Colloidal sols can be formed by dispersion methods (e.g. by mechanical subdivision of larger particles or by dissolution in the case of lyophilic sols) or by condensation methods (from supersaturated solutions or supercooled vapours, or as the product of chemical reactions) or by a combination of these two (e.g. in an electrical discharge).
When a condensation method is applied, molecules (or ions) are deposited on nuclei, which may be of the same chemical species as the colloid (homogeneous nucleation) or different (heterogeneous nucleation).
An aggregate of a small number of atoms, molecules or ions is called an embryo. A critical embryo has that size at which the Gibbs energy at constant pressure and temperature is a maximum. A larger embryo is called a homogeneous nucleus.
A nucleating agent is a material either added to or present in the system, which induces either homogeneous or heterogeneous nucleation.
The rate of nucleation is the number of nuclei formed in unit time per unit volume.
Dialysis is the process of separating a colloidal sol from a colloid-free solution by a membrane permeable to all components of the system except the colloidal ones, and allowing the exchange of the components of small molar mass to proceed for a certain time.
The colloid-free solution obtained at equilibrium in dialysis is called equilibrium dialysate. Its composition approaches that of the dispersion medium (more precisely, the limit to which the composition of the dispersion medium tends at large distances from the particles). In the dialysis equilibrium an osmotic pressure difference exists between sol and equilibrium dialysate.
After (complete or incomplete) dialysis two solutions are obtained. The one free from colloidal material is called dialysate; the other one, containing the colloidal particles may be called retentate, dialysis residue, or simply residue, but should not be called dialysate.
The ultrafiltrate, prepared by ultrafiltration (filtration through a dialysis membrane), is in general not of the same composition as the equilibrium solution.
If dialysis is conducted in the presence of an electric field across the membrane(s) it is called electrodialysis. Electrodialysis may lead to local differences in concentration and density. Under the influence of gravity these density differences lead to large scale separation of sols of high and of low (often vanishingly low) concentrations. This process is called electrodecantation (electrophoresis convection).
Flotation is the removal of matter by entrainment at an interface. In particular, froth flotation is the removal of particulate matter by foaming (frothing).
Foam fractionation is a method of separation in which a component of the bulk liquid is preferentially adsorbed at the L/V interface and is removed by foaming.