CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, 55 East Zhongguancun Road, P. O. Box 2735, Beijing 100190, China.
Soft Matter. 2020 Jan 2;16(2):411-420. doi: 10.1039/c9sm02115a.
The thermotropic phase behavior of ionic liquids and ionic liquid crystals based on novel N-alkyl-3-methylpyridinium halides, trihalides and dichloroiodates was experimentally studied by polarized optical spectroscopy (POM) and differential scanning calorimetry (DSC) as well as by molecular dynamics (MD) simulation. In the experiments, the existence and thermal range of stability of the smectic phase of these ionic liquid crystals are found to strongly depend on the volume ratio between the cation and anion, that is their relative size. Only compounds with a relatively large volume ratio of the cation to anion, i.e., those with longer cationic alkyl chains and monoatomic halide anions, have a stable smectic A phase. Both melting points and clearing points increase with such a ratio. The MD simulation results qualitatively agree very well with the experimental data and provide molecular details which can explain the experimentally observed phenomena: the stronger van der Waals interactions from the longer alkyl chains and the stronger electrostatic interactions from the smaller anions with a higher charge density increase the stability of both the crystal phase and the smectic phase; this also prevents the ionic layers from easily mixing with the hydrophobic regions, a mechanism that ultimately leads to a nanosegregated isotropic liquid phase.
新型 N-烷基-3-甲基吡啶卤化物、三卤化物和二氯碘酸盐基离子液体和离子液晶的热致相行为通过偏光显微镜（POM）和差示扫描量热法（DSC）以及分子动力学（MD）模拟进行了实验研究。在实验中，发现这些离子液晶的层状相的存在和热稳定性范围强烈依赖于阳离子和阴离子之间的体积比，即它们的相对大小。只有阳离子与阴离子的体积比相对较大的化合物，即那些具有较长的阳离子烷基链和单价卤化物阴离子的化合物，才具有稳定的层状 A 相。熔点和清亮点都随此比例增加。MD 模拟结果与实验数据定性非常吻合，并提供了可以解释实验观察到的现象的分子细节：较长的烷基链产生更强的范德华相互作用，具有更高电荷密度的较小阴离子产生更强的静电相互作用，这两种相互作用都增加了晶体相和层状相的稳定性；这也防止了离子层与疏水区轻易混合，这种机制最终导致纳米分离各向同性液相。
