Brooks Nicholas J, Castiglione Franca, Doherty Cara M, Dolan Andrew, Hill Anita J, Hunt Patricia A, Matthews Richard P, Mauri Michele, Mele Andrea, Simonutti Roberto, Villar-Garcia Ignacio J, Weber Cameron C, Welton Tom
Department of Chemistry , Imperial College London , London , SW7 2AZ , UK . Email:
Department of Chemistry , Materials and Chemical Engineering "Giulio Natta" , Politecnico di Milano , Piazza L. da Vinci 32 , 20133 Milan , Italy.
Chem Sci. 2017 Sep 1;8(9):6359-6374. doi: 10.1039/c7sc01407d. Epub 2017 Jul 11.
The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [CCim] cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.
离子液体（IL）混合物的形成已被提议作为一种合理微调离子液体物理化学性质以用于各种应用的方法。然而，形成此类混合物对液体最终性质的影响才刚刚开始被理解。为了更全面地理解离子液体混合的热力学以及混合这些液体对其结构和物理化学性质的影响，我们使用小角X射线散射（SAXS）、正电子湮没寿命谱（PALS）和氙核磁共振（Xe NMR）技术系统地探索了含有常见[CCim]阳离子和不同阴离子的离子液体混合物的空间排列和自由体积。就非极性域的大小和离子间距离而言，阴离子大小对离子液体及其混合物的空间排列影响最大。研究发现，由于阴离子原子间电荷密度分布导致的相反电荷离子之间库仑吸引力的差异也显著影响这些离子间距离。尽管这些技术的时间尺度差异很大，但发现与这些混合物自由体积相关的PALS和Xe NMR结果彼此密切相关。此外，从这些测量中计算出的过量自由体积与所研究离子液体混合物的混合过量体积非常吻合。这些技术的一致性表明这些液体混合物的静态和动态自由体积紧密相连。因此，与由离子的大小、形状和库仑吸引力产生的离子空间排列相比，诸如氢键等流动过程对这些液体的自由体积贡献不大。鉴于自由体积与诸如粘度和电导率等传输性质之间的关系，这些结果为离子液体混合物的结构、混合热力学及其物理化学性质之间提供了联系。
