(194d) Material Structure Parameters for Transport Properties

The understanding of the role of the porosity architecture on the transfer properties of a fluid through a porous media is one of the key factors of many industrial and environmental processes that cover a very broad range of specialities (geology, engineering, chemistry and physics). There is a need to model or predict the effect of the porosity on the fluid flow with the help of a few parameters. Surface area, porosity and pore size distributions are the most commonly measured parameters, but they do not reflect the complexity of most porous networks that consist of labyrinths of interconnected pores with irregular shapes and cross-sections. This complexity has direct consequences on the transfer properties of charge and compounds through porous media. Hence, there is a need to define a limited number of parameters that i) quantify the complexity, (ii) may be introduced in models and (iii) may be measured in a straightforward manner. In this paper we want to focus on such a parameter, very naturally called tortuosity, which can be obtained by many different methods. In this work, the intra- and extra-particle tortuosities are determined by a new method based on electrical measurements as well as by the analysis of the shape of chromatographic peaks obtained in the conditions of inverse size exclusion chromatography. In that aim, the mass transfer kinetics of toluene and polystyrenes in a tetrahydrofuran mobile phase through silica columns were studied in non-adsorbing conditions. The tortuosity value obtained by conductivity is supposed to reflect the topology of the porous network, whereas the effective tortuosity determined by liquid chromatography depends not only on topology but also on the pore steric hindrance parameter which depends on the probe/pore size ratio. The effective tortuosity inside the silica particles are calculated from diffusion coefficient determined by using chromatographic models based on band broadening. The comparison of the two methods should give access to the pore steric hindrance parameter. The results are compared to diffusion hindrance models such as the Renkin equation calculated for a sphere in a cylindrical pore as a function of relative sphere/cylinder radius.