Bulk aggregation versus surface adsorption of charged colloidal silica spheres on a flat glass surface in water

Surface coatings are used in many applications. An advantage of coating a surface using particles from a dispersion is that a change in, for example, temperature can control the compactness of the surface coverage. In this research the competition between bulk aggregation and surface adsorption of colloidal silica spheres was investigated. The system used contained aqueous dispersions of industrial Ludox silica particles and flat silica surfaces. The attractive force between the particles and between a particle and the surface was varied by adding polyelectrolyte to induce a depletion flocculation. Bulk aggregation was investigated and phase diagrams were established by observing phase separation visually and using dynamic light scattering. Above a certain polymer concentration the systems showed a liquid-liquid or a liquid-solid phase separation. The dynamic light scattering experiments, corrected for changes in viscosity, showed that below the phase boundary no particle aggregates formed. Surface adsorption was monitored using mainly ellipsometry and a quartz crystal microbalance. Surface adsorption occurred at lower polymer concentrations than bulk aggregation. For different Ludox concentrations, the concentration of polymer required to make the particles adsorb at the surface (the adsorption boundary) showed the same trend as the bulk phase boundary. Between the two boundaries an increase of polymer gave an increase in surface adsorption. A raise of the concentration of Ludox particles also increased the adsorption. Below the phase boundary no aggregation was found. In the theory of Wennerstrom and Linse [1] surface adsorption occurs at a lower interaction strength than bulk aggregation. This research is in agreement with the latter theory.