Simple solvent small-molecule motion

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. 

Neat liquids are, in a way, difficult objects for NMR relaxation studies. The simple modelling of reorientational motion as small-step rotational diffusion is based on hydrodynamics (large body immersed in continuum solvent) and becomes problematic if we deal with a liquid consisting of molecules of a single kind. Deviations from the models based on few discrete correlation times can therefore be expected. 

This section examines motion (diffusion, conductance, electrophoretic mobility) of rigid probes through simple solvents and small-molecule solutions. Experiments test the validity of Stokes law / and the Stokes-Einstein form D T/rjR. 

Translational motions of solvent and other small molecules in polymer solutions are quite different from their behaviors in viscous liquids. The self-diffusion coefficient of the solvent has a transition at a polymer volume fraction 0.4. At smaller (j), Ds follows a simple exponential exp(-a) in polymer concentration, but at larger Ds(c) follows a stretched exponential with large exponent. The exponential factor a is independent of polymer molecular weight, while rj depends strongly on M, so Ds and A must be nearly independent of solution rj. Probes somewhat... 

We consider a mixture of two simple liquids 1 and 2 are small and spherically symmetric and the ratio of their sizes is close to unity. We suppose that the arrangement of the molecules in each pure liquid is that of a regular array all the molecules are situated on lattice points that are equidistant from one another. Molecular motion is limited to vibration about the equilibrium positions and is not affected by the mixing process. We suppose further that for a fixed temperature, the lattice spacing for the two pure liquids and for the mixture is the same, independent of composition. These assumption having been accepted as required, we consider the total number of ways of arranging the Ui identical molecules of the solvent and U2 identical molecules of the solute on the lattice comprising =72 + n. cells. This just the... 

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