State of Hydrophobic and Hydrophilic Ionic Liquids in Aqueous Solutions: Are the Ions Fully Dissociated?

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• 作者：Patrick Yee ; Jindal K. Shah ; Edward J. Maginn
• 刊名：Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：October 17, 2013
• 年：2013
• 卷：117
• 期：41
• 页码：12556-12566
• 全文大小：451K
• 年卷期：v.117,no.41(October 17, 2013)
• ISSN：1520-5207

文摘

Molecular dynamics simulations were performed for aqueous solutions of five ionic liquids (ILs): 1-ethyl-3-methylimidazolium ([C₂mim]) bis(trifluoromethanesulfonyl) imide ([NTf₂]), 1-n-butyl-3-methylimidazolium ([C₄mim]) [NTf₂], 1-n-hexyl-3-methylimidazolium ([C₆mim]) [NTf₂], [C₂mim] ethylsulfate ([C₂H₅SO₄]), and [C₂mim] chloride (Cl) to determine whether the ions of these ILs are associated at relatively high dilutions and whether the association is governed by hydrophobicity/hydrophilicity of the ILs. The adaptive biasing force technique was applied to calculate the potential of mean force (PMF) for each IL ion pair. For all of the ILs, the PMF is characterized by two distinct contact minima in which the ions have different relative conformations. The hydrophobic ILs bearing the anion [NTf₂]^{鈭?/sup> exist predominantly in the associative state; the strength of the association of these ILs increases with increase in the alkyl chain length. The most hydrophilic IL [C₂mim] Cl was determined to be almost fully dissociated at the concentration examined in the study. [C₂mim] [C₂H₅SO₄] showed hydration behavior that was intermediate between that exhibited by the ILs in which the anion is substituted with either Cl鈥?/sup> or [NTf₂]鈭?/sup> paired with [C₂mim]+. Association constants for these ILs were also computed. Radial distribution functions calculated by constraining the ions at the contact minima showed that hydration of the anion plays the dominant role in determining the microscopic behavior of these ILs in aqueous solutions.}