for seperations

If membrane selectivity of two dissolved species were solely determined by hindered (Knudsen) diffusion, estimate pore diameter needed for the effective separation of species A from species C in medium B (solvent) if it requires AC=1000 to be feasible. Species A has a radius of gyration when dissolved in medium B of 1.5nm; similarly, molecule C is 2.5nm; the average molecular diameter of the solvent (water) is 0.28nm. Use the Stokes-Einstein equation for an easy measure of molecular diffusivity for A and C in water, respectively (though this would be a gross error). Membrane porosity is 70% with roughly straight (tracketched) pathways. Clearly state any additional requirements, limitations, or fixed parameters in making reasonable, quantitative estimates. [Note, the setup and process understanding are key herein rather than exact numerical values.]