Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, USA.
Phys Chem Chem Phys. 2012 Sep 21;14(35):12157-64. doi: 10.1039/c2cp41964e. Epub 2012 Aug 7.
Using molecular dynamics simulations, the melting points and liquid phase dynamic properties were studied for four alkyl-imidazolium-based ionic liquids, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-n-butyl-2,3-dimethylimidazolium hexafluorophosphate ([BMMIM][PF6]), 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF6]), and 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate ([EMMIM][PF6]), respectively. Experimentally it has been observed that the substitution of a methyl group for a hydrogen at the C2 position of the cation ring leads to an increase in both the melting point and liquid phase viscosity, contrary to arguments that had been made regarding associations between the ions. The melting points of the four ionic liquids were accurately predicted using simulations, as were the trends in viscosity. The simulation results show that the origin of the effect is mainly entropic, although enthalpy also plays an important role.
利用分子动力学模拟，研究了四种烷基-咪唑鎓基离子液体的熔点和液相动态性质，分别为 1-正丁基-3-甲基咪唑六氟磷酸盐([BMIM][PF6])、1-正丁基-2,3-二甲基咪唑六氟磷酸盐([BMMIM][PF6])、1-乙基-3-甲基咪唑六氟磷酸盐([EMIM][PF6])和 1-乙基-2,3-二甲基咪唑六氟磷酸盐([EMMIM][PF6])。实验观察到，在阳离子环的 C2 位置上用甲基取代氢原子会导致熔点和液相粘度都增加，这与关于离子之间的缔合的观点相反。通过模拟准确预测了这四种离子液体的熔点以及粘度的趋势。模拟结果表明，这种效应的起源主要是熵的，尽管焓也起着重要作用。
