Vapor pressure of ionic liquids

Ionic liquids are compounds in which one of the ions is a large, organic ion that prevents the liquid from crystallizing at ordinary temperatures. The low vapor pressures of ionic liquids make them desirable solvents that reduce pollution. 

Rebelo, L.RN. et al.. On critical temperature, normal boiling point, and vapor pressures of ionic liquids, ]. Phys. Chem. B, 109, 6040, 2005. 

It is also possible to carry out reactions in ionic liquids at temperatures much higher than the melting point of the IL. Called ionothermal reactions, these differ from hydrothermal and solvothermal reactions in that they may be carried out at atmospheric pressure, due to the low vapor pressures of ionic liquids. Ionothermal reactions have led to novel materials containing cations with unusual coordination numbers (e.g., four-, five-, and six-coordinate aluminum) and morphologies. 

It is very remarkable that most of the non-synthetic applications rely on only a few key success factors and those are very similar in each group. While electrochemical applications benefit mostly from the vyide electrochemical vyindow of ionic liquids, analytical applications often profit from the special solubility properties of ionic liquids. The steeply growing area of engineering fluid applications includes material processing with ionic liquids, separation technologies and applications in process machinery and plant applications. All these applications make use - in a more or less pronounced manner - of the negligible vapor pressure of ionic liquids. 

As a consequence of these findings, the vapor pressure of an ionic liquid can no longer be considered to be zero. As, from a physicochemical perspective, all matter (even a stone ) has some vapor pressure, this former perception was questionable anyway. However, Rebelo s study also showed that the vapor pressure of ionic liquids is indeed negligible at near-ambient conditions. For applications under harsh temperature and high vacuum conditions, however, the vapor pressure of the applied ionic liquids will have some relevance and should be known. Thus, we expect that in the future the vapor pressure of ionic liquid structures will be determined experimentally and that these data will just add to the property and application profile of each specific ionic liquid. 

The lack of a measurable vapor pressure of ionic liquids (ILs) and their tailor-made physicochemical properties are important reasons for their widespread use as green solvents and reaction media [1], In the same way, the selectivity of catalyzed processes is just as important because of the desire to avoid undesired reactions and/or by-products and facilitate product recovery. Both features are also key to understanding the high potential of ILs in catalytic processes and particularly in enzyme-catalyzed reactions [2, 3]. 

Photochemical reactions can be important mechanisms to reduce the vapor-phase concentrations of conventional organic solvents (see Chapter 16.1). However, as discussed in Section 16.2.4.2, the vapor pressure of ionic liquids is negligible suggesting that it is unlikely that typical ionic liquids will occur as a vapor phase in the atmosphere. It is possible that ionic liquids could be transported in the atmosphere as an adsorbed phase on particulate matter that is subject to dry and wet deposition, but this pathway is only conjecture at this time. 

Rebelo LPN, Lopes INC, Esperan9a JMSS, Filipe E (2005) On the critical temperature, normal boiling point, and vapor pressure of ionic liquids. J Phys Chem B 109 6040-6043, supporting information Table A... 

It is important to realize that these oligomers have similarly low vapor pressures as ionic liquids. To illustrate the effect of extrinsic charge carrier formation, the evolution of proton conductivity with triflic acid doping is shown for the oligomeric system Imi-2 in Figure For such systems, the mobility of... 

Gallium(III) bromide is a hygroscopic, white solid which sublimes readily and melts at 122.5° to a covalent, dimeric liquid. The solid is ionic and its electrical conductivity at the melting point is twenty-three times that of the liquid.5 The vapor pressure of the liquid at T°K is given by the equation log p(mm.) = 8.554 — 3129/T and the heat of dissociation of the dimer in the gas phase is 18.5 kcal./mol.3 At 125° the liquid has the following properties 5,6 density, 3.1076 dynamic viscosity, 2.780 c.p. surface tension, 34.8 dynes/cm. and specific conductivity, 7.2 X 10-7 ohm-1 cm.-1 Gallium(III) bromide readily hydrolyzes in water and forms addition compounds with ligands such as ammonia, pyridine, and phosphorus oxychloride. 

They have no effective vapor pressure. Since ionic liquids are salts, they cannot evaporate. For this reason, they cannot contaminate the atmosphere and are a medium of choice for green chemistry. Reaction products can be separated by simple distillation. 

Experimental vapor pressures of l-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and a correlation Scheme for estimation of vaporization enthalpies of ionic liquids. 

A. Heintz,/. Phys. Chem. A, 110, 7303 (2006). Experimental Vapor Pressures of l-Alkyl-3-methylmidazolium bis(trifluoromethylsulfonyl) Imides and a Correlation Scheme for Estimation of Vaporization Enthalpies of Ionic Liquids. 

Zaitsau DH, Kabo GJ, Strechan AA, Paulechka YU, Tscherisch A, Verevkin SP, Heintz A (2006) Experimental vapor pressures of l-alkyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imides and a correlation scheme for estimation of vaporization enthalpies of ionic liquids. J Phys Chem A 110 7303-7306... 

The lack of significant vapor pressure prevents the purification of ionic liquids by distillation. The counterpoint to this is that any volatile impurity can, in principle, be separated from an ionic liquid by distillation. In general, however, it is better to remove as many impurities as possible from the starting materials, and where possible to use synthetic methods that either generate as few side products as possible, or allow their easy separation from the final ionic liquid product. This section first describes the methods employed to purify starting materials, and then moves on to methods used to remove specific impurities from the different classes of ionic liquids. 

Notwithstanding their very low vapor pressure, their good thermal stability (for thermal decomposition temperatures of several ionic liquids, see [11, 12]) and their wide operating range, the key property of ionic liquids is the potential to tune their physical and chemical properties by variation of the nature of the anions and cations. An illustration of their versatility is given by their exceptional solubility characteristics, which make them good candidates for multiphasic reactions (see Section 5.3.4). Their miscibility with water, for example, depends not only on the hydrophobicity of the cation, but also on the nature of the anion and on the temperature. 

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. 

When the products are partially or totally miscible in the ionic liquid, the separation of the products can be more complicated. It is however possible to reduce the solubility of typical organic products in the ionic liquid by introducing a more polar solvent that can be separated by distillation afterward at a lower temperature (27). Because of the low vapor pressure of the ionic liquid, direct distillation can be applied without azeotrope formation (28). However, such operation is often limited to highly volatile or thermally labile products because of the general thermal instability of organometallic catalysts. 

We begin with a brief summary of some of the review articles that have been written on the subject of ionic liquids. Wilkeswrote a short history of ionic liquids describing the chronological development of ionic liquids with an emphasis on listing the names and pictures of those involved in the research. Holbrey and Seddon and Earle and Seddon reviewed the literature of ionic liquids composed entirely of ions which were mainly of interest to electrochemists. Recently, however, it has become apparent that, inter alia, their lack of measurable vapor pressure characterizes them as green solvents, and that a wide range of chemical reactions (reviewed here) can be performed in them. Wassercheid and Keim reviewed the literature of ionic liquids, not only the synthesis and physical properties of the ILs, but also their use as... 

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