Behavior of Ionic Liquids

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubility of ionic liquids in other compounds and the solubility of other compounds in the ionic liquids. For instance, an increase in allcyl chain length decreases the mutual solubility with water, but some anions ([BFJ , for example) can increase mutual solubility with water (compared to [PFg] , for instance) [1-3]. While many mixture properties and many types of phase behavior are important, we focus here on the solubility of gases in room temperature IFs. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

However, investigations up to now have mainly concentrated themselves on ambient environments even though it is known that ionic liquids have a very low vapor pressure, making them suitable for vacuum applications such as in space mechanisms, the disk drive industry, and microelec-tromechanical systems (MEMS). Due to the ultra-low vapor pressure of most ionic liquids, they have been expected to be good lubricants in vacuum. Further experimental works are required to evaluate lubrication behavior of ionic liquids under ultra-high vacuum conditions and in inert atmospheres. 

However, it is emphasized that the reported polarity values do not provide a rigorous basis for a prediction of the behavior of ionic liquids in catalysis, as the measurements of polarity values are particularly dependent on the methods used in some cases the values are not consistent. For example, in one report the polarity of selected ionic liquids was stated to increase in the order [BMIM]PFg<[BMIM]Tf2N< [OMIM]PFg (75), whereas in another the order was just the opposite (77). In any case, the differences are small. 

Domarfska, U. and Bogel-Lukasik, R., Solubility of ethyl-(2-hydroxyethyl)-dimeth-ylammonium bromide in alcohols (C2-CJ2), Fluid Phase Equilib., 233, 220, 2005. Doman ska, U., Thermophysical properties and thermodynamic phase behavior of ionic liquids, Thermochim. Acta, 448,19, 2006. 

Domariska, U., Zolek-Tryznowska. Z., and Krolikowski, M., Thermodynamic phase behavior of ionic liquids, /. Chem. Eng. Data, 52, 1872, 2007. 

Crosthwaite, J.M. et al.. Liquid phase behavior of ionic liquids with alcohols experimental studies and modeling, ]. Phys Chem. B, 110,9354,2006. 

The miscibility behavior of ionic liquids and organic solvents is also rather unpredictable dichloromethane and THF mix with, e.g., [BMIm][ Tf2N], whereas alkanes and ethers do not, and ethyl acetate seems to be a borderline case [35]. Supercritical carbon dioxide (scC02) does not mix with ionic liquids such as [BMIm][PF6] and [OMIm][BF4], but is absorbed in the ionic liquid phase in huge amounts (up to a molar fraction of 0.7) [36]. No ionic liquid dissolves in the C02 phase. 

The goal of this chapter is to compile existing knowledge on the behavior of ionic liquids and their influence on solvation and chemical reactivity. The intent is not to list reactions and their outcomes, but rather to review the results of studies that offer physical insight into the microscopic environment of ILs and their interaction with solute species. While many excellent reviews of ILs have been written [1, 4, 23, 30, 38 -0], this chapter is distinct in its attempt to identify the basic physical principles relevant to solvation in ILs. 

The goal of this chapter is to understand the behavior of ionic liquids as solvents and their influence on reaction based on their chemical structure and microscopic environment. We will therefore provide only a basic overview of their macroscopic physical properties. An online database, compiled by a research team operating under the auspices of the International Union of Pure and Applied Chemists (IUPAC), is now available detailing the physical properties of many known IL species [52],... 

At a fixed temperature, the only parameter determining the mean hole size is the surface tension. Though one is aiming at a microscopic (structural) explanation of the behavior of ionic liquids, one goes ahead and uses the macroscopic value of surface tension. The mean hole radius then turns out to have the same order of magnitude as the mean radius of ions comprising the liquid. 

Wang H, Lu Q, Ye C, et al. Friction and wear behaviors of ionic liquid of alkylimidazolium hexafluorophosphates as lubricants for steel/steel contact. Wear. 2004. 256, 44-48. 

Lee, J. K., Kim, D.-C., Song, C. E., Lee, S.-g. Thermal behaviors of ionic liquids under microwave irradiation and their application on microwave-assisted catalytic Beckmann rearrangement of ketoximes. Synth. Commun. 2003, 33, 2301-2307. 

The overall conclusion from these studies is that, in spite of essential progress in understanding the collective behavior of ionic liquids, many problems are still open for further work. In particular, it is important to perform an additional study of a binary mixture of neutral particles with antiferromagnetic ... 

The use of thermal conductivity, heat capacity and rheological properties for [C4mim][NTf2] was also shown by Chen et al. [123] to correlate with Shah s equation for forced convective heat transfer in the laminar flow regime, indicating that knowledge of these parameters can successfully be used to model heat transfer behavior of ionic liquid systems at the larger scale. 

Z.F. Zhang, W.Z. Wu, H.X. Gao, B.X. Han, B. Wang and Y. Huang, Tri-phase behavior of ionic liquid-water-CO2 system at elevated pressures, Phys. Chem. Chem. Phys. 6, 5051-5055(2004). 

Blanchard LA, Gu Z, Brennecke JF. High-pressure phase behavior of ionic liquid/C02 systems. J Phys Chem B 2001 105 2437-2444. 

Kondrat S, Bier M, Harnau L (2010) Phase behavior of ionic liquid crystals. J Chem Phys 132 184901... 

Li JL, Feng DP, Liang YM et al (2010) Synthesis and tribological behavior of ionic liquid substituted lluoroalkoxycyclophosphazene derivatives in steel-steel contacts. Ind Lubr Tribol... 

Chen SM, Kobayashi K, Miyata Y et al (2009) Morphology and melting behavior of ionic liquids inside single-walled carbon nanotubes. J Am Chem Soc 131 14850-14856... 

The phase behavior of ionic liquids can be complicated. Some are crystalline at low temperatures and show a sharp transition from crystal to liquid state (a true melting point) as the temperature is raised, but others exist as a glass at low temperatures and convert to a liquid at the glass-liquid transition temperature, denoted by a small change in heat capacity. Still others are glasses at very low temperatures, transform to crystals as the temperature is raised, and finally become liquid at a still higher temperature. See Reference 3 for a discussion of the types of phase behavior. 

The heating behavior of ionic liquids combined with organic solvents under microwave irradiation has also been the subject of a recent paper by Ondruschka and coworkers [84]. Kappe et al. have used microwaves for inter and intramolecular hetero Diels-Alder reactions of 2(ll-f)-pyrazinones using dichloroethane as a solvent with small amounts of ionic liquid as heating aids (Scheme 7.26) [85], The conventional reaction can take up to 2 days to reach completion. With the micro-wave heating the reaction time could be reduced to 50 min in the absence of the ionic liquid and only 18 min in its presence. Product yields were reported to be similar to those obtained by use of conventional heating. Interestingly, when the... 

For industrial implementation the corrosion behavior of ionic liquids must ofcourse be investigated as materials corrosion can be a risk to plant integrity, reduce plant efficiency and may require costly maintenance. A recently published report describes the corrosion behavior of a number of metallic materials in seven commercially available ionic liquids in the absence and presence of water under flow conditions at temperatures up to 90 °C (Scheme 2.2-3, Fig. 2.2-6) [25]. 

The dynamic behavior of ionic liquids is important for both practical and theoretical reasons. From a practical standpoint, bulk transport properties such as the viscosity, self-diffUsivity, thermal conductivity and electrical conductivity govern the effectiveness of these liquids in any application. For example, mass transfer of reactants and products is critical to the performance of ionic liquid solvents, and is highly correlated with the self-difiiisivity and viscosity. Viscosity also plays a role in the cost of pumping the liquid and its performance as a lubricant. Thermal conductivity is a key parameter for thermal fluid applications, and electrical conductivity is obviously important in electrochemical applications. 

Shen, Y. F, Zhang, Y. ]., Kuehner, D., Yang, G. F, Yuan, F. Y., and Niu, L. (2008]. ion-responsive behavior of ionic-liquid surfactant aggregates with applications in controlled release and emulsification. Chem. Phys. Chem., 9, pp. 2198-2202. 

Q. Zhou, P. D. Boyle, L. Malpezzi, A. Mele, J.-H. Shin, S. Passerini, W. A. Henderson, Chem. Mater. 2011, 23, 4331-4337. Phase behavior of ionic liquid-LiX mixtures Pyrtolidinium cations and TFSl anions - Linking structure to transport properties. 

J. Huang, A. F. HoUenkamp, J. Phys. Chem. C 2010,114,21840-21847. Thermal behavior of ionic liquids containing the FSl anion and the Li+ cation. 

Huang, J. Hollenkamp, A. R, Xhermal Behavior of Ionic Liquids Conteiining the FSI Anion and the Li Cation, J. Phys. Chem. C, 2010, 114,21840-21847. 

Henderson, W.A. Passerini, S., Phase Behavior of Ionic Liquid-LiX Mixtures Pyrrolidinium Cations and XFSI-Anions, Chem. Mater, 2004,16,2881-2885. 

---