Translational mobility in vitreous matrices

The question about the difference between the macroscopic and microscopic values of the quantities characterizing the translational mobility (viscosity tj, diffusion coefficient D, etc.) has often been discussed in the literature. Numerous data on the kinetics of spin exchange testify to the fact that, with the comparable sizes of various molecules of which the liquid is composed, the microscopic translational mobility of these molecules is satisfactorily described by the simple Einstein-Stokes diffusion model with the diffusion coefficient determined by the formula 

On the basis of these considerations it can be expected that the Einstein Stokes model will be applicable for vitrified solutions consisting of the particles with comparable sizes. In other words, for the order of magnitude estimations of the microscopic translational mobility of molecules in such solutions, the value of the diffusion coefficient, which is obtained by using the macroscopic viscosity t] in eqn. (1), can be used. 

The given considerations of the character of the translational mobility of molecules in vitreous matrices, of course, do not claim to be theoretically strict. Nevertheless, they are in qualitative agreement with the existing ideas about the structure of vitreous matrices and, apparently, can be useful for rough preliminary order of magnitude estimates of the mobility. 

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