Relationship between pore structure and diffusion tortuosity of ZrO₂ colloidal aggregates

The performance of chromatographic materials is extremely sensitive to variations in pore architecture. In this study, porous zirconia particles (5-10 μm) manufactured with an oil emulsion process have been characterized with the aim of explaining its success in protein separations with liquid chromatography (HPLC). Its chromatographic performance is surprising since it is an aggregate of colloidal spheres; if the spheres are near close packed, small constrictions in the pore network should drastically reduce the effective diffusion coefficient and thus diminish the chromatographic resolution of proteins. Moreover, if small constrictions are avoided, such materials may serve as catalyst supports requiring reaction of large molecules (e.g., block copolymers). To characterize the pore structure we use electron microscopy, SEM and TEM; nitrogen adsorption; and mercury porosimetry. Since the latter techniques are compromised by the presence of pore constrictions, we also use the NMR spin lattice relaxation and NMR self diffusion experiment to determine the average hydraulic diameter and tortuosity of the pore space. It is shown that while many small constrictions are present, the spheres are quite loosely packed. There is still a high degree of connection between pore spaces through larger constrictions than would be expected for close packed spheres. The particles are very porous and the effective tortuosity for diffusion is surprisingly low.