Hindered Diffusion in Microporous Membranes with Known Pore Geometry

The hindrance effect on the aqueous diffusion rate of solutes within membrane pores of molecular size has been accurately determined. Mica membranes, 3 to 5 micrometers thick, were prepared with uniform, straight pores from 90 to 600 angstroms in diameter. With these membranes a direct estimation was possible of the interaction between pore size and molecular diffusion rates. There were no uncertainties due to wide pore size distributions or nonuniform tortuous channels as in previously used model microporous materials such as dialysis tubing or gels. Aqueous diffusion rates through these mica membranes were measured for a series of compounds with molecular diameters from 5.2 to 43 angstroms and were corrected for "liquid film resistances" adjacent to the membrane-solution interface to obtain estimates of molecular diffusivities within the micropores of the membrane. Definite evidence is presented showing that, even when molecular size is a small fraction of pore size, diffusion rates decrease markedly. The apparent reduction in solute diffusivity in the microporous membrane can be quantitatively estimated by means of the Renkin equation for hindered diffusion.