BROWNIAN AND ELECTROPHORETICAL MOBILITY OF SILICA PARTICLES COATED WITH ADSORBED LAYERS OF POLYETHYLENOXIDES

For two silica sols having mean particle radii of 19 and 69 nm, the Brownian and electrophoretical mobilities were measured using dynamic light scattering. Measurements were made in water solutions of polyethylenoxide of two molecular weights (4.10(4) and 6.10(5)) at different NaCl concentrations and pH values. From the difference in Brownian mobilities of particles with and without an adsorption layer of polymers it was possible to calculate the hydrodynamic thickness of the saturated adsorption layer delta-H. On the basis of this value, electrical potentials PSI-1 of the silica surface under the adsorption layers were determined. It was shown that PSI-1 values are much lower than the corresponding values before adsorption. Decrease in surface charge can be connected with a lowering in dielectric permeability in the dense part of the adsorption layer. Calculations of the free energy of pair interaction of particles covered with saturated adsorption layer using PSI-1 values, and taking into account steric repulsion and molecular forces, are in agreement with stability data. The bridging of particles takes place at low polymer concentrations in the cases where delta-H is larger than the Debye radius.