamics simulation, we show that, with increasing cationic side-chain length, some local pieces of non-polar domains are gradually formed by side chains aligned in parallel inside the NSL phase, before an abrupt percolation phase transition happens when the system transforms into the ILC phase, manifesting that it is a critical phenomenon. Percolation phase transition is applied to ILs, providing new insights into many recent observations both in experiments and simulations.