Stokes—Einstein correlation, ionic liquids

This work, along with all other diffusion studies in ionic liquids, depends on the validity of the Stokes-Einstein equation and here lies the main discrepancy with the analysis of most difhision coefficient data in ionic liquids. To be strictly accurate, the distance term, R, in Eqnation 1.3 should be replaced by the correlation length, which is only really equivalent to the radius of the diffusing species when the size of the diffusing particle is large conpared with the solvent particles [27]. Eqnation 1.3 was initially derived to describe the random movement of... 

Classical diffusion can be described by Equation 1.3 when the radius of the sphere is small conpared with the mean free path. With ionic liquids, the mean free path can be less than the radius of the ion, and hence the ion can be considered as moving via a series of discrete jumps where the correlation length is a measure of the size of the hole into which the ion can junp. Appreciating why deviations from the Stokes-Einstein equation occur shows why a model based on holes becomes appropriate. The approximate nature of the Stokes-Einstein equation is often overlooked and is discussed in detail by Bockris and Reddy [5, p. 379]. There are numerous aspects that need to be taken into account, including that it is derived for non-charged particles, it is the local viscosity rather than the bulk that is required, and the ordering effect of the ions exhibits an additional frictional force that needs to be explained. 

Harris KR (2010) Relations between the fractional stokes- einstein and nemst- einstein equations and velocity correlation coefficients in ionic liquids and molten salts. J Phys ChemB 114 9572-9577... 

Jeong, D., Choi, M., Kim, H. Jung, Y. (2010). Fragility, Stokes-Einstein violation, and correlated local excitations in a coarse-grained model of an ionic liquid, Phys. Chem. Chem. Phys. 12 2001-2010. 

We would like to stress that in all these publications the authors investigated peculiarities of the rotational and translational diffusion of spin-probe molecules in various room temperature ionic liquids, comp)ared them with molecular dynamics in common organic solvents. Correlations with Stokes-Debye-Einstein or Stokes-Enstein laws were foimd. Areas in RTILs (polar, non-polar), in which spin probes (hydropElic, charged, hydrophobic) are localized were determined. Just recently, attention of the scientists was attracted to another type of molecular motions in the ionic liquids (Tran et al., 2007a, 2009). Such processes as well as solvent effects on them can be examined in detail by EPR sp>ectroscopy with the use of stable nitroxide biradicals (Parmon et al., 1977a, 1980). 

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