Ionic liquids solvation effects

Anderson, J.L. et al.. Surfactant solvation effects and micelle formation in ionic liquids, Chem. Comm., 2444, 2003. 

Kinetic studies have been reported of the reactions of a series of 2-substituted-5-nitrothiophenes (substituent = Br, OMe, OPh, OC6H4-4-NO2) with secondary amines in room-temperature ionic liquids. The kinetic behaviour is similar to that of the corresponding reactions in methanol so that most reactions do not show base catalysis. The observation that reactivity is higher in the ionic liquids than in methanol (or benzene) is attributed to relatively poor solvation of the reagents by the ionic liquids. As in conventional solvents, 2-bromo-3-nitrothiophene shows higher reactivity than 2-bromo-5-nitrothiophene.42 Solvent effects on the kinetics of the alkaline hydrolysis of 2-phenylthio-3,5-dinitropyridine in aqueous organic solvents have been analysed.43... 

Chaumont, A., Wipff, G. (2004), Solvation of Uranyle 11, Europium 111 Cations and their Chloro complexes in a Room-Temperature Ionic Liquid. A Theoretical Study of the effect of Solvent Humidity, Inorg. Chem. 43, in press. 

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. 

The addition of LiCl to the ionic liquid was found to have only a small effect upon the conductivity of the liquid, but it did affect the speciation [121], producing more of the [CrCLp2H20]. It was anticipated that the small Li+ ion would have a high mobility in the liquid but the conductivity is less than expected, suggesting that the ion must be strongly solvated or highly associated with the anion. 

Ionic liquids have polarities comparable to lower alcohols, which are amongst the most widely used solvents in which to conduct homogeneously catalysed reactions. In contrast to lower alcohols, however, many ionic liquids are non-nucleophilic, which can have a pronounced effect on a catalysed reaction. For example, solvated catalytic intermediates are unlikely to form, and this feature could lead to different reaction mechanisms, and hence to different yields and selectivities. The non-nucleophilic environment presented by many ionic liquids is also less likely to deactivate a catalyst and can lead to increased turnover numbers, which is essential in biphasic processes where catalyst recycling and reuse is required. 

It was seen (Seetion 2.4) that in aqueous solutions, the solvent eould not be relegated to the status of an unobtrusive background. The solvent molecules, by entering into the solvation sheaths of ions, participated in their drift. Thus, in addition to the flows of the positive and negative ions, there was a flux of the solvent. This complication of solvent flux is absent in pure ionic liquids. There is, however, an interesting effect when a current is passed through a fused salt. 

Wishart JF, LaU-Ramnarine SI, Raju R, Scumpia A, Bellevue S, Ragbir R, Engel R. (2005) Effects of functional group substitution on electron spectra and solvation dynamics in a family of ionic liquids. Radiat Phys Chem 72 99-104. 

HS-GC has been developed to serve as a sensitive tool to determine even small differences in the solvation properties of ionic liquids using a choice of model solutes featuring specific interactions molecular ion-dipol interactions, hydrogen bond donor and acceptor interactions, and n- and n-electron dispersion forces can be probed by model solutes such as acetonitrile, 1,4-dioxane, n-propanol, n-heptane and toluene, respectively. Bearing in mind that no solute exhibits exclusively one specific interaction, the systematic investigation of the effect of the variation of the structural elements of ionic liquids, i.e. choice of cation, cation substitution and anion, lead to the following conclusions. 

It should be noted that many of the scales whose use is reported below are based on the effect of the ionic liquids on a single probe molecule. While the response of the probe to the solvent in which it is dissolved is determined by all possible solvent-probe interactions, there is no reason for there to be an equal contribution from all of them for all of probes. Hence, it is important to be as aware as possible of which interactions are likely to have a strong effect on the particular probe used and which have a lesser effect. Since ionic liquids are composed of both anions and cations, either of which may preferentially solvate a particular probe molecule, this problem will be even greater than is usually the case in molecular solvents. It is probably foolhardy to take a scale generated by any one probe as a measure of what is generally understood by the term polarity. However, very useful information can still be gleaned from these studies. 

It can be seen for the [N(Tf)2] ionic liquids, that the hydrogen-bond acidity does indeed vary with cation with [BMIM]+ being the most acidic followed by [BMPYJ+ and finally [BMMIMJ+ in both studies. However, changing to more basic anions leads to a dramatic drop in the acidity measurements in the solvation study, whereas it has only a limited effect in the Kamlet-Taft experiment. That is, the solvation measurement is anion dominated, whereas the Kamlet-Taft measurement is cation dominated. 

In each case, an extensive, and informative investigation has been carried out on the solvation effects in ionic liquids [44],... 

Most catalytic applications using ionic liquids aim to realize beneficial effects with the lowest possible amount of ionic liquid present in the system. A fully soluble ionic liquid would thus lead to the situation where a very small amount of ionic liquid would be dissolved in a large volume of reaction mixture. In such a scenario the ions of the ionic liquid would be present as either fully dissociated and solvated cations/anions or as solvated ion-pairs depending on the polarity of the reactants. 

Different P Fg or NTfj imidazolium-based ionic liquids have been used as solvents and electrolytes for several typical electrochemistry reactions. Although the structure of molecular solvents and ILs are expected to be quite different, the main result is that the use of ionic hquids does not modify the nature of the mechanisms investigated using conventional organic media. An effect of the structure of ILs can nevertheless be observed in the case of bimolecular reactions (e.g., oxidative electrodimerization), as kinetic rate constants are lower in ionic liquids than in conventional polar solvents. This phenomenon cannot be simply attributed to the high viscosity of ILs but may be explained by a specific solvation of the reactants due to a high degree of ion association in ILs [59]. 

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