High viscosity of ionic liquids

Lipase-catalyzed transesterification to prepare polyesters (replacing the traditional chemical polymerization at >200 °C) has received considerable attention in recent years. CaLB was found to mediate polyester synthesis in the ionic liquids [BMIm][BF4], [BMIm][PF6], and [BMIm][ Tf2N] at 60°C [110, 111, 112], but the molecular weight of the product was rather low compared with that in a solventless system [113], perhaps owing to the high viscosity of ionic liquid media. 

The high viscosity of ionic liquids is also problematic computational methods that aim to determine thermodynamic properties rely on sampling a large number of configurations or snap-shots of the constituent molecules (or ions) in different positions or orientations within the liquid [10]. In a viscous liquid this motion is limited and thus it can take a very long time, and be computationally expensive, to build up the required number of snap-shots that wiU produce accurate predictions [11,12]. 

Therefore, the conparatively high viscosities of ionic liquids should lead to these reactions being much slower than their equivalents in free-flowing molecular solvents [28]. 

The viscosity of ionic liquids is often quite high. In addition, impurities can have a marked influence and may increase the viscosity of the ionic liquid. In the worse case scenario the addition of a catalyst and substrate to an ionic liquid can increase the viscosity to such an extent that it becomes gel-like and therefore difficult to process. 

The viscosity of ionic liquids is high compared with molecular solvents and increases with the chain length. Consequently, diffusion is bound to be slow in ionic liquids. The effects on biocatalytic transformations seem to be insignificant, however, except in extreme cases, presumably because the reaction times are measured in hours rather than minutes. 

Hence the ionic liquids with the lowest viscosity tend to have highly fluorinated anions as these shield the charge density and result in low surface tensions. The cation also affects the viscosity of ionic liquids. For imidazolium cations, the viscosity initially decreases as the length of the R group increases, as the ion-ion interactions decrease and hence the surface tension decreases. However, as the alkyl group increases in size its mobility will decrease due to a lack of suitably sized voids for the cations to move into. This can be seen in the data presented by Tokuda ct al. who showed a minimum in viscosity for ethyl methyl imidazolium salts [129]. 

In general, the published data on the viscosity of ionic liquids is scarce. Most of this published literature on ionic liquids viscosity deals with the first generation ionic liquids. The viscosity of any fluid is highly dependent on both the measuring technique used and the purity of the samples. Given this difficulty the reported values in the literature are often neither comparable or reproducible. 

The size of the cation in the chloroaluminate ionic liquids also appears to have an impact on the viscosity. For ionic liquids with the same anion(s) and compositions, the trend is for greater viscosity with larger cation size (Table 3.2-2). An additional contributing factor to the effect of the cation on viscosity is the asymmetry of the alkyl substitution. Highly asymmetric substitution has been identified as important for obtaining low viscosities [17]. 

The highly detailed results obtained for the neat ionic liquid [BMIM][PFg] clearly demonstrate the potential of this method for determination of molecular reorienta-tional dynamics in ionic liquids. Further studies should combine the results for the reorientational dynamics with viscosity data in order to compare experimental correlation times with correlation times calculated from hydrodynamic models (cf [14]). It should thus be possible to draw conclusions about the intermolecular structure and interactions in ionic liquids and about the molecular basis of specific properties of ionic liquids. 

The spectrum of physical and chemical properties of ionic liquids is much larger than that of classical organic or inorganic solvents. As well as wide liquid ranges, they offer ranges of physical properties such as density and viscosity and also have high heat conductivities. In addition, the miscibilities of ionic liquids with organic... 

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