Ionic liquid-water interactions

In principle, there is no difference between the pretreatment that a metal should undergo before immersion in an ionic liquid or in an aqueous solution. The sole difference is that the workpiece must be dry before immersion in the ionic liquid. The sensitivity of the ionic liquid to water content is dependent upon the ionic liquid. Eutectic-based ionic liquids are less sensitive to water content than liquids with discrete anions. This is thought to be due to the ability of the chloride anions in the former interacting strongly with the water molecules, decreasing their ability to be reduced. Especially with AICI3-based ionic liquids water has to be strictly avoided. 

Ions comprise another class of solutes which have been studied extensively by Wipff and coworkers, who have been particularly interested in lanthanide and uranyl ions and their chloro-complexes [44,142]. Their studies show that the chloro-complexes are stabHized by solvation in the ionic liquids based on imidazolium cations and the [PFe]" anion. The principal interaction of the naked ions is with the anions, but on chlorination the cations move closer to complexes such as [EuQe] and [U02Cl4] . In a further study [45] they showed that in an equimolar mixture of water and ionic liquid, water molecules tend to fill the first solvation shell of naked ions in preference to [PFs]" ions, and tend to solvate the chloro-complexes in preference to the imidazolium cations. 

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. 

Wu B, Liu Y, Zhang YM, Wang H (2009) Probing intermolecular interactions in ionic liquid-water mixtures by near-infrared spectroscopy. Chemistry 15 6889-6893... 

In order to look at the effect of water on the structure of both hydrophobic and hydrophilic room-temperature ionic liquids, SFG measurements were taken at water partial pressures of 5 X 10 Torr and 20 Torr. Results showed that ionic liquid behaviour at the surface differed depending on whether the ionic liquid was hydrophobic or hydrophilic. For hydrophobic ionic liquids, the imidazolium ring reorients towards the surface normal upon addition of water, while for hydrophilic ionic liquids, the ring remains flat on the surface. The process was found to be reversible, with the tilting of the cation attributed to the interaction of water with C(2)—H. Moreover, for water-miscible ionic liquids, water molecules were said to be... 

Aono, M, Imai, Y., Abe, H., Matsumoto, H. Yoshimura, Y., Optical Interaction of Room Temperature Ionic Liquid- Water Mixtures N, N-diethyl-N-methyl-N- (2-methoxyethyl) Ammonium Tetrafluoroborate, submitted to Thermochim. Acta. 

The solubility of water vapor in ionic liquids is of interest because ionic liquids are extremely hygroscopic. In addition, the solubility of water vapor in ILs is an excellent test of the strength of molecular interactions in these fluids. By using the gravi-... 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

Friedel-Crafts acylation reactions usually involve the interaction of an aromatic compound with an acyl halide or anhydride in the presence of a catalyst, to form a carbon-carbon bond [74, 75]. As the product of an acylation reaction is less reactive than its starting material, monoacylation usually occurs. The catalyst in the reaction is not a true catalyst, as it is often (but not always) required in stoichiometric quantities. For Friedel-Crafts acylation reactions in chloroaluminate(III) ionic liquids or molten salts, the ketone product of an acylation reaction forms a strong complex with the ionic liquid, and separation of the product from the ionic liquid can be extremely difficult. The products are usually isolated by quenching the ionic liquid in water. Current research is moving towards finding genuine catalysts for this reaction, some of which are described in this section. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

Mele, A., Tran, C. D., and De Paoli Lacerda, S. H., The structure of a room-temperature ionic liquid with and without trace amounts of water The role of C-H...O and C-H...F interactions in l-n-butyl-3-methylimidazolium tetrafluo-roborate, Angew. Chem., Int. Ed., 42, 4364, 2003. 

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