Ionic liquids Stokes-Einstein relation

The Walden rule is interpreted in the same manner as the Stokes-Einstein relation. In each case it is supposed that the force impeding the motion of ions in the liquid is a viscous force due to the solvent through which the ions move. It is most appropriate for the case of large ions moving in a solvent of small molecules. However, we will see here that just as the Stokes-Einstein equation applies rather well to most pure nonviscous liquids [30], so does the Walden rule apply, rather well, to pure ionic liquids [15]. When the units for fluidity are chosen to be reciprocal poise and those for equivalent conductivity are Smol cm, this plot has the particularly simple form shown in Figure 2.6. 

Validity of the Stokes-Einstein Relation in Ionic Liquids... 

This equation was deduced in Section 4.4.8. It is of interest to inquire here about its degree of appiicabiiity to ionic liquids, i.e., fused salts. To make a test, the experimental values of the self-diffusion coefficient D and the viscosity tj are used in conjunction with the known crystal radii of the ions. The product D r//T has been tabulated in Table 5.22, and the plot of D tj/T versus 1/r is presented in Fig. 5.31, where the line of slope k/6n corresponds to exact agreement with the Stokes-Einstein relation. ... 

In polar liquids, ionic mobilities have been extensively studied, for instance by conductometry. They are known for many solvents (Robinson and Stokes, 1959 Janz and Tomkins, 1972). In low polar liquids, K has been deduced most often from transit time measurements, after irradiation of a thin layer of liquid or photoexcitation of the cathode, or after applying a voltage step. K values are now available in different liquids (Tables 1 and 2) but the nature of the corresponding ion is often unknown. So, to estimate the value of the mobility of a given ion in a liquid, the relation, Kt) = e/6nr, between K and the liquid viscosity Vi (Stokes-Einstein relation) is often used, where r is the hydrodynamic radius of the ion. In fact, this relation is only a crude approximation even for rather large ions, since the viscous forces are not the only retarding force acting on the ion, and the solvent is... 

E. Herald, M. Strauch, D. Michalik, A. Appelhagen, R. Ludwig, Dynamics of methanol in ionic liquids validity of the Stokes-Einstein and Stokes-Einstein-Debye relations, ChemPhysChem 15 (14) (2014) 3040-3048. 

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... 

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