Comparison of the Ionic Liquid Crystal Phase of [C12C1im][BF4] and [C12C1im]Cl by Atomistic MD Simulations

Giacomo Saielli

doi:10.3390/cryst10040253

Abstract

We present fully atomistic molecular dynamics (MD) simulations at 450 K of two ionic liquid crystals in the smectic phase: 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12C1im][BF4]) and 1-dodecyl-3-methylimidazolium chloride ([C12C1im]Cl). The two systems experimentally exhibit different ranges of thermal stability of the ionic smectic phase: The chloride salt has a more stable LC phase, between 270.3 K and 377.6 K, with a range of SmA of more than 107 K. In contrast, the tetrafluoroborate salt has a smectic phase between 299.6 K and 311.6 K, with a range of mesophase of just 12 K. The MD simulations show that the chloride system is stable in the smectic phase for the 5 ns of simulation, while the tetrafluoroborate salt melts into an isotropic ionic liquid, in qualitative agreement with the experiments. Comparison of the electrostatic and van der Waals energetic contributions enables one to rationalize the observed behavior as being due to the large size of the [BF4] anion: a larger size of the anion means a lower charge density, and therefore a weaker electrostatic interaction in the ionic layer.

Highlights

Ionic liquid crystals (ILCs) are materials formed by ions and exhibiting thermotropic liquid crystal (LC) behavior; that is, the mesophases are observed as a function of the temperature for the pure samples [1,2]
We proposed that the stability of the ionic smectic phase is governed by a balance between the energetic stability of the two alternating layers, the ionic and the hydrophobic one: it is possible to destabilize one layer, for example, the ionic layer, by making the anion larger without destroying the ionic smectic phase, provided that we increase the stability of the hydrophobic layer, mostly due to van der Waals interaction, by increasing the chain length
In this work we extend our previous investigation with a direct comparison of two imidazolium ionic liquid crystals, 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12 C1 im][BF4 ])

Summary

Introduction

Ionic liquid crystals (ILCs) are materials formed by ions and exhibiting thermotropic liquid crystal (LC) behavior; that is, the mesophases are observed as a function of the temperature for the pure samples [1,2]. The key molecular feature which is necessary in order to have ionic liquid crystal behavior is a relatively long alkyl chain attached to the ionic part. This will guarantee nano-segregation between the ionic parts and the hydrophobic chains, leading, almost invariably, to layered, that is smectic, ILC phases, wherein ionic layers are alternated with hydrophobic alkyl layers, though no positional order exists within each layer. A few years ago, Nelyubina et al reported an interesting correlation, experimentally

Methods

Results

Discussion

Conclusion