Stokes' Radius

is the hydrodynamic radius Rh does not describe the effective radius of a hydrated molecule in solution as often mentioned. It rather describes the radius of a hard sphere that diffuses at the same rate as the molecule. The behavior of this sphere includes hydration and shape effects. Since most molecules are not perfectly spherical, the Stokes radius is smaller than the effective radius (or the rotational radius). A more extended molecule will have a larger Stoke's Radii compared to a more compact molecule of the same molecular weight. In liquids where there are considerable interactions between solute and solvent molecule, Stokes radius is proportional to frictional coefficient f and inverse proportional to viscosity η:

Frictional coefficient is determined by the size and shape of the molecule under consideration.

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