An ionic liquids miscibility with water is of great interest for biphasic

An ionic liquids miscibility with water is of great

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factors, the polarity of the ions and the possible degrees of coordination. An ionic liquid’s miscibility with water is of great interest for biphasic reactions where the ionic liquid replaces the usual organic solvent. The degree of miscibility of water in an ionic liquid depends mainly on the degree of coordination possible with the ions. Basic ions ([NO 3 ] - ) can strongly coordinate with water, whilst acidic ions are non-coordinating and pH neutral ions ([BF 4 ] - ) and ([NTf 2 ] - ) are weakly coordinated with water. The length of the alkyl chains on the cation also affects the miscibility of water in an ionic liquid. Longer chains result in a more hydrophobic ionic liquid.
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6 CHAPTER 1. BACKGROUND Ionic liquids can also be chosen to be miscible or immiscible in organic solvents. How miscible the ionic liquid is depends largely on the polarity of the ionic liquid. Generally speaking ionic liquids are miscible in polar organic solvents and immiscible in non-polar organic solvents. For any given ionic liquid the solubility of an alkene decreases with increased alkyl chain length on the alkene [ 21 ]. Gases can be soluble in ionic liquids to a variable extent and is depending on both cation and anion [ 22 , 23 , 24 ]. The solubility of gases in a given ionic liquid can be important for gas separation in a mixture of gases, for storing gases in ionic liquids for carrying out reactions such as oxygenation. Highly soluble gases decrease in solubility with increased temperature, the reverse being true for gases with low solubility [ 25 ]. 1.1.6 Dissociation of ionic liquid When discussing ionic species one must also discuss ion dissociation at some point. The degree of disassociation for an ionic liquid shows how large a fraction of the nominally neutral species has separated into cations and anions. Some degree of dissociation must exist as shown by the non-zero conductivity of ionic liquids. There exist several empirical solvent polarity scales to describe solvents and their physicochemical properties, one of which is Gutmann’s donor and acceptor number. Gutmann [ 2 ] showed that the strength of the Lewis base and the Lewis acid can be used to describe solvent effects. Numerous groups have experimentally [ 17 , 26 , 27 , 28 , 29 ] studied and related physicochemical properties of ionic liquids to their structure. In general it appears that ionic liquids typically behave as weak Lewis bases and weak Lewis acids. This in turn will lead to a weak coordination between ions and the surrounding molecules. The degree of disassociation can thus be seen as one parameter to characterize ionic liquids as it will directly affect the solvatization capabilities and polarity of the ionic liquid. There are, however, many problems with measuring the dissociation of ionic liquids and not many studies has thus far been made on the subject. In one publication [ 1 ] Tokuda et al. combine conductivity measurements with diffusion NMR to obtain information on the degree of association of the investigated ionic liquids. In this work, electrophoretic NMR is used to directly measure the effective charge of the anions and cations in an ionic liquid and thus characterize the association of the ionic liquid.
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