Diffusion in ionic liquids

Diffusion in Ionic Liquids and Correlation with Ionic Transport Behavior... 

DIFFUSION IN IONIC LIQUIDS AND CORRELATION WITH IONIC TRANSPORT BEHAVIOR... 

Ionic Liquids in Synthesis, ed. P. Wasserscheid and T. Welton, Wiley-VCH Verlag GmbH Co. KGaA, Weinheim, Germany, 2003 R 225 J. Richter, A. Leuchter and G. Palmer, Translational Diffusion [in Ionic Liquids] , p. 162... 

Figure 8.12 Two-step thermal diffusion in ionic liquids. Energy transfer is fast within a local structure, hut is slow across the two local structures.

Taylor, A.W., Licence, P. and Abbott, A.P., Non-classical diffusion in ionic liquids, Phys. Chem. Chem. Phys. 13,10147-10154 (2011). 

McLean, A.J., Muldoon, M.J., Gordon, C.M. and Dunkin, I.R., Bimolecular rate constants for diffusion in ionic liquids, Chem Commun., 1880-1881 (2002). 

Wang, Y. Voth, G. A. (2006). Tail aggregation and domain diffusion in ionic liquids., /. Phys. Chem. B 110 18601-18608. 

Tsuzuki, S. Tokuda, H. Hayamizu, K Watanabe, M. (2005). Magnitude and directionality of interaction in ion pairs of ionic liquids relationship with ionic conductivity. Journal of Physical Chemistry B, 109,16474-16481 Turton, D. A. Hunger, J. Stoppa, A. Hefter, G. Thoman, A. Walther, M. Buchner, R Wynne, K. (2009). Dynamics of imidazolium ionic liquids from a combined dielectric relaxation and optical Kerr effects study evidence for mesoscopac aggregation. Journal of the American Chemical Society, 131,11140-11146 Urahata, S. M. Ribeiro, M. C. C. (2005). Single particle dynamics in ionic liquids of 1-alkyl-3-methylimidazolium cations. Journal of Chemical Physics, 122,024511/1-9 Wang, Y. Voth, G. A. (2006). Tail aggregation and domain diffusion in ionic liquids. Journal of Physical Chemistry B, 110,18601-18608 Wasserscheid, P. Welton, T., Eds. (2008). Ionic Liquids in Synthesis, 2nd Ed., WILEY-VCH, ISBN 978-3-527-31239-9, Weinheim... 

The diffusion coefficients of the constituent ions in ionic liquids have most commonly been measured either by electrochemical or by NMR methods. These two methods in fact measure slightly different diffusional properties. The electrochemical methods measure the diffusion coefficient of an ion in the presence of a concentration gradient (Pick diffusion) [59], while the NMR methods measure the diffusion coefficient of an ion in the absence of any concentration gradients (self-diffusion) [60]. Fortunately, under most circumstances these two types of diffusion coefficients are roughly equivalent. 

Since this is just the beginning of investigations into the diffusion behavior and intermolecular forces in ionic liquid systems, further experimental work needs to be done both with pure ionic liquids and with systems of mixtures of ionic and organic liquids. 

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). 

As we have seen, the electric state of a surface depends on the spatial distribution of free (electronic or ionic) charges in its neighborhood. The distribution is usually idealized as an electric double layer one layer is envisaged as a fixed charge or surface charge attached to the particle or solid surface while the other is distributed more or less diffusively in the liquid in contact (Gouy-Chapman diffuse model, Fig. 3.2). A balance between electrostatic and thermal forces is attained. 

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. 

Due to the lack of a reliable description, the diffusion of an ionic species in a molecular species is usually represented by the effective ionic diffusivity in the liquid phase [52]. The calculation of the diffusion coefficient for an ionic component in another ionic species is reduced to the arithmetical mean of both effective ionic diffusivities [52]. 

Mass transport may influence material growth in ionic liquids. 1-Butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, for example, is, at room temperature, about 60 times more viscous than water. At temperatures above 150°C its viscosity is similar to most molecular solvents at ambient conditions. Indeed, temperatures between 150 and 200 °C were best to deposit tantalum from TaFs in the presence of LiF. One has to keep in mind that the deposition of tantalum from TaFs or an anionic complex delivers one Ta atom and 5-7 fluorides. If the deposition is too fast F may not diffuse rapidly enough from the surface to the bulk of the solution and may be trapped in the deposit. This might explain why we only got crystalline tantalum layers at low current densities. 

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]. 

Morgan, Ferguson, and Scovazzo find. Eng. Chem. Res. 44, 4815 (2005)] They studied diffusion of gases in ionic liquids having moderate to high viscosity (up to about 1000 cP) at 30°C. Their range was limited, and the empirical equation they found was... 

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