to Fluids
to Dispersions
Dr. Dmitri Kopeliovich
The size of dispersed phase particles in Colloids is within the range 1 - 1000 nm (4*10-8 - 4*10-5 inch).
There are two principal ways of preparation of colloids:
Grinding and milling techniques are commonly used for subdivision large solid particles.
The methods are capable to disperse particles to a a size, which is a result of an equilibrium between the two processes: subdivision and aggregation under mechanical force.
Further subdivision may be achieved by the following methods:
Liquid aerosols are prepared by atomizing a liquid with a gas jet.
Emulsions are prepared by mechanical agitation (stirring) of the components (dispersed phase and dispersion media together with a small amount of a surfacant (emulsifier) stabilizing the emulsion.
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The method involves preparation of a supersaturated solution, from which the dispersed particles precipitate as a second phase.
As any other new phase formation (eg. Crystallization) the process consists of two stages:
Nucleation may occur by either homogeneous or heterogeneous mechanism depending on the value of supersaturation of the solution.
Presence of foreign particles or other foreign substance in the solution allows precipitation at minor value of supersaturation. This is heterogeneous nucleation.
If there are no solid particles present higher supersaturation is required in order to form stable nuclei, providing following particle growth (homogeneous nucleation)
Number of stable nuclei per unit volume of the solution determines the dispersed particles size.
The particles grow by two mechanisms: transfer of the dissolved molecules to the new phase and aggregation/coalescence of colliding particles.
The rate of the new phase growth is determined by the following factors:
- Concentration of the solution. Higher concentrations provides more molecules for the growing particles.
- Diffusion coefficient of the dissolved substance. Higher diffusion coefficient results in a faster growth.
- Presence of surfacants in the solution, which depress the particle growth.
- Particles concentration, which determines the frequency of their collisions.
- Viscosity of the dispersion media. Low viscosity promotes both: diffusion of the molecules to the growing particles and diffusion of the particles caused by Brownian motion.
- Mechanical agitation, which is simultaneously disperses the particles and cause them to collide and reunite.
- Temperature. Higher temperature increases the particle growth rate.
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