AS 4784:2003 Sample preparation for particle size analysis––Dispersing procedures for powders in liquids
8.2 General principles
Subclauses 8.2 to 8.4 explain the principles used in developing the decision charts in 8.5. Complete dispersion of a powder in a liquid occurs when the individual particles that made up the original clumps have become separated, move independently of each other, and remain separated from one another. This requires that there be no attractive force between the particles as they approach one another. If there is an attraction then the solid/dispersion will exhibit non-Newtonian flow and have a yield stress (i.e. the dispersion will be able to support a finite shear stress without any flow occurring.) Most of the indirect tests of dispersion rely on this effect. For example, a dispersion with a yield stress enables settling particles to form an open structure which does not collapse under the force of gravity. Such a dispersion will settle to form a higher sedimentation volume (lower sediment density) than a completely dispersed system would.
Highly anisotropic particles form a more or less rigid gel at very low concentrations of solids when there is a net attractive force between the particles.
8.3.1 Introduction
Particles which bear a surface charge will repel each other if the electrostatic repulsion is larger than the polarizability attraction (also called the Hamaker or Van der Waals attraction). A surface charge corresponding to a zeta potential greater than 30 mV is generally sufficient to provide a stable dispersion. Charge stabilization is the best way to stabilize dispersions in which the liquid has a relative dielectric permittivity greater than 30 (methanol at room temperature has a relative dielectric permittivity of 33, water of about 80) and an ionic strength less than 0,1 mol/l (i.e. a low concentration of ions in solution).
8.3.2 Surface ionization
The charge on the particle may arise from ionization of surface groups (influenced by the pH of the solution). For example, surface amine groups will adsorb a hydrogen ion from solution and become positively charged if the pH is below the pK b for the powder. Surface carboxyl groups will lose a hydrogen ion and become negatively charged if the pH is above the pK a for the powder. Amphoteric surface groups, such as the OH groups found on a metal oxide or hydroxide, will adsorb a hydrogen ion and become positive if the pH is below the pH iso for that oxide and will lose a hydrogen ion and become negative if the pH is above the pH iso.