Tuning the melting point of selected ionic liquids through adjustment of the cation's dipole moment
Tuning the melting point of selected ionic liquids through adjustment of the cation's dipole moment
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DOI：
10.1039/d0cp01214a
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发表时间：
2020-06-07
影响因子：
3.3
通讯作者：
Davis, James H., Jr.
中科院分区：
文献类型：
--
作者：
Rabideau, Brooks D.;Soltani, Mohammad;Davis, James H., Jr.
在先前使用热稳定盐的工作中[Cassity等人，物理化学化学物理，2017，19，31560]注意到阳离子的偶极矩的增加通常导致熔点的降低。分子动力学模拟的液体状态显示，增加的偶极矩减少阳离子-阳离子排斥通过偶极-偶极排列。据信这降低了液相焓，这将倾向于降低IL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测试这一原理。这使我们能够改变阳离子的静电，而基本上不影响空间。此外，可以通过选择阳离子内的不同位置进行置换来控制偶极矩的强度。我们研究了四种不同的母体阳离子与bistriflimide配对的变体，并通过DSC实验测定了它们的熔点、熔点和熔融熵。熔点的降低被确定为由化学驱动的。我们发现，由量子化学计算确定的阳离子的偶极矩与给定化合物的熔点呈负相关。两种异构体的结晶和固态的分子动力学模拟显示，它们的融合能力与实验结果非常吻合。此外，这种熔化焓的降低被确定为是由结晶状态的焓的增加引起的。我们提供的证据表明，偶极-偶极阳离子之间的相互作用导致在结晶状态下的阳离子域的形成。这些阳离子缔合部分地阻断有利的阳离子-阴离子相互作用，其在熔融时恢复。然而，如果阳离子之间的偶极-偶极相互作用太强，它们就有形成玻璃的倾向。该研究为通过改变离子液体的偶极矩来降低其熔点提供了一个设计准则。
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.