水对离子液体微观结构和传输性能的影响

Effect of water on microstructure and transport properties of ionic liquids

离子液体以独特的结构性质和优异的物化性能而显示出日益广阔的应用前景,但实际应用中,不可避免的混入一些水,水的存在会对离子液体结构和性质产生显著影响。本工作采用分子动力学模拟方法系统研究了不同含量的水对三种离子液体([Bmim][B(CN)_(4)],[Bmim][PF_(6)]和[Bmim][Tf_(2)N])微观结构、传输性能和相互作用的影响规律。通过氢键数目的分析可发现,随水含量增加,阴离子与水之间的氢键数目显著增加,而阴离子与阳离子之间氢键数目显著下降;阴离子与水之间的氢键数目大于阳离子与水之间的氢键数目。进一步计算了自扩散系数,发现随水浓度的升高,离子液体的自扩散系数逐渐增大,且水溶性较高的离子液体扩散系数显著增加。径向分布函数和配位数的计算结果表明,阴离子与水的相互作用强于阳离子与水的作用,但水的存在对阴离子与阳离子之间结构几乎没有影响。空间分布函数的结果表明,阴离子和水分子在阳离子咪唑环上H5和H4位置的分布区域存在竞争,减弱了阴离子与阳离子之间的相互作用。本研究有助于进一步理解离子液体与水体系的微观作用机理和推进离子液体的应用。

Due to the unique structure and excellent physicochemical properties, ionic liquids have shown a broad application prospect. However, some water inevitably exists in ionic liquids during applications. The presence of waters has a significant impact on the structure and properties of ionic liquids. In this work, the microstructure, interactions and transport properties of [Bmim][B(CN)_(4)], [Bmim][PF_(6)] and [Bmim][Tf_(2)N] with different contents of water were systematically studied by molecular dynamics simulations. By analyzing the number of hydrogen bonds for the system of ionic liquids and water, it was found that the number of hydrogen bonds between anions and water increased significantly with increasing water content, while the number of hydrogen bonds between anions and cation decreased gradually with the increase of water content. The number of hydrogen bonds between anions and water were greater than those between cations and water. The self-diffusion coefficients for the mixture of ionic liquids and water were calculated. It was found that the self-diffusion coefficient of the ionic liquid increased gradually with the increase of the water content. The more hydrophilic the ionic liquids were, the more the diffusion coefficient increased. The radial distribution function and coordination number results showed that the interaction between anions and water was stronger than that between cations and water. However, addition of water to ionic liquids was found to have no impact on the structure between anions and cations. In addition, the results of the spatial distribution function suggested that the distribution of anions and water around H5 and H4 of imidazolium cation ring were competitive, which reduced the interaction between cations and anions. This research results can help to further understand the microscopic mechanism of ionic liquids and water system and promote the application of ionic liquids.