Rivera José L, Molina-Rodríguez Luis, Ramos-Estrada Mariana, Navarro-Santos Pedro, Lima Enrique
Graduate School of Physics Engineering, Universidad Michoacana de San Nicolás de Hidalgo Morelia C.P. 58000 Michoacán Mexico
Faculty of Chemical Engineering, Universidad Michoacana de San Nicolás de Hidalgo Morelia C.P. 58000 Michoacán Mexico.
RSC Adv. 2018 Mar 13;8(18):10115-10123. doi: 10.1039/c8ra00915e. eCollection 2018 Mar 5.
We carried out molecular dynamics simulations of the liquid/vacuum equilibrium of the ionic liquid [bmim][triflate] in a wide range of temperatures (323.15 to 573.15 K). The results showed liquid phases with high densities even at temperatures close to the decomposition temperature of the liquid. The density and surface tension behaviors are linear across this wide range of temperatures, which is an extension of the behaviors of these systems at low temperatures, where these properties have been experimentally measured. The interfacial region shows peaks of adsorption of the ions; they are ordered, with the alkyl chains of the [bmim] cations pointing out of the liquid, and the tailing angle of the chains becomes 90° at higher temperatures. The alkyl chains are part of the outermost interfacial region, where intra- and intermolecular tangential forces are in equilibrium; thus, they do not contribute to the total surface tension. Unlike simpler organic liquids, the surface tension is composed of positive normal contributions of intermolecular interactions; these are almost in equilibrium with the negative normal contributions of intramolecular interactions, which are mainly vibrations of the distance and the angle of valence. The pressure profiles show that the molecules are in 'crushed' conformations internally in the bulk liquid and even more so in the normal direction at the interface. The total pressure profiles show values with physical meaning, where the tangential peaks show higher values than normal pressures and give rise to the surface tension. Short cutoff radii for the calculation of intermolecular forces (less than 16.5 Å) produce a system that is not mechanically stable in the region of the bulk liquid (confirmed by radial distribution function calculations); this produces a difference between the normal pressure and the average of the tangential pressures, which affects the calculation of the surface tension due to overestimation by up to 20% when using the global expression, which is extensively used for the calculation of surface tension. The use of a sufficiently long cutoff radius avoids these mechanical balance problems.
我们对离子液体[bmim][triflate]在很宽的温度范围（323.15至573.15 K）内的液/真空平衡进行了分子动力学模拟。结果表明，即使在接近液体分解温度的温度下，液相仍具有高密度。在这个很宽的温度范围内，密度和表面张力行为呈线性关系，这是这些系统在低温下行为的一种延伸，在低温下这些性质已通过实验测量。界面区域显示出离子吸附峰；它们是有序的，[bmim]阳离子的烷基链指向液体外部，并且在较高温度下链的尾角变为90°。烷基链是最外层界面区域的一部分，在该区域内分子内和分子间的切向力处于平衡状态；因此，它们对总表面张力没有贡献。与更简单的有机液体不同，表面张力由分子间相互作用的正的法向贡献组成；这些贡献几乎与分子内相互作用的负的法向贡献处于平衡状态，分子内相互作用主要是距离和价键角的振动。压力分布表明，分子在本体液体内部处于“挤压”构象，在界面处的法向更是如此。总压力分布显示出具有物理意义的值，其中切向峰显示的值高于法向压力，并产生表面张力。用于计算分子间力的短截止半径（小于16.5 Å）会导致系统在本体液体区域内机械不稳定（通过径向分布函数计算得到证实）；这会导致法向压力与切向压力平均值之间存在差异，当使用广泛用于计算表面张力的全局表达式时，由于高估，这会影响表面张力的计算，高估幅度高达20%。使用足够长的截止半径可避免这些机械平衡问题。
