Porous Structure of Silica Colloidal Crystals

We prepared silica colloidal crystals with different pore sizes using isothermal heating evaporation-induced self-assembly in quantities suitable for nitrogen porosimetry and studied their porous structure. We observed pores of two types in agreement with the description of silica colloidal crystals as face-centered cubic packed structures containing octahedral and tetrahedral voids. We calculated the sizes of these pores using the Derjaguin-Broekhoff-de Boer theory of capillary condensation for spherical pores. We also described the pore geometry mathematically and showed that the octahedral pore radii measured experimentally matches closely the radii of the spheres of the same volume. In the case of the tetrahedral pores, the proposed approach underestimated the pore radius by ca. 40%. Overall, this simple geometrical description provides a good representation of the porous system in silica colloidal crystals.