Solvation properties, ionic liquids

In a series of papers published throughout the 1980s, Colin Poole and his co-workers investigated the solvation properties of a wide range of alkylammonium and, to a lesser extent, phosphonium salts. Parameters such as McReynolds phase constants were calculated by using the ionic liquids as stationary phases for gas chromatography and analysis of the retention of a variety of probe compounds. However, these analyses were found to be unsatisfactory and were abandoned in favour of an analysis that used Abraham s solvation parameter model [5]. 

Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

One particular feature of ionic liquids lies in their solvation properties, not only for hydrophobic compounds but also for hydrophilic compounds such as carbohydrates. Park and Kazlauskas reported the regioselective acylation of glucose in 99 % yield and with 93 % selectivity in [MOEMIM][BF4] (MOE = CH3OCH2CH2), values much higher than those obtained in the organic solvents commonly used for this purpose (Entry 18) [22] (Scheme 8.3-4). 

Theoretical considerations based upon a molecular approach to solvation are not yet very sophisticated. As in the case of ionic solvation, but even more markedly, the connection between properties of liquid mixtures and models on the level of molecular colculations is, despite all the progress made, an essentially unsolved problem. Even very crude approximative approaches utilizing for example the concept of pairwise additivity of intermolecular forces are not yet tractable, simply because extended potential hypersurfaces of dimeric molecular associations are lacking. A complete hypersurface describing the potential of two diatomics has already a dimensionality of six In this light, it is clear that advanced calculations are limited to very basic aspects of intermolecular interactions,... 

Because ionic liquids are in general weakly coordinating, they display low nu-cleophilicity. Such an environment favors the stabilization of electron-deficient intermediates (7). This unique property allows ionic liquids to be used as nonsolvating media for the stabilization of strongly acidic species. It is this property that has given rise to the superacidity of non-solvated protons in acidic [AMIM]Al2Clv 103). 

Many ionic liquids have been widely investigated with regard to applications other than as liquid solvents such as electrolytes, phase-transfer reagents, surfactants, and fungicides and biocides. The physical and chemical properties of ionic liquids can be varied over a wide range by the selection of suitable cations and anions. Some of the properties that depend on the cation and anion selection includes melting point, viscosity, density, acidity and coordination ability, solvation strength and solubility characteristics. ... 

Examination of ionic liquid solvation interactions and thermodynamic properties using gas-liquid chromatography... 

ILs is in the range of 230-250 nm) which make them suitable to be used as solvents for spectroscopic measurements especially in the visible region. Because of their ionic origin, ILs allow the coordination of a complex compound in a liquid sfafe to be sfudied. An additional advanfage of ILs is that their solvating properties can be designed in such a way that differently coordinating solvents are obtained. A lot of examples can be presented on spectroscopic studies with lanthanides and actinides. 

The sieving effect of the carbon host was also demonstrated by measuring the capacitance values of an AC in a series of solvent-free ionic liquids (ILs) of increasing cation size [17], Since ions are not solvated in pure ILs, it was easy to interpret the electrochemical properties by comparing the nanoporous characteristics of carbon and the size of cations calculated by molecular modeling. It was found that the overall porosity of the carbon is noticeably underused, due to pores smaller than the effective size of the cations. The results with ILs confirm that the optimal pore size depends on the kind of electrolyte, i.e., the dimensions of pores and ions must match each other. 

A number of studies have attempted to characterize ionic liquids through their dielectric constant, and all have observed inconsistencies between the measured dielectric constant and the solvation properties of the liquid. Recent experiments making use of dielectric reflectance spectroscopy [214] indicate dielectric constants in the range of 10-15 for a series of imidazolium-based ILs, substantially lower than those for molecular solvents observed to possess comparable polarities as estimated by solvatochromism. Weingartner [215] has recently published a series of static dielectric constants obtained from dielectric reflectance spectroscopy, and compared them with those of common molecular liquids. The analysis includes comparison with the Kamlet-Taft ji parameter for the liquids from Eq. (11) we have prepared a plot of n versus dielectric constant in Fig. 6. The relationship between n and e for molecular liquids... 

Ionic liquids are an intriguing class of solvents. Their macroscopic solvation properties make it possible to recreate many conventional processes in these novel materials, but their convenience in this regard should not lull researchers into complacency. As salts, they represent fundamentally different media than molecular liquids and conventional chemical logic must be modified to account for these differences in their underlying physics. 

The chemistry of gold is more diversified than that of silver. Six oxidation states, from -I to III and V, occur in its chemistry. Gold(-I) and Auv have no counterparts in the chemistry of silver. Solvated electrons in liquid ammonia can reduce gold to give the Au" ion which is stable in liquid ammonia (E° = -2.15 V). In the series of binary compounds MAu (M = Na, K, Rb, Cs), the metallic character decreases from Na to Cs. CsAu is a semiconductor with the CsCl structure and is best described as an ionic compound, Cs+Au . The electron affinity of gold (—222.7 kJ mol"1) is comparable to that of iodine (-295.3 kJ mol-1). Gold in the oxidation state -I is also found in the oxides (M+ Au O2 (M = Rb, Cs) these, too, have semiconducting properties.1... 

Room-temperature ionic liquids (RTTLs) have become in recent years more widely applied as solvents, hence their solvating properties required investigation. Foremost among... 

An example is the question of the pH-value of acidic solutes in ionic liquids. From a practical point of view, this issue is extremely difficult to resolve, as pH-electrodes cannot be used in ionic liquids. Furthermore, the pH-value is defined as the negative logarithm of the H30+ activity, but in a dry ionic liquid, water is not present to form this species with dissociated protons. Does this then mean that protons are naked , tending to exhibit super-acidic character The answer to this question lies of course in the solvation properties of the ionic liquid under investigation. 

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