A colloid is any substance which is dispersed throughout another substance very evenly, to the point of even distribution on the microscopic level. In order to be distributed in this way, the colloidal mixture has to be broken down into very small particles, called colloidal particles, which are too small to be directly seen by a conventional microscope. There are both biological and man-made examples of colloids which are evenly distributed throughout another substance in the world, such as within milk, fog, smoke and pears, or aerosol sprays, marshmallows, styrofoam, and shaving lather.
A colloid is prepared by reducing large particles to colloidal size (generally between 1 nanometer and 1 micrometer), or increasing small particles (usually single molecules) to the size of colloidal particles. There are a wide variety of techniques in practice, and the type of method used depends on what form state of colloid is desired. Aerosols are produced by using a jet of pressured gas to tear away liquid from the mixture, sometimes facilitated by the ionization of the liquid, then using the repulsion between the similarly ionized liquid to separate into easily removed droplets. Emulsions are prepared by vigorously shaking the two constituent liquids together, sometimes with the use of surfactants like soap in order to help emulsify and stabilize the product formed here.
Semi-solid colloids are what are known as gels, sometimes formed by taking lycophilic (attracting solvents) sols (a stable dispersed mixture of a solid and a liquid) and cooling them, such that their large linear molecules and their natural viscosity cause the solution to disperse. Colloids can be purified through dialysis, which is the process of removing any ionic material that may have accompanied the colloid during its formation. In order to carry out this dialysis, a membrane that allows ions and solvents to pass through, but which stops colloids is used, exhibiting the concepts of diffusion, osmosis, and ultrafiltration.
Colloids have a number of unique properties. Colloids have strong absorption qualities, caused by the large exposed surface area when colloidal particles are finely divided. Colloidal particles can carry an electrical charge, resulting in attraction or repulsion among itself, known as electrostatic interaction.
Stable colloidal systems are simply colloids with the capability to remain in the colloidal state. Unstable colloidal dispersion results in aggregation, where the dispersed substance builds up, and becomes unevenly distributed. Aggregation can be avoided through the use of electrostatic stabilization, by which the colloidal substance is induced with a like electrical charge throughout, resulting in repulsion between all dispersed particles.