Ian Morrison, Cabot Corporation
Table of Contents
Abstract 1. Classification 2. Methods to Produce Dispersions 3. Chemical Processing Aids 4. Dispersed Particles 5. Flow of Dispersions 6. Stability of Dispersions 7. Model Dispesions 8. Bibliography.
A dispersion is a mixture of particles suspended in a liquid. Examples are blood, ceramics, concrete, grease, inks, paints, paper coatings, pesticides and photographic “emulsions”. The descriptor, dispersion, is sometimes used broadly to include mixtures of liquids in liquids (emulsions), gases in liquids (foams), powders in gases (dusts), and liquids in gases (aerosols). A dispersion generally has a large contact area between the two phases. This interfacial area is a region of high chemical and physical activity.
Special equipment is necessary to make dispersions. High speed stirrers are used to combine dry powders with liquids, to mix two liquids, or to make a foam. High shear mills such as colloid mills and homogenizers produce finer dispersions. The finest dispersions are produced with media mills.
Special chemicals, called surfactants, are needed to make dispersions. Examples, are dishwasher soap, engine oil additives, hydrogenated vegetable oil, laundry detergent, and shampoo. Surfactants prevent aggregation, flocculation, or coalescence. They have specialized chemical structures. One part of the surfactant is highly interactive with the particle and another part is highly interactive with the liquid.
Dispersed particles can be characterized by size, by electric charge, and by their interparticle forces. Size can be measured by observation, single particle detection, light scattering, and acoustic properties. Interparticle forces can be studied by rheological measurements.
Dispersed particles are small enough to collide by Brownian
motion. If the particles stick when they collide, the dispersion is unstable.
If the particles bounce apart when they collide, the dispersion is stable.
Theories that account for the effect of particle-particle attraction,
electrostatic repulsion, and polymer repulsion explain many observations.
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