Solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid: a molecular dynamics study.
Solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid: a molecular dynamics study.
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DOI：
10.1021/jp071347s
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发表时间：
2007-06
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通讯作者：
N. Sieffert;G. Wipff
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作者：
N. Sieffert;G. Wipff
我们报道了氯化钠在 1-丁基-3-甲基-咪唑鎓双(三氟甲基磺酰基)亚胺离子液体 ([BMI][Tf2N] IL) 中溶剂化的分子动力学研究。我们首先考虑解离单个 Na+Cl- 离子对的平均力的潜力，表明后者更喜欢不解离而不是解离（约 9 kcal/mol），自由能垒约为 10 kcal/mol。 5 kcal/mol（d 约为 5.2 A）用于缔合过程。从浓缩溶液的 100 ns 分子动力学模拟中也观察到对 Na+Cl- 缔合的偏好，其中 Na+Cl- 离子倾向于在 IL 中形成低聚物和微晶。相反，模拟 Na13Cl14- 和 Na14Cl13+ 立方微晶（分别在顶点处具有 Cl- 和 Na+）不会导致在 IL 中溶解。其中，Na14Cl13+ 被发现比 Na13Cl14- 更好地溶剂化，这主要是因为与立方体顶点处的 Cl-...BMI+ 相互作用相比，Na+...Tf2N- 相互作用更强。最后，我们考虑了 NaCl 和 IL 100 面之间的固/液界面，结果表明，尽管具有极性，晶体表面却被极性较小的 IL 组分（CF3(Tf2N) 和丁基 (BMI) 基团）而不是极性组分（O(Tf2N) 和咪唑鎓 (BMI) 环）溶剂化。描述了 Tf2N- 阴离子和 BMI+ 阳离子在界面处的特定排序。在第一层离子液体中，离子与表面相当平行，而在第二“层”中，离子则更加垂直。在全中性 Na0Cl0 固体类似物的表面发现了类似的 IL 结构，证实由于 IL 和晶体离子之间的不匹配，晶体的溶剂化相当“非极性”。与水、甲醇或不同的基于 BMI+ 的离子液体作为溶剂进行了一些比较，使我们能够更好地了解离子液体-NaCl 相互作用的特异性。
We report molecular dynamics studies on the solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid ([BMI][Tf2N] IL). We first consider the potential of mean force for dissociating a single Na+Cl- ion pair, showing that the latter prefers to be undissociated rather than dissociated (by ca. 9 kcal/mol), with a free energy barrier of ca. 5 kcal/mol (at d approximately 5.2 A) for the association process. The preference for Na+Cl- association is also observed from a 100 ns molecular dynamics simulation of a concentrated solution, where the Na+Cl- ions tend to form oligomers and microcrystals in the IL. Conversely, the simulation of Na13Cl14- and Na14Cl13+ cubic microcrystals (with, respectively, Cl- and Na+ at the vertices) does not lead to dissolution in the IL. Among these, Na14Cl13+ is found to be better solvated than Na13Cl14-, mainly due to the stronger Na+...Tf2N- interactions as compared to the Cl-...BMI+ interactions at the vertices of the cube. We finally consider the solid/liquid interface between the 100 face of NaCl and the IL, revealing that, in spite of its polar nature, the crystal surface is solvated by the less polar IL components (CF3(Tf2N) and butyl(BMI) groups) rather than by the polar ones (O(Tf2N) and imidazolium(BMI) ring). Specific ordering at the interface is described for both Tf2N- anions and BMI+ cations. In the first IL layer, the ions are rather parallel to the surface, whereas in the second "layer" they are more perpendicular. A similar IL structure is found at the surface of the all-neutral Na0Cl0 solid analogue, confirming that the solvation of the crystal is rather "apolar", due to the mismatch between the IL and the crystal ions. Several comparisons with water, methanol, or different BMI+-based ILs as solvents are presented, allowing us to better understand the specificity of the ionic liquid-NaCl interactions.