Computational studies of room temperature ionic liquid–water mixtures

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](-) 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(-) or [NTf2](-) 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.

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“…The interested reader is referred to an excellent review article published recently by Bhargava et al 39 and references therein.…”

Section: Introductionmentioning

confidence: 99%

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

2013

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“…[30] Many computational models have been developed that examined changes in IL solvent structure by dissolved water, often over the full concentration range. [31][32][33][34][35] At low water concentration, the models predict that the IL nanostructure is relatively unperturbed, but at high water content the system resembles aqueous solutions of ionic surfactants. However, these studies have overwhelmingly investigated aprotic ILs; largely absent are corresponding studies of PILs and experimental verification of the findings.…”

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confidence: 99%

How Water Dissolves in Protic Ionic Liquids

2012

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In recent years, ionic liquids (ILs) have emerged as useful chemical solvents for an enormous number of processes and technologies. [1,2] Their ions have more complex chemical structures than inorganic salts; by incorporating large, sterically mismatched anions and cations, ILs melt at low temperatures because, compared to typical inorganic salts, Coulombic attractions are weakened and lattice-packing arrangements frustrated. [3] ILs are regarded as "designer solvents", as molecular control over liquid properties is possible depending on how the ions are functionalized. Hydrogen bonding can play a key role in IL chemistry. [4][5][6] Whereas most inorganic salts cannot form hydrogen bonds and are dominated by electrostatic interactions between ions, many ILs have extensive H-bonding capacity. For example, H-bond donor and acceptor sites are created during synthesis of protic ionic liquids (PILs). [2] This enables some PILs to develop dense Hbond networks and thus mirrors a number of remarkable structural [5] and solvent [2] properties of water. Finally, ILs have the capacity to self-assemble, forming well-defined nanostructures in the bulk phase [7][8][9][10][11][12][13] as well as at interfaces. [3,[14][15][16] IL nanostructure arises because at least one of the ions (frequently the cation) is amphiphilic, with distinct charged and uncharged moieties. [9] This drives segregation of ionic and nonionic groups in ILs, reminiscent of self-assembly in aqueous surfactant mesophases. [3,12] Here we elucidate the bulk solvent structure of mixtures of a PIL, ethylammonium nitrate (EAN), and water ( Figure 1). EAN is one of the oldest known [17] and most extensively studied PILs. As EAN is completely miscible with water, this raises questions such as: how do EAN and water mix? Are the forces that lead to self-assembly in pure EAN [9] sufficient to maintain a solvophobic nanostructure? What is the nature of ion solvation in such mixtures? If nanostructure persists in aqueous mixtures and key solvent properties are retained, this will increase PIL utility by offering an additional mechanism for tuning liquid behavior and lowering the overall cost of the solvent medium.While primitive (continuum solvent) models of dilute aqueous electrolyte solutions are generally successful, understanding ion-water interactions and concentrated solutions has proved challenging, and is complicated in part by the absence of a satisfactory model for liquid water. [18,19] The structure in aqueous electrolyte solutions is understood in terms of Hofmeister [20] and hydrophobic [21] effects, which can only be probed using sophisticated experimental [22][23][24] and computational [25][26][27] techniques. Solvated ions induce a different local structure of water molecules in the first, and even the second or third solvation shells, to accommodate the dissolved species. This leads to ions being classified as either "structure making" or "structure breaking" through the creation of "solute cavities". [28] Recent, growing interest in IL/water mixtures h...

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“…As a matter of fact, IL -water mixtures as well as interface systems are of tremendous technological importance and have thus garnered considerable attention. [39][40][41][42][43][44] Figure 1 summarizes the studied liquid/liquid interfaces. As a reference, we also present results obtained on the liquid/vapor interface (technically liquid-vacuum interface due to the low vapor pressure of ILs) of both ionic liquids.…”

Section: Introductionmentioning

confidence: 99%

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

et al. 2015

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This version is available at https://strathprints.strath.ac.uk/54829/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. We investigate by means of Molecular Dynamics simulations how the intrinsic surface structure of liquid/liquid interfaces involving ionic liquids depends on the opposite phase of varying polarity. We study 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF 6 ) and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid (BMIM NTf 2 ). The opposite phase is either cyclohexane or water, but as a reference, IL -vacuum interfaces are also studied. We combine a distance-based cluster search algorithm with the ITIM intrinsic analyzing method to separate liquid phases showing non-negligible mutual miscibility and to identify atoms residing at the instantaneous surface. In contrast to the well structured surface of IL -vacuum systems, at liquid/liquid interfaces of ILs density correlations, ionic associations and orientational preferences are all weakened, this effect being much more pronounced when the other species is water. In such systems we observe a drastic reduction in the presence of the cation at the surface and an increase of appearance of polar moieties (of both the cations and anions) leading to decreased apolar character of the interface. Furthermore, cations are mostly found to turn with their butyl chains toward the bulk while having their methyl groups sticking towards water. Anion-cation associations are reduced and partially replaced by water-anion and rarely also water-cation associations.

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Computational studies of room temperature ionic liquid–water mixtures

Cited by 105 publications

References 115 publications

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

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

How Water Dissolves in Protic Ionic Liquids

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

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