Direct measurement of the depletion interaction in a charged colloidal dispersion

Direct measurement of the depletion interaction between electrically charged surfaces has been obtained using the optical technique of total internal reflection microscopy (TIRM). The long-range interaction between a polystyrene sphere and a polished glass plate was measured in aqueous solutions of non-adsorbing colloidal silica particles (all surfaces were negatively charged). The measured potential-energy profiles displayed a repulsive potential at large separations, followed by a depletion attraction at closer distances. At increasing silica concentrations, evidence of a long-range oscillatory behaviour was observed. Comparison of the measured energies to those predicted using a force-balance model indicate that the non-adsorbing silica particles contribute slightly to the screening of the electrostatic interaction between the polystyrene particle and plate. Adjustment for this screening produces good agreement between theory and experiment.