Abstract
Five distinct crystal structures, based on experimental data or constructed manually, of ionic liquid [C14Mim][NO3] were heated in NPT molecular dynamics simulations under the same pressure such that they melted into the liquid crystal (LC) phase and then into the liquid phase. It was found that the more entropy-favored structure had a higher solid-LC transition temperature: Before the transition into the LC, all systems had to go through a metastable state with the side chains almost perpendicular to the polar layers. All those crystals finally melted into the same smectic-A LC structure irrelevant of the initial crystal structure.
Highlights
Ionic liquid crystals (ILCs) are the intersections of ionic liquids (ILs) and liquid crystals (LCs), with the features of ILs being comprised of pure ions and of LCs having order(s) in one or two dimensions, whose application prospects [1,2] have been appreciated in the most recent twenty years.Long-chain imidazolium-based ILs [Cn Mim]X, with a proper combination of alkyl-chain length n and anion type X, are well known to be able to form LC structures and have been intensively studied
Transition temperature, and a metastable state always occurred during the solid-LC transition
If CrI does not turn to CrII at the intersection of their free energy curves in Figure 1a in a finite-time simulation, it turns to LC at a lower temperature with a smaller entropy near the transition points, and CrII turns to LC at a higher temperature with a larger entropy, which coincides with the simulation data of the IL system here: Generally, the IL
Summary
Ionic liquid crystals (ILCs) are the intersections of ionic liquids (ILs) and liquid crystals (LCs), with the features of ILs being comprised of pure ions and of LCs having order(s) in one or two dimensions, whose application prospects [1,2] have been appreciated in the most recent twenty years.Long-chain imidazolium-based ILs [Cn Mim]X, with a proper combination of alkyl-chain length n and anion type X, are well known to be able to form LC structures and have been intensively studied. IL samples usually undergo several heating-cooling cycles, heated from a crystal phase to reach LC and/or liquid phases and cooled down to recrystallize, during which crystal polymorphism is often reported, which is potentially of great importance in materials science and industry because physiochemical properties of polymorphs, such as thermal stability [6,9,18], ion conductivity [9,12], etc., can significantly affect productions and applications of ILs and ILCs. Due to the complexity of IL and ILC systems, the microscopic details of the phase behaviors related to IL polymorphs can only be speculative solely based on experimental data, in which molecular dynamics (MD) simulations can play a vital role [14,18,19]
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