DIFFUSION OF LARGE MOLECULES IN POROUS-GLASS

Effective diffusion coefficients for perylene and vanadyl porphyrine in acetone and six polystyrenes with molecular weight in the range 1200-19,000 g/mol in tetrahydrofuran were determined from the responses of a chromatographic column packed with porous glass (pore diameter 11 nm). The contribution of axial dispersion was evaluated from the measurements with polystyrenes which cannot penetrate the pores. To obtain the diffusion time constants a newly developed method for matching the experimental peaks in the time domain was utilized. The steric factors, S (partition coefficients), of the solutes evaluated from the first moments of the column responses were used for the determination of ratios of solute and pore sizes, lambda. The enhanced drag coefficient, which characterizes the interaction of the solute with the pore wall, changes with (1 - lambda)1.34. Thus, the obtained tortuosity of the porous glass (2.71) is well within the expected interval. The effective diffusion coefficients are related to the bulk diffusivities of the solute-solvent pair, D, as D = epsilon(1 - lambda)3.34 D/q.