The extent to which cations and anions in ionic liquids (ILs) and ionic liquid solutions are dissociated is of both fundamental scientific interest and practical importance because ion dissociation has been shown to impact viscosity, density, surface tension, volatility, solubility, chemical reactivity, and many other important chemical and physical properties. When mixed with solvents, ionic liquids provide the unique opportunity to investigate ion dissociation from infinite dilution in the solvent to a completely solvent-free state, even at ambient conditions. The most common way to estimate ion dissociation in ILs and IL solutions is by comparing the molar conductivity determined from ionic conductivity measurements such as electrochemical impedance spectroscopy (EIS) (which measure the movement of only the charged, i.e., dissociated, ions) with the molar conductivity calculated from ion diffusivities measured by pulse field gradient nuclear magnetic resonance spectroscopy (PFG-NMR, which gives movement of all of the ions). Because the NMR measurements are time-consuming, the number of ILs and IL solutions investigated by this method is relatively limited. We have shown that use of the Stokes-Einstein equation with estimates of the effective ion Stokes radii allows ion dissociation to be calculated from easily measured density, viscosity, and ionic conductivity data (ρ, η, λ), which is readily available in the literature for a much larger number of pure ILs and IL solutions. Therefore, in this review, we present values of ion dissociation for ILs and IL solutions (aqueous and nonaqueous) determined by both the traditional molar conductivity/PFG-NMR method and the ρ, η, λ method. We explore the effect of cation and anion alkyl chain length, structure, and interaction motifs of the cation and anion, temperature, and the strength of the solvent in IL solutions.

Ion Dissociation in Ionic Liquids and Ionic Liquid Solutions.

Current bottlenecks in the large-scale commercial use of many ionic liquids (ILs) include their high costs, low biodegradability, and often unknown toxicities. As a proactive effort to better understand the molecular mechanisms of ionic liquid toxicities, the work herein presents a comprehensive molecular simulation study on the interactions of 1-n-alkyl-3-methylimidazolium-based ILs with a phosphatidylcholine (PC) lipid bilayer. We explore the effects of increasing alkyl chain length (n = 4, 8, and 12) in the cation and anion hydrophobicity on the interactions with the lipid bilayer. Bulk atomistic molecular dynamics (MD) simulations performed at millimolar (mM) IL concentrations show spontaneous insertion of cations into the lipid bilayer regardless of the alkyl chain length and a favorable orientational preference once a cation is inserted. Cations also exhibit the ability to “flip” inside the lipid bilayer (as is common for amphiphiles) if partially inserted with an unfavorable orientation. Moreover, structural analysis of the lipid bilayer show that cationic insertion induces roughening of the bilayer surface, which may be a precursor to bilayer disruption. To overcome the limitation in the timescale of our simulations, free energies for a single IL cation and anion insertion have been determined based on potential of mean force calculations. These results show a decrease in free energy in response to both short and long alkyl chain IL cation insertion, and likewise for a single hydrophobic anion insertion, but an increase in free energy for the insertion of a hydrophilic chloride anion. Both bulk MD simulations and free energy calculations suggest that toxicity mechanisms toward biological systems are likely caused by ILs behaving as ionic surfactants. [Yoo et al., Soft Matter, 2014].

Amphiphilic interactions of ionic liquids with lipid biomembranes: a molecular simulation study.

A series of tetraalkylphosphonium 2-cyanopyrrolide ([Pnnnn][2-CNPyr]) ionic liquids (ILs) were prepared to investigate the effect of cation size on physical properties and CO2 solubility. Each IL was synthesized in our laboratory and characterized by NMR spectroscopy. Their physical properties, including density, viscosity, and ionic conductivity, were determined as a function of temperature and fit to empirical equations. The density gradually increased with decreasing cation size, while the viscosity decreased noticeably. In addition, the [Pnnnn][2-CNPyr] ILs with large cations exhibited relatively low degrees of ionicity based on analysis of the Walden plots. This implies the presence of extensive ion pairing or formation of aggregates resulting from van der Waals interactions between the long hydrocarbon substituents. The CO2 solubility in each IL was measured at 22 °C using a volumetric method. While the anion is typically known to be predominantly responsible for the CO2 capture reaction, the [Pnnnn...

Effect of Cation on Physical Properties and CO2 Solubility for Phosphonium-Based Ionic Liquids with 2-Cyanopyrrolide Anions.

The industrial application of ionic liquids (ILs) requires the knowledge of their physical properties and phase behavior. This work addresses the experimental determination of the vapor−liquid equilibria (VLE) of binary systems composed of water + imidazolium-based ILs. The ILs under consideration are 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1- butyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium tosylate, 1-butyl-3-methylimidazolium trifluoroacetate, 1- butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium methanesulfonate, and 1-butyl-3-methylimidazolium acetate, which allows the evaluation of the influence of the IL anion through the phase behavior. Isobaric VLE data were measured at 0.05, 0.07, and 0.1 MPa for IL mole fractions ranging between 0 and 0.7. The observed increase in the boiling temperatures of the mixtures is related with the strength of the interaction between the IL anion and water. The Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was further used to describe the obtained experimental data. The ILs were treated as molecular associating species with two association sites per IL. The model parameters for the pure fluids and the binary interaction parameter kij between water and ILs were determined by a simultaneous fitting to pure-IL densities, water activity coefficients at 298.15 K and VLE data at 0.1 MPa. Pure-IL densities, water activity coefficients, and VLE data were well described by PC-SAFT in broad temperature, pressure, and composition ranges. The PC-SAFT parameters were applied to predict the water activity coefficients at infinite dilution in ILs, and a satisfactory prediction of experimental data was observed.

/pdf/vapor-liquid-equilibria-of-water-alkylimidazolium-based-ux3ic8idvl.pdf

Vapor–Liquid Equilibria of Water + Alkylimidazolium-Based Ionic Liquids: Measurements and Perturbed-Chain Statistical Associating Fluid Theory Modeling

Quantifying ion association and collective dynamical processes in organic electrolytes is essential for fundamental property interpretation and optimization for electrochemical applications. The extent of ion correlation depends on both the ion concentration and dielectric strength of the solvent; ions may be largely uncorrelated in sufficiently high-dielectric solvents at low concentration, but properties of concentrated electrolytes are dictated by correlated and collective ion processes. In this work, we utilize molecular dynamics simulations to characterize ion association and collective ion dynamics in organic electrolytes composed of binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM+][BF4–] and 1,2-dichloroethane, acetone, acetonitrile, and water solvents. We illustrate different physical regimes of characteristically distinct ion correlations for the systematic range of electrolyte concentrations and solvent dielectric strengths. Dilute electrolytes composed of low-dielectric s...

/pdf/ion-correlation-and-collective-dynamics-in-bmim-bf4-based-3l8ggtkqld.pdf

Ion Correlation and Collective Dynamics in BMIM/BF4-Based Organic Electrolytes: From Dilute Solutions to the Ionic Liquid Limit.

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 dis...

State of hydrophobic and hydrophilic ionic liquids in aqueous solutions: are the ions fully dissociated?

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State of hydrophobic and hydrophilic ionic liquids in aqueous solutions: are the ions fully dissociated?