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The transport of oligomeric molecules in silica spheres with cylindrical mesopores has been quantified and related to the structural features of the spherical particles and the interactions at the solid-liquid interface. An emulsion-solvent evaporation method was used to produce silica spheres having cylindrical mesopores with an average pore diameter of 6.5 nm. The transport of dextran molecules (fluorescently tagged) with molecular weights of 3000 and 10,000 g/mol was quantified using confocal laser scanning microscopy (CLSM). The intraparticle concentration profiles in the dextran-containing spheres were flat at all times, suggesting that the release is not isotropic and not limited by diffusion. The release of dextran into the solution is characterized by an initial burst, followed by long-term sustained release. The release follows a logarithmic time dependency, which was rationalized by coupling concentration-dependent effective diffusion constants with adsorption/desorption.

We introduced the novel technique of AFM-porosimetry and applied it to measure the total pore volume of porous particles with a spherical geometry. The methodology is based on using an atomic force microscope as a balance to measure masses of individual particles. Several particles within the same batch were measured, and by plotting particle mass versus particle volume, the bulk density of the sample can be extracted from the slope of the linear fit. The pore volume is then calculated from the densities of the bulk and matrix materials, respectively. In contrast to nitrogen sorption and mercury porosimetry, this method is capable of measuring the total pore volume regardless of pore size distribution and pore connectivity. In this study, three porous samples were investigated by AFM-porosimetry: one ordered mesoporous sample and two disordered foam structures. All samples were based on a matrix of amorphous silica templated by a block copolymer, Pluronic F127, swollen to various degrees with poly(propylene glycol). In addition, the density of silica spheres without a template was measured by two independent techniques: AFM and the Archimedes principle.