Gathering of charged colloidal particles near a like-charged glass plate

The distribution of negatively charged colloidal particles near a like-charged glass plate was investigated using a confocal laser scanning microscope and video imagery. It was found that the number of particles about 5 mu m from the glass surface was higher than that inside the dispersion and decreased gradually with distance. At shorter distances, such as 1 mu m, there were no particles due to an electrostatic repulsion between the glass plate and the particles. The ''condensation'' near the glass surface was observed at low ionic strengths in the dispersion, such as NaCl concentrations of 10(-5) M or lower, and the peak in the particle distribution profile as a function of distance from the glass surface disappeared at 10(-4) M. The peak height increased with the number of surface charges on the particles and with the surface xi-potential of the plate. The greater the number of charges on the plate and particles, the more pronounced became the condensation. The dependencies of the peak height indicated that the driving force for the condensation was an electrostatic attractive interaction between the like-charged plate and particles, though such an interaction is not predicted by the conventional DLVO theory.