Viscosity, of ionic liquids

The viscosity of a fluid arises from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow. With respect to viscosity there are two broad classes of fluids Newtonian and non-Newtonian. Newtonian fluids have a constant viscosity regardless of strain rate. Low-molecular-weight pure liquids are examples of Newtonian fluids. Non-Newtonian fluids do not have a constant viscosity and will either thicken or thin when strain is applied. Polymers, colloidal suspensions, and emulsions are examples of non-Newtonian fluids [1]. To date, researchers have treated ionic liquids as Newtonian fluids, and no data indicating that there are non-Newtonian ionic liquids have so far been published. However, no research effort has yet been specifically directed towards investigation of potential non-Newtonian behavior in these systems. 

Experimentally determined viscosities are generally reported either as absolute viscosity (q) or as Idnematic viscosity (u). Kinematic viscosity is simply the absolute viscosity normalized by the density of the fluid. The relationship between absolute viscosity (q), density (p), and Idnematic viscosity (u) is given by Equation 3.2-1. 

The unit of absolute viscosity is the Poise (P, g cm s or mPa s), while the unit for Idnematic viscosity is the Stoke (St, cm s Because of the large size of these viscosity units, absolute viscosities for ionic liquids are usually reported in centipoises (cP) and Idnematic viscosities reported in centistokes (cSt). 

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The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

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. 

Viscosities of ionic liquids are several tens to hundreds times higher than that of water at room temperature. The structure of the cation strongly influences viscosity longer alkyl chains in the cation make the liquid more viscous. The viscosity of an... 

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. 

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. 

We have fitted the viscosity of ionic liquids using hole theory [123]. The theory was developed for molten salts but has been shown to be very useful for ionic liquids. It was shown that the value of En is related to the size of the ions and the size of the voids present in the liquid [103]. The viscosity of ionic liquids is... 

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]. 

Since the viscosity of ionic liquids is large in some cases and concomitantly the diffusion is slow, ionic liquids generally exhibit a lower conductivity than aqueous electrolytes. To improve the mass transport it has been suggested to add diluents like benzene, toluene or acetonitrile. Water may also be a suitable diluent in some cases, acting as both a ligand and a viscosity improver. 

Tochigi K, Yamamoto H (2007) Estimation of ionic conductivity and viscosity of ionic liquids using a QSPR model. J Phys Chem C 111 15989-15994... 

To sum up, enzymes in ionic liquids could maintain their activity over a much longer period than in molecular organic solvents. This stabilization has been explained on the basis of the interaction of the ionic liquid ions as well as higher viscosity of ionic liquids with respect to conventional organic solvent, which could cause slower migration of protein domains from the active conformation into the inactive one. [33]. 

The conductivity and viscosity of ionic liquids are often combined into what is termed Walden s rule [52],... 

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]. 

The viscosity of ionic liquids can be lowered by the addition of a small amount of organic co-solvent. It is noteworthy that the lowering of viscosity does not depend on the nature of the solvent added, but only on its mole fraction. Furthermore, a slight increase in temperature can diminish the viscosity of these solvents notably ... 

Supercritical carbon dioxide (scCOa) can be combined with ionic liquids providing an efficient, simple, and environmentally friendly separation method. For example, methanol is miscible with the ionic liquids and forms one phase. When this phase is saturated with scCOa, the formation of two phases could be observed and the upper phase contains most of the methanol. It should be noted that SCCO2 reduces the viscosity of ionic liquids and could facilitate mass transfer. The scCOa-philic, less polar products are easy to extract from reaction media. 

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. 

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