Separation of colloidal particles in a packed column using depletion and structural forces

The depletion and structural forces produced by nonadsorbed nanoparticles were used to separate an equinumber binary dispersion of 1.5 and 0.82 μm polystyrene sulfate (PS) particles in a flow-through packed bed. Experiments consisted of injecting a pulse of the binary dispersion into a carrier fluid at the inlet of a cylindrical column packed with 0.5 mm silica collector beads. The carrier fluids used were either an aqueous electrolyte solution or a dispersion of silica nanoparticles. When the carrier fluid was a dispersion of silica nanoparticles, the ratio of PS particles in the column outlet would change from 1:1 large-to-small particles to slightly over 2:1, implying that more of the smaller, 0.82 μm particles were being trapped on the surface of the collector beads due to depletion forces. Experiments with a single particle size (either 1.5 or 0.82 μm PS particles only) were also performed and the results were consistent with those obtained with the binary mixture. Equilibrium potential energy profiles between a PS particle and a flat silica plate were calculated using the equilibrium force-balance model of Walz and Sharma [J. Colloid Interface Sci. 168 (1994) 485]. The long-range, secondary energy barrier (arising from structuring of the nanoparticles in the gap region) for the 1.5 μm particles was two times greater than that for the 0.82 μm particles, meaning that the 0.82 μm particles were far more likely to overcome the energy barrier and get trapped on the surface of the collector beads. Although not explored, separation of particles based on other properties, such as surface charge density, should also be possible with this approach, as the depletion and structural forces are sensitive to such properties.