MD simulation of the mesomorphic behaviour of 1-hexadecyl-3-methylimidazolium nitrate: assessment of the performance of a coarse-grained force field

Abstract

We report on the thermal behaviour of a model system based on a coarse-grained force field (CG-FF) developed for ionic liquids here extended to the description of the ionic liquid crystal 1-hexadecyl-3-methylimidazolium nitrate. The phase diagram has been investigated as a function of the temperature in the NPT ensemble. We have identified three distinct phases, a crystal phase, the smectic A phase and the isotropic liquid phase. The various phases have been characterized by several pair distribution functions, density profiles and order parameters. A comparison is made with the experimental data available and suggestions on how to re-parameterise the CG-FF in order to achieve a better accuracy for the description of ionic mesophases are presented. On the other hand the results reported in this work demonstrate that the model potential is sufficiently accurate, at a qualitative level, to obtain useful insights into the relation between structural parameters, such as anion and cation core size, chain lengths and others, and the thermal range of stability of the ionic mesophases.