Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India.
Phys Chem Chem Phys. 2011 Feb 21;13(7):2714-22. doi: 10.1039/c0cp01272f. Epub 2010 Dec 13.
The influence of alkyl chain symmetry of the imidazolium cation on the structure and properties of the ionic liquid-vapour interface has been addressed through molecular dynamics simulations. The anion chosen is bis(trifluoromethylsulfonyl)imide (NTf(2)). Profiles of number densities, orientation of cations, charge density, electrostatic potential, and surface tension have been obtained. At the interface, both cations and anions were present, and the alkyl chains of the former preferred to orient out into the vapour phase. A large fraction of cations preferred to be oriented with their ring-normal parallel to the surface and alkyl chains perpendicular to it. These orientational preferences are reduced in ionic liquids with symmetric cations. Although the charge densities at the interface were largely negative, an additional small positive charge density has been observed for systems with longer alkyl chains. The electrostatic potential difference developed between the liquid and the vapour phases were positive and decreased with increasing length of the alkyl group. The calculated surface tension of the liquids also decreased with increasing alkyl chain length, in agreement with experiment. The surface tension of an ionic liquid with symmetric cation was marginally higher than that of one with an asymmetric, isomeric cation.
通过分子动力学模拟研究了离子液体-蒸气界面中咪唑阳离子的烷基链对称性对结构和性质的影响。所选择的阴离子是双（三氟甲基磺酰基）亚胺（NTf(2)）。得到了数密度、阳离子取向、电荷密度、静电势和表面张力的分布。在界面处，既有阳离子又有阴离子，前者的烷基链倾向于进入气相。很大一部分阳离子优先与环面平行于表面，烷基链垂直于表面。这些取向偏好会在对称阳离子的离子液体中减少。尽管界面处的电荷密度主要为负，但对于具有较长烷基链的系统，观察到了额外的小正电荷密度。在液相和气相之间形成的静电位差为正，并随烷基链长度的增加而减小。计算出的液体表面张力也随烷基链长度的增加而减小，这与实验结果一致。具有对称阳离子的离子液体的表面张力略高于具有不对称、异构阳离子的离子液体的表面张力。
