Obstructed diffusion in silica colloidal crystals.

The hindered diffusion in silica colloidal crystals was studied experimentally, both by fluorescence recovery after photobleaching and by measurement of ionic conductivity. Particle size was varied to include 120, 220, 470, and 1300 nm, and the porosities were determined by flow measurements. For fluorescein, the results showed that the obstruction factor, which is the ratio of the diffusion coefficients inside the media and in open solution, is equal to the porosity within experimental error. For proteins, the same conclusion is made after correction for size exclusion of the pores. The obstruction factors for these media are 2-fold lower than those measured for chromatographic media, 60% higher than theoretical predictions, and equal to what is assumed for electrophoretic sieving in random fibers.