Room temperature ionic liquids properties

So far, there have been few published simulation studies of room-temperature ionic liquids, although a number of groups have started programs in this area. Simulations of molecular liquids have been common for thirty years and have proven important in clarifying our understanding of molecular motion, local stmcture and thermodynamics of neat liquids, solutions and more complex systems at the molecular level [1 ]. There have also been many simulations of molten salts with atomic ions [5]. Room-temperature ionic liquids have polyatomic ions and so combine properties of both molecular liquids and simple molten salts. 

Room temperature ionic liquids arc currently receiving considerable attention as environmentally friendly alternatives to conventional organic solvents in a variety of contexts.144 The ionic liquids have this reputation because of their high stability, inertness and, most importantly, extremely low vapor pressures. Because they are ionic and non-conducting they also possess other unique properties that can influence the yield and outcome of organic transformations. Polymerization in ionic liquids has been reviewed by Kubisa.145 Commonly used ionic liquids are tetra-alkylammonium, tetra-alkylphosphonium, 3-alkyl-l-methylimidazolium (16) or alkyl pyridinium salts (17). Counter-ions are typically PF6 and BF4 though many others are known. 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

Room-temperature ionic liquids exhibit many properties that make them potentially attractive as solvents for catalysis [11]. They dissolve a wide range of organic, inorganic and organometallic compounds as well as gases, e.g., CO2, HCl, NH3, and to a much lower extent also H2 and CO. Many ionic liquids show temperature operating ranges of more than 300 °C compared... 

Room temperature ionic liquids continue to attract interest by both fundamental and applied researchers. Several general review articles have been published in recent years that describe not only their physical properties but also discuss how these physical properties can be applied for solvents used in separations and as replacement for organic solvents for homogeneously-catalyzed reactions. In this review, we focus our attention on those physical... 

Xiong, H.Y., Chen, T., Zhang, X.H., and Wang, S.R, Electrochemical property and analysis application of biosensors in miscible nonaqueous media-room temperature ionic liquid, Electrochem. Commun., 9,1648-1654,2007. 

Poole, C.R, Chromatographic and spectroscopic methods for the determination of solvent properties of room temperature ionic liquids, /. Chromatogr. A, 1037, 49-82, 2004. 

Nishi, N., Kawakami, T., Shigematsu, E, Yamamoto, M., Kakiuchi, T., Fluorine-free and hydrophobic room-temperature ionic liquids, tetraalkylammonium bis(2-ethylhexyl)sulfosuccinates, and their ionic liquid-water two-phase properties, Green Chem., 8, 349-355, 2006. 

Lu, J., Liotta, G.L., Eckert, G.A., Spectroscopically probing microscopic solvent properties of room-temperature ionic liquids with the addition of carbon dioxide, /. Phys. Chem. A, 107, 3995-4000, 2003. 

Tokuda, H., Hayamizu, K., Ishii, K. et al.. Physicochemical properties and structures of room temperature ionic liquids. 1. variation of anionic species, /. Phys. Chem. B, 108,16593,2004. 

Room-temperature ionic liquids are attractive due to their chemical and thermal stability, negligible vapor pressure, high ionic conductivity, and ample electrochemical window. Their properties can be varied by a rational choice of the cations and of the anions and can represent an important iodide source for an I /I3 -based electrolyte (Fig. 17.12). 

Recently, room-temperature ionic liquids (molten salts) have been extensively studied in order to replace volatile organic solvents by them in electrochemical devices such as batteries. Interest in these materials is stimulated by their properties (e.g., high ionic conductivity, good electrochemical stability, and low volatility). Among these properties, the low volatility is the most critical for ensuring the long-term stability of electrochemical devices. Room-temperature ionic liq-... 

Apart from Section 12.7, which deals with supercritical fluids and room-temperature ionic liquids, only molecular liquid solvents are considered in this book. Thus, the term solvents means molecular liquid solvents. Water is abundant in nature and has many excellent solvent properties. If water is appropriate for a given purpose, it should be used without hesitation. If water is not appropriate, however, some other solvent must be employed. Solvents other than water are generally called non-aqueous solvents. Non-aqueous solvents are often mixed with water or some other non-aqueous solvents, in order to obtain desirable solvent properties. These mixtures of solvents are called mixed solvents. 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

This principle serves as the basis for a number of models of fused salt systems. Perhaps the best known of these is the Temkin model, which uses the properties of an ordered lattice to predict thermodynamic quantities for the liquid state [79]. However, certain other models that have been less successful in making quantitative predictions for fused salts may be of interest for their conceptual value in understanding room temperature ionic liquids. The interested reader can find a discussion of the early application of these models in a review by Bloom and Bockris [71], though we caution that with the exception of hole theory (discussed in Section II.C) these models are not currently in widespread use. The development of a general theoretical model accurately describing the full range of phenomena associated with molten salts remains a challenge for the field. 

This discussion of the structure and dynamics of fused salts provides only the briefest overview of their properties. However, even this minimal background will prove a useful reference point as we turn our attention to room temperature ionic liquids. 

Gan, Q., Xue, M., and Rooney, D. (2006) A study of fluid properties and microfiltration characteristics of room temperature ionic liquids [C10-min][NT 2] and N8881[NTf2] and their polar solvent mixtures. Sep. Purif. Technol, 51 (2), 185-192. 

Ionic liquids are quite simply liquids that are composed entirely of ions [96, 97]. They are generally salts of organic cations, e.g. tetraalkylammonium, alkylpyridi-nium, 1,3-dialkylimidazolium, tetraalkylphosphonium (Fig. 7.28). Room temperature ionic liquids exhibit certain properties which make them attractive media for performing green catalytic reactions. They have essentially no vapor pressure and are thermally robust with liquid ranges of e.g. 300 °C, compared to 100 °C for water. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion, which has earned them the accolade, designer solvents . 

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