Yan Tianying, Li Shu, Jiang Wei, Gao Xueping, Xiang Bing, Voth Gregory A
Institute of New Energy Material Chemistry and Department of Material Chemistry, Nankai University, Tianjin 300071, China.
J Phys Chem B. 2006 Feb 2;110(4):1800-6. doi: 10.1021/jp055890p.
Molecular dynamics simulations for the liquid-vacuum interface of the ionic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+/NO3-) were performed for both electronically polarizable and nonpolarizable potential energy surfaces. The interfacial structural properties, such as the oscillation in the number density profile, the orientational ordering, and the local clustering in the interfacial region, were calculated. The simulations with both the polarizable and nonpolarizable model demonstrate the existence of an inhomogeneous interfacial structure normal to the surface layer. It was found for both models that the ethyl tail group on EMIM+ is likely to protrude outward from the surface. In the outmost surface layer, the cation is likely to lie on the surface with the imidazolium ring parallel to the interface, while there is a second region with enhanced density from that in the bulk where the cation preferably slants with the imidazolium ring tending to be perpendicular to the surface. The results also reveal that the electronic polarization effect is important for the ionic liquid interface. It is found that the cation is likely to be segregated at the ionic liquid surface for the polarizable model, while for the nonpolarizable model, the anion is found to be more likely to exhibit such behavior. The surface tension of the polarizable model (58.5 +/- 0.5 mN/m) is much smaller than that of the nonpolarizable model (82.7 +/- 0.6 mN/m), in better agreement with extrapolated experimental measurements on similar ionic liquid systems.
针对离子液体1-乙基-3-甲基咪唑鎓硝酸盐（EMIM⁺/NO₃⁻）的液-真空界面，在电子可极化和不可极化势能面条件下都进行了分子动力学模拟。计算了界面结构性质，如数密度分布中的振荡、取向有序性以及界面区域的局部聚集情况。可极化模型和不可极化模型的模拟均表明，垂直于表面层存在不均匀的界面结构。对于这两种模型都发现，EMIM⁺上的乙基尾基可能从表面向外突出。在最外层表面，阳离子可能以咪唑环平行于界面的方式位于表面，而在 bulk 中密度增强的第二个区域，阳离子更倾向于倾斜，咪唑环趋于垂直于表面。结果还表明，电子极化效应对于离子液体界面很重要。发现对于可极化模型，阳离子可能在离子液体表面偏析，而对于不可极化模型，阴离子更有可能表现出这种行为。可极化模型的表面张力（58.5±0.5 mN/m）远小于不可极化模型的表面张力（82.7±0.6 mN/m），这与对类似离子液体系统的外推实验测量结果更相符。
