Ionic liquids viscosity effect

Fontaine, O., J. Ghilane, P. Martin, J.-C. Lacroix, and H. Randriamahazaka. Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium salts. Langmuir 26, 2010 18542-18549. 

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. 

The conductivity of ionic liquids can be modeled in the same manner as the viscosity, i.e. despite the high ionic strength of the liquid, ionic migration is limited by the availability of suitably sized voids [130]. Since the fraction of suitably sized holes in ambient temperature ionic liquids is effectively at infinite dilution, migration should be described by a combination of the Stokes-Einstein and Nernst-Einstein equations. This is explained in greater detail in Chapter 11.3 on process scale-up but it is sufficient to say that an expression can be derived for the conductivity, k... 

The ionic liquid viscosity change effect was obtained by the bipolar solvent (DMF). Influence of the ionic liquid viscosity has been detected by the fluorescence emission and absorption spectra. Important information is that the new electronic states of P-carotene were found in the mixture of RTIL and DMF (Bialek-Bylka, 2008). 

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 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 electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

Once the two salts are mixed in solution (acetone is a common solvent for this), the sodium chloride precipitates and is removed by filtration. The solvent is then removed under reduced pressure and, since salts have no vapour pressure, the ionic liquid remains in the flask. The problem with this reaction is that it is almost impossible to remove the last traces of chloride ions. The chloride not only influences the physical properties of the liquid such as melting point and viscosity, but is also a good nucleophile and can deactivate catalysts and affect reproducibility. A great deal of effort has been directed towards removal of the chloride contamination, including washes and chromatography, but none have proved to be completely effective [9], This has led to the development of some alternative synthetic routes. Simply exchanging Na[BF4]... 

These changes in anion and cation were not merely a case of methyl, ethyl, propyl, butyl, and then futile. The change of anion dramatically affects the chemical behavior and stability of the ionic liquid the change of cation has a profound effect on the physical properties, such as melting point, viscosity, and density readily can be seen by examining the phase diagrams for the hexafluorophosphate and tetrafluoroborate salts (see Figures 5.5 and 5.6, respectively). 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

The temperature dependence of viscosity of 23 room-temperature ionic liquids was investigated. The size and symmetry of the cations and anions were shown to have a marked effect on viscosity (79). 

D.8.3. Cl Impurity. Trace amounts of chloride impurities, which may be present at levels between 0.1 and 0.5mol/kg, have significant effects on the physical properties of ionic liquids, such as viscosity and density. Increases in viscosity are of particular concern in biphasic processes because of the formation of emulsions that affect the interface between the two phases 88). 

An issue that has been explored is how the relative distribution of charge and mass affect the viscosity of an ionic liquid. Kobrak and Sandalow [183] pointed out that ionic dynamics are sensitive to the distance between the centers of charge and mass. Where these centers are separated, ionic rotation is coupled to Coulomb interactions with neighboring ions where the centers of charge and mass are the same, rotational motion is, in the lowest order description, decoupled from an applied electric field. This is significant, because the Kerr effect experiments and simulation studies noted in Section III. A imply a separation of time scales for ionic libration (fast) and translation (slow) in ILs. Ions in which charge and mass centers are displaced can respond rapidly to an applied electric field via libration. Time-dependent electric fields are generated by the motion of ions in the liquid... 

Ionic liquids with discrete anions have a fixed anion structure but in the eutectic-based liquids at some composition point the Lewis or Bronsted acid will be in considerable excess and the system becomes a solution of salt in the acid. A similar scenario also exists with the incorporation of diluents or impurities and hence we need to define at what composition an ionic liquid is formed. Many ionic liquids with discrete anions are hydrophilic and the absorption of water is found sometimes to have a significant effect upon the viscosity and conductivity of the liquid [20-22], Two recent approaches to overcome this difficulty have been to classify ionic liquids in terms of their charge mobility characteristics [23] and the correlation between the molar conductivity and fluidity of the liquids [24], This latter approach is thought by some to be due to the validity of the Walden rule... 

Cation Cationic structure and size will affect the viscosity and conductivity of the liquid and hence will control mass transport of metal ions to the electrode surface. They will also be adsorbed at the electrode surface at the deposition potential and hence the structure of the double layer is dominated by cations. Some studies have shown that changing the cationic component of the ionic liquid changes the structure of deposits from microcrystalline to nanocrystalline [27]. While these changes are undeniable more studies need to be carried out to confirm that it is a double layer effect. If this is in fact the case then the potential exists to use the cationic component in the liquid as a built-in brightener. 

The choice of anion will have a known effect on the physicochemical properties of the ionic liquid. To demonstrate the anion effect, selected data on properties of general interest to electrochemists (density, viscosity, conductivity and electrochemical window) have been gathered in Table 2.2. In each case, the anion is paired with the same cation l-ethyl-3-methylimidazolium. Certain trends from this data can be generalized, as well as in other collections of such data (for example, see Ref. [61] and references therein), that hold true regardless of the identity of the cation. For example, the effect of the anion on density follows the trend ... 

One issue is that most metal complexes formed in ionic liquids are anionic and these will have a significant effect on viscosity and mass transport. The effect of metal ion concentration on reduction current will therefore not be linear. Relative Lewis acidity will affect mass transport, ionic strength and speciation and accordingly the nucleation and growth mechanism of metals would be expected to be concentration dependent. 

Abbott et al. studied the deposition of zinc from a 1 2 choline chloride (ChCl) ZnCl2 ionic liquid [109] at 60°C and found deposits with a similar morphology to that shown by Sun. The optimum current density was found to be between 2 and 5 Am 2 and higher current densities led to powdery, non-adherent deposits. This is due primarily to the high viscosity and low conductivity of the choline-based liquids. The current plating efficiency in this liquid was found to be effectively 100% and the deposition process was shown to be almost totally reversible, with only the UPD material remaining on the surface. 

When ionic liquids are used, this will have a significant effect on the viscosity and hence the conductivity and rate of ion diffusion within the ionic liquids. Growth of conducting polymers at reduced temperatures (as low as — 28 ° C) [4,24] in molecular solvent systems is generally accepted to result in smoother, more conductive films, but we have found that in ionic liquids the significant increase in the viscosity can be problematic. In addition, the temperature used for the conducting polymer synthesis may be limited by the melting point of the ionic liquid [25]. 

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