Thermodynamique et Interactions Moléculaires, Université Blaise Pascal Clermont-Ferrand - CNRS, 24 avenue des Landais, 63177 Aubière, France.
J Phys Chem B. 2009 Nov 5;113(44):14708-18. doi: 10.1021/jp905585e.
Molecular dynamics simulations of the liquid-vacuum interface of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide were performed with an all-atom force field. Structural properties of the interface, such as orientational ordering and density profiles, were calculated. The hexyl side chain of the cation is likely to protrude outward from the surface layer. There is a region with enhanced density from that of the bulk where the cation preferably slants with the imidazolium ring tending to be perpendicular to the interface. The surface tensions are calculated using mechanical and thermodynamic definitions via profiles along the direction normal to the interface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The use of local pressure profiles provides an explanation to the systematic problems encountered by several researchers to obtain accurate values of the surface tension at low temperature. Even when macroscopically the system looks in equilibrium, locally this is not accomplished.
采用全原子力场对离子液体 1-己基-3-甲基咪唑双（三氟甲基磺酰基）酰胺的液-真空界面进行了分子动力学模拟。计算了界面的结构特性，如取向有序和密度分布。阳离子的己基侧链很可能从表面层向外突出。在密度高于体相的区域，阳离子优选倾斜，其中咪唑环趋于垂直于界面。通过沿垂直于界面的方向的轮廓，使用机械和热力学定义计算表面张力。我们还讨论了由于排斥-分散和静电相互作用导致的表面张力的不同贡献。使用局部压力分布为一些研究人员在获得低温下准确的表面张力值时遇到的系统问题提供了解释。即使从宏观上看系统处于平衡状态，局部上也并非如此。
