Determination of the Effective Hydrodynamic Radii of Small Molecules by Viscometry

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Abstract
The effective hydrodynamic radii of small uncharged molecules in dilute aqueous solution were determined using Einstein's classical theory of viscosity. The radii thus obtained are those of a hypothetical sphere whose hydrodynamic behavior is the same as that of the solute molecule plus that water of hydration which is too firmly bound to partake in the viscous shearing process. The results obtained compare favorably with radii determined from molecular models constructed in accordance with atomic dimensions compiled by Pauling. Although the application of the Einstein theory to molecules whose size is comparable to that of water represents a considerable extrapolation, the results suggest that this deviation from the assumptions of the theory, in the case of the molecules studied, is of second order importance.