Colloidal suspension behaviour is strongly influenced by the particle – particle interaction forces operative in the system. In aqueous systems, suspension stability can be easily altered either by the addition of electrolyte or by changes in the pH. Such changes result in changes to either the net surface charge of the particles or the thickness of the diffuse electrical double layer around the particles. If the charge or the double layer is sufficiently reduced, the ubiquitous attractive van der Waals interaction forces can dominate and the dispersion is destabilised.

QCM-D is used to determine the stability of particle suspensions in different solution conditions. Mono-dispersed silica suspensions were prepared for investigation, at various pH and KNO3 electrolyte concentrations. The conditions used here were chosen on the basis of measurements of the particle zeta potentials as a function of pH at various salt concentrations. This study shows that by varying the suspension stability, the frequency shift of the resonating crystal, recorded as the energy input into the QCM cell is increased, is significantly greater for an un-stable silica suspension compared to a stable suspension.

Keywords: Quartz Crystal Microbalance, Colloid Stability