Percolation Phase Transition from Ionic Liquids to Ionic Liquid Crystals

Shen Li,Yanting Wang

doi:10.1038/s41598-019-49493-3

Abstract

Due to their complex molecular structures and interactions, phase behaviors of complex fluids are quite often difficult to be identified by common phase transition analysis methods. Percolation phase transition, on the other hand, only monitors the degree of connection among particles without strict geometric requirements such as translational or orientational order, and thus suitable for pinpointing phase transitions of complex fluids. As typical complex fluids, 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) state, which will eventually transform into the ionic liquid crystal (ILC) structure 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. This work not only identifies that the NSL to ILC phase transition is a critical phenomenon, but also demonstrates the importance of percolation theory to complex fluids.

Highlights

In contrast to simple liquids, complex fluids are composed of molecules with complex structures and interactions, whose physical properties are beyond the description of simple liquid theories
The van der Waals (VDW) interaction between cationic side chains increases with side-chain length, and when the side chains are adequately long, the strength of their VDW interaction allows the side chains to align in parallel and form ionic liquid crystal (ILC) that have unique features beyond traditional liquid crystals[18,19], e.g., polar layers are connected by charged atomic groups[20]
This Ionic liquids (ILs) to ILC phase transition is a critical phenomenon, which is difficult or even impossible to be unambiguously identified by common phase transition analysis methods, demonstrating that percolation phase transition is useful for analyzing phase behaviors of complex fluids

Summary

Introduction

In contrast to simple liquids, complex fluids are composed of molecules with complex structures and interactions, whose physical properties are beyond the description of simple liquid theories. The corresponding physical picture is that, with increasing cationic side-chain length, the IL structures first gradually change from the globally isotropic NSL state to a liquid state with some local small clusters formed by cationic side chains aligned in parallel, and go through a sharp percolation phase transition when the majority of the cationic side chains are globally aligned in parallel This IL to ILC phase transition is a critical phenomenon, which is difficult or even impossible to be unambiguously identified by common phase transition analysis methods, demonstrating that percolation phase transition is useful for analyzing phase behaviors of complex fluids

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Conclusion