Solutes in ionic liquids

Although pure ionic liquid properties are important, many applications envision the use of ionic liquids as solvents or separation agents, where properties of ionic liquids mixed with other species are paramount. Relatively few simulations have been carried out to study the properties of ionic liquids with dissolved solutes, but the number of studies on this topic has increased recently. Here we review several of these studies. 

Hanke and Lynden-Bell carried out another study of water dissolved in hydrophilic [Cimim][Cl] and hydrophobic [CimimJlPFg] in which isochoric-isothermal MD simulations were run at 127°C on mixtures containing water mole fractions of 0.05, 25, 50, 75, and 99.5%. They computed excess volumes, enthalpies, and internal energies of mixing and compared the results between the two ionic liquids and found that the excess properties between the two systems differed qualitatively, as might be expected. Unfortunately, no experimental data was available for these systems against which to compare their results. The authors pointed out, however, that the magnitude of computed quantities such as excess molar volumes were consistent with those for other ionic liquid systems for which data was available. 

Shah and Maginn carried out a Monte Carlo study of CO2 solubility in [C4mim][PF6] to compute the Henry s law constant of CO2 using the test particle insertion free energy perturbation method. The Henry s law constant for solute 2 dissolved in solvent 1, Hi,i, is defined as 

Part of the inaccuracy of these results can be attributed to the force field used, which included a united-atom model for the cation and an explicit-atom model for the anion. This model,unlike Shah and Maginn s previous united-atom model, underestimated the pure liquid density by about 5%, which should lead to higher gas solubilities on the basis of free volume arguments. The other source of error rests with the simulation method itself. Kofke and Cummings have shown that single-stage free energy perturbation methods 

NO3 anions are surrounded by four to five imidazolium cations. Chaumont, Engler and Wipff employed the same technique to examine the solvation of uranyl and strontium nitrates as well as uranyl chlorides in the same ionic liquids.As was observed in the previous study, the anions of the ionic liquids solvate the strontium and uranyl ions preferentially. Detailed radial distribution functions and coordination numbers were computed. This group has extended this work by making comparisons of computed solvation ordering with data from experimental spectroscopic studies. 

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Bike, D.M., Brennecke, J.F., and Maginn, E.J., Rredicting infinite-dilution activity coefficients of organic solutes in ionic liquids, Ind. Eng. Chem. Res., 43, 1039, 2004. 

Anode material In aqueous solutions the anodic processes are either breakdown of the electrolyte solution (with oxygen evolution at an inert anode being favored) or the use of soluble anodes. The use of soluble anodes is limited by the passivation of many metals in aqueous solutions. In ionic liquids, however, the first option is not viable due to the cost and the nature of the anodic breakdown products. New strategies will therefore have to be developed to use soluble anodes where possible or add a sacrificial species that is oxidized to give a benign gaseous product. Preliminary data have shown that for some metals the anodic dissolution process is rate limiting and this affects the current distribution around the cathode and the current density that can be applied. 

As with other solutes in ionic liquids, the general rule of like dissolving like is applicable i.e. ionic species will generally be soluble as will species capable of interacting with the anion. Aromatic species tend to exhibit poor solubility in ionic liquids consisting of aliphatic cations and vice versa. 

An example is the question of the pH-value of acidic solutes in ionic liquids. From a practical point of view, this issue is extremely difficult to resolve, as pH-electrodes cannot be used in ionic liquids. Furthermore, the pH-value is defined as the negative logarithm of the H30+ activity, but in a dry ionic liquid, water is not present to form this species with dissociated protons. Does this then mean that protons are naked , tending to exhibit super-acidic character The answer to this question lies of course in the solvation properties of the ionic liquid under investigation. 

The extent of mixing and the distribution of solutes in ionic liquids depend, therefore, on the relative solute-solute and solute-solvent interactions, which can have significant consequences on chemical reactivity and stabihty. In many ionic liquids, water-sensitive catalysts and chemical reactions are less sensitive to water compared with the situation in organic solvents because water dispersed throughout the ionic liquid cannot act like bulk water. 

The deposition of metals has been observed in a number of catalytic processes in ionic liquids and this will be discussed in Section 6.3. This has encouraged the exploration of the possibility of the chemical deposition of metals from solutions in ionic liquids. For instance, lr(0) nanoparticles 2.3 0.4 run in diameter have been prepared by the reduction of [Ir(cod)Q]2 in [BMIMjfPFs] with H2 [24]. In a particularly elegant experiment, palladium nanoparticles were first formed by the thermal decomposition of paUadium(ii) acetate in the presence of triphenylphosphine (to give a regular particle size of 1 run) in [BMIM][Tf2N]. A silica aerogel support was then formed around the nanoparticles by adding (EtO)4Si and formic acid to the mixture [25],... 

GG measurements have also been used to derive infinite dilution activity coefficients (y ) for a range of potential solutes in ionic liquids. For an ideal mixture, y = 1 if y > 1 solvent-solute interactions are less favorable than the solvent-solvent interactions and if y < 1 solvent-solute interactions are more favorable than the solvent-solvent interactions. Nonpolar solutes such as alkanes, alkenes, and simple... 

Mutelet, F. and Jaubert, ).N. (2006) Accurate measurements of thermodynamic properties of solutes in ionic liquids using inverse gas chromatography. J. Ckromatogr. A, 1102 (1-2), 256-267. 

Experimental yj Determinations for Organic Solutes in Ionic Liquid Solvents... 

Nonpolar, polar and associated solutes in Ionic Liquids. J Phys Chem B 110 16816-16818... 

Two distinct groups of experimental techniques were presented in COlL-2 to study the behaviour of different solutes in ionic liquids, spectroscopic and thermodynamic, giving access to different scales of the properties studied - one microscopic and the other macroscopic. It was possible to explain microscopically the phase behaviour of ionic liquid solutions by balancing the effects of the solute-solvent interactions and the dynamics of the solutions. Bases were established to assess the microscopic mechanisms responsible by the properties observed and so to open the way to the rapid advancement of the field contributing to the development of novel applications in a growing variety of disciplines including catalysis, synthesis, nanomaterial synthesis or pharmaceutics. 

Canongia Lopes, J.N., Costa Gomes, M.F. and Padua, A.A.H., Nonpolar, polar, and associating solutes in ionic liquids, J. Phys. Chem. B110,16816-16818 (2006). 

Some of the MD codes mentioned above (such as DL POLY, AMBER, and NAMD) can perform thermodynamic integration and various free energy perturbation calculations. These techniques can be used to compute excess chemical potentials of solutes in ionic liquids and thereby obtain information on solvation. We believe MC methods are better suited for these types of calculations, however. 

Mutelet, F., Jaubert, J.N. (2006). Accurate measurements of thermodynamic properties of solutes in ionic liquids using inverse gas chromatography. Journal of Chromatography A1102., 1-2,256-267., 00219673 Ohno, H. (2005). Electrochemical Aspects of Ionic Liquids, 2 Ed., John Wiley and Sons, Inc., ISBN 978-0-471-64851-2, Hoboken, New Jersey, USA Plechkova, N.V., Seddon, K.R. (2008). Applications of ionic liquids in the chemical industry. 

The large dataset of partition coefficients (or activity coefficients at infinite dilution) published in the literature may be used to present a general behaviour of solutes in ionic liquids. The values of activity coefficients at infinite dilution () for the -alkanes increase with an increase in carbon number. In most ionic liquids, the high y" values observed with n-alkanes indicate their low solubility in ionic liquids. The values of n-alkanes are higher than the values obtained with cyclohexane, alkenes, alkynes and aromatics. Introduction of a double or triple bond in the n-alkanes decreases the values. 

Computer simulations of ionic liquids have largely focused on the development of force field parameters specific to an ionic liquid or an ionic liquid family [17-23]. In addition to simulation of structural, dynamical, electric, and thermodynamic properties of several pure ionic liquids [24-26], the solvation of small solutes in ionic liquids has also been investigated [27-31]. Compatibility of ionic liquids and cellulose... 

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