What is a colloid?

Colloidal Silver (An Analytical - Investigative Report & Theoretical Overview)

(2) What is a colloid?

Colloid n A particle in the size range 10^8 - 10^6 m, or, roughly in the range from molecular size to that visible under high magnification. The definition must be regarded as rather arbitrary. ?colloidal adj. See Graham, Thomas.

Becher, Paul
Dictionary Of Colloid and Surface Science Copyright 1990 MARCEL DECKER, INC.
ISBN 0 ? 8247 ? 8326 ? 3

This definition is the most accurate I have found, because as you will see the term colloidal is used rather arbitrarily to describe the colloidal state of matter. Different substances have different physical attributes in different dispersion mediums, and different size particle clusters of the same element or substance often have completely different attributes to clusters of different sizes, even in the same dispersion medium.

Many things can be considered colloidal, human cells, plant cells, pathogens, clay used as casting slip in ceramics, polymer latex, nanoparticles, emulsions, micro-emulsions, liposomes, paints, inks, cosmetics, self leveling concrete just to name a few generalizations.

Many industries employ colloidal chemistry, such as paper mills, electronic component manufacturing, geology, pharmacology, medicine, food industry, water purification etc.

A generalization is that a colloidal particle ranges from as few as eight atoms in a cluster (a few nanometers) to particles as large as 30 microns.

Normally it is presumed the particles are ultimately only truly considered colloidal when they are in a liquid (dispersion medium). The dispersion medium that the particles are in can be anything from water, oils to organic solvents.

Colloidal particles, when they are in a liquid (dispersion medium), result in an interaction of the particle cluster surface charge, with ions of opposite charge (gegenions or counterions) in the dispersion medium, a double layer of ions are then formed.

Put another way an electrokinetic phenomena is manifest in the form of a charge on the surface of the particle cluster, that then attracts ions of the opposite charge, and pulls them close to the surface of the particle cluster, creating a double layer of oppositely charged ions.

The charge at the surface of the particle may be an inherent attribute of the particles in the cluster, or may result from an interaction between the particles and the dispersion medium.

Depending on the substance considered a colloid, the double layer may be divided into the Stern Layer (close to the particle) and the Gouy ? Chapman layer, which extends into the dispersion medium.

The random repulsion of like charged ions at the surface of the double layer of a particle cluster, combined with random fluctuations in the density of the liquid, cause the particles to be dispersed, spinning around in what is know as ?Brownian Motion? named after the discoverer of the phenomena. Originally observed in 1827 by Mr. Brownian, as a random, erratic oscillation of pollen grains in a liquid. An example could be likened to how dust particles appear when captured in a beam of light from the sun.

This phenomenon is really what is normally meant when the term colloid is used, primarily as a way to identify a sustainable suspended particle in a liquid. Ultimately, the animated attribute, gives colloids a lively quality. Though I could say much more, let this explanation suffice for the moment.

Now with that out of the way, lets move on...

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2009-07-13