Dispersion Stability of Mixed Microparticle Systems Studied by Their Morphological Observations and Interacting Surface Force Measurements

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To understand the key stability parameters of mixed microparticle dispersion systems, the interaction forces between model particle surfaces in solution were measured by using a technique of atomic force microscopy in connection with the in-situ observation of dispersed particle morphology. Prior to study those of the mixed systems, the adsorption role of polyelectrolyte and surfactant in silica surfaces were studied by electrokinetic potential and surface force measurements. A cationic polyelectrolyte strongly adsorbs on negatively charged silica over a wide range of pH.At a high concentration of the polyelectrolyte the silica reversed the charge from negative to positive. The positively charged silica modified with the polyelectrolyte in water permits the strong binding of surfactant counterions, which led to the charge sign from positive to negative. In ethanol, the same silica surface modified with the polyelectrolyte does not show any charge reversal in the presence of the same surfactant counterions. These results suggested that the mixed particles in water are stabilized by electrostatic forces, when their surfaces are modified with polyelectrolytes to give a high surface charge. In ethanol, surfactant ions can be used as supporting electrolyte to adjust the electrostatic forces.In the mixed dispersion systems with asymmetrical surfaces in the co-presence of ionic surfactant micelles in water, the surface forces revealed a periodic, steric force at small surface separations in addition to an ordinary electrostatic force. The steric force may be utilized for the stabilization in the mixture of micro-particle dispersion.