Gómez-González Víctor, Docampo-Álvarez Borja, Cabeza Oscar, Fedorov Maxim, Lynden-Bell Ruth M, Gallego Luis J, Varela Luis M
Grupo de Nanomateriais e Materia Branda, Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n, E-15782 Santiago de Compostela, Spain.
Facultade de Ciencias, Universidade da Coruña, Campus A Zapateira s/n, E-15008 A Coruña, Spain.
J Chem Phys. 2015 Sep 28;143(12):124507. doi: 10.1063/1.4931656.
We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of a protic (ethylammonium nitrate) and an aprotic (1-butyl-3-methylimidazolium hexaflurophosphate [BMIM][PF6]) room-temperature ionic liquids doped with magnesium and calcium salts with a common anion at 298.15 K and 1 atm. The solvation of these divalent cations in dense ionic environments is analyzed by means of apparent molar volumes of the mixtures, radial distribution functions, and coordination numbers. For the protic mixtures, the effect of salt concentration on the network of hydrogen bonds is also considered. Moreover, single-particle dynamics of the salt cations is studied by means of their velocity autocorrelation functions and vibrational densities of states, explicitly analyzing the influence of salt concentration, and cation charge and mass on these magnitudes. The effect of the valency of the salt cation on these properties is considered comparing the results with those for the corresponding mixtures with lithium salts. We found that the main structural and dynamic features of the local solvation of divalent cations in ionic liquids are similar to those of monovalent salts, with cations being localized in the polar nanoregions of the bulk mixture coordinated in monodentate and bidentate coordination modes by the NO3 and PF6 anions. However, stronger electrostatic correlations of these polar nanoregions than in mixtures with salts with monovalent cations are found. The vibrational modes of the ionic liquid (IL) are seen to be scarcely affected by the addition of the salt, and the effect of mass and charge on the vibrational densities of states of the dissolved cations is reported. Cation mass is seen to exert a deeper influence than charge on the low-frequency vibrational spectra, giving a red shift of the vibrational modes and a virtual suppression of the higher energy vibrational modes for the heavier Ca(2+) cations. No qualitative difference with monovalent cations was found in what solvation is concerned, which suggests that no enhanced reduction of the mobility of these cations and their complexes in ILs respective to those of monovalent cations is to be expected.
我们报告了一项分子动力学研究，该研究针对在298.15 K和1 atm条件下，掺杂有镁盐和钙盐（具有共同阴离子）的质子型（硝酸乙铵）和非质子型（1-丁基-3-甲基咪唑六氟磷酸盐[BMIM][PF6]）室温离子液体混合物的结构和单粒子动力学。通过混合物的表观摩尔体积、径向分布函数和配位数，分析了这些二价阳离子在致密离子环境中的溶剂化作用。对于质子型混合物，还考虑了盐浓度对氢键网络的影响。此外，通过盐阳离子的速度自相关函数和振动态密度研究了盐阳离子的单粒子动力学，明确分析了盐浓度、阳离子电荷和质量对这些量的影响。通过将结果与相应锂盐混合物的结果进行比较，考虑了盐阳离子价态对这些性质的影响。我们发现，离子液体中二价阳离子局部溶剂化的主要结构和动力学特征与单价盐相似，阳离子定位于本体混合物的极性纳米区域，通过NO3和PF6阴离子以单齿和双齿配位模式配位。然而，发现这些极性纳米区域的静电相关性比与单价阳离子盐的混合物更强。离子液体（IL）的振动模式几乎不受盐添加的影响，并报告了质量和电荷对溶解阳离子振动态密度的影响。阳离子质量对低频振动光谱的影响比电荷更深，对于较重的Ca(2+)阳离子，振动模式出现红移，较高能量的振动模式几乎被抑制。在溶剂化方面，未发现与单价阳离子有质的差异，这表明与单价阳离子相比，预计这些阳离子及其配合物在离子液体中的迁移率不会有增强的降低。
