Influence of molecule size on its transport properties through a porous medium

The mass transfer kinetics of toluene and polystyrenes (of which the M(w) varies from 162 to 1.85 x 10(6) g mol(-1)) through columns filled with silica porous spheres were studied by inverse size exclusion chromatography. The mass transfer parameters were measured by modeling the band broadening of the chromatograms. The experimental height equivalent to a theoretical plate (HETP) data were analyzed using the general rate model in order to determine the effective diffusion coefficient in porous particles as a function of molecular size. The bulk molecular diffusion coefficients were experimentally determined by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA). The topological tortuosity of the porous particles was determined by electrical measurements. The effective molecular diffusion coefficient through porous particles was modeled taking into account exclusion, friction, and at last tortuosity effects. A phenomenological law is proposed to model the evolution of the tortuosity experienced by a molecule in a porous particle as a function of its size. It gives a good prediction of the evolution of effective diffusion coefficient with the molecule/pore size ratio.