Rabideau Brooks D, Soltani Mohammad, Parker Rome A, Siu Benjamin, Salter E Alan, Wierzbicki Andrzej, West Kevin N, Davis James H
Department of Chemical & Biomolecular Engineering, The University of South Alabama, Mobile, Alabama 36688, USA.
Department of Chemistry, The University of South Alabama, Mobile, Alabama 36688, USA.
Phys Chem Chem Phys. 2020 Jun 4;22(21):12301-12311. doi: 10.1039/d0cp01214a.
In previous work with thermally robust salts [Cassity et al., Phys. Chem. Chem. Phys., 2017, 19, 31560] it was noted that an increase in the dipole moment of the cation generally led to a decrease in the melting point. Molecular dynamics simulations of the liquid state revealed that an increased dipole moment reduces cation-cation repulsions through dipole-dipole alignment. This was believed to reduce the liquid phase enthalpy, which would tend to lower the melting point of the IL. In this work we further test this principle by replacing hydrogen atoms with fluorine atoms at selected positions within the cation. This allows us to alter the electrostatics of the cation without substantially affecting the sterics. Furthermore, the strength of the dipole moment can be controlled by choosing different positions within the cation for replacement. We studied variants of four different parent cations paired with bistriflimide and determined their melting points, and enthalpies and entropies of fusion through DSC experiments. The decreases in the melting point were determined to be enthalpically driven. We found that the dipole moment of the cation, as determined by quantum chemical calculations, is inversely correlated with the melting point of the given compound. Molecular dynamics simulations of the crystalline and solid states of two isomers showed differences in their enthalpies of fusion that closely matched those seen experimentally. Moreover, this reduction in the enthalpy of fusion was determined to be caused by an increase in the enthalpy of the crystalline state. We provide evidence that dipole-dipole interactions between cations leads to the formation of cationic domains in the crystalline state. These cationic associations partially block favourable cation-anion interactions, which are recovered upon melting. If, however, the dipole-dipole interactions between cations is too strong they have a tendency to form glasses. This study provides a design rule for lowering the melting point of structurally similar ILs by altering their dipole moment.
在之前关于热稳定性良好的盐的研究工作中[卡西蒂等人，《物理化学化学物理》，2017年，第19卷，31560页]，人们注意到阳离子偶极矩的增加通常会导致熔点降低。液态的分子动力学模拟表明，增加的偶极矩通过偶极 - 偶极排列减少了阳离子 - 阳离子排斥。据信这会降低液相焓，从而倾向于降低离子液体的熔点。在这项工作中，我们通过在阳离子内的选定位置用氟原子取代氢原子来进一步检验这一原理。这使我们能够改变阳离子的静电作用而基本不影响空间位阻。此外，可以通过选择阳离子内不同的取代位置来控制偶极矩的强度。我们研究了与双三氟甲磺酰亚胺配对的四种不同母体阳离子的变体，并通过差示扫描量热法（DSC）实验测定了它们的熔点、熔化焓和熔化熵。确定熔点的降低是由焓驱动的。我们发现，通过量子化学计算确定的阳离子偶极矩与给定化合物的熔点呈负相关。两种异构体的晶体和固态的分子动力学模拟显示，它们的熔化焓差异与实验观察到的差异紧密匹配。此外，熔化焓的这种降低被确定是由晶体态焓的增加引起的。我们提供证据表明阳离子之间的偶极 - 偶极相互作用导致在晶体态形成阳离子域。这些阳离子缔合部分阻碍了有利的阳离子 - 阴离子相互作用，这种相互作用在熔化时得以恢复。然而，如果阳离子之间的偶极 - 偶极相互作用太强，它们就有形成玻璃态的倾向。这项研究为通过改变结构相似的离子液体的偶极矩来降低其熔点提供了一条设计规则。
