What are colloids?

A colloid is a microheterogeneous system in which a component has dimensions in the range of 1 nanometer (10-9 m) to 1 micron (10-6 m). This makes the ratio of area to volume (A/V) exceptionally large, so they have a lot of surface energy which confers instabilities and therefore tend to aggregate.

They consist of microscopically dispersed insoluble particles suspended in another substance. They have been known since ancient times and have peculiar properties and are used in everyday life. The name colloid comes from the Greek root colaire meaning "that can stick together".

The term colloidal suspension refers to the total mixture. A colloid is characterized by a dispersed phase (the suspended particles) and a continuous phase (the suspension medium).

Although some definitions specify that the particles must be dispersed in a liquid, others extend it to include substances such as aerosols and gels.

Examples of colloids

Numerous examples of colloidal systems can be found in nature: blood, milk, latex, etc.

example beer foam colloid
An example of a colloid is beer foam where gas particles (bubbles) are dispersed in a liquid medium.

Other examples include: inks, cement, paints, dyes, foams, plastics, rubbers, soil.

Colloidal systems are used in a large number of technological applications:

  • Food (mayonnaise, sauces, ice cream, creams, functional foods, etc.).
  • Adhesion (ultra-glues, adhesives for dental implants, biomaterials, hydrophobic suits, etc.).
  • Building (asphalts, insulating materials, adhesives, shock absorbers).
  • Biomedicine (diagnostic tests).
  • Pharmaceuticals (new drug transport and release systems), etc.

Classification of colloids

Three general classifications are recognized:

  1. Colloidal dispersions. Thermodynamically unstable systems with high surface free energy that cannot be reconstituted after phase separation (see emulsion, lyophobic colloid, sol).
  2. Solution of macromolecular materials (see polymerization, polymers).
  3. Association colloids or electrolyte colloids that form micelles. Colloids are formed by the degradation of bulk matter or the aggregation of molecules and ions.

Properties of colloids

Colloids cannot be filtered, they are separated by dialysis. Some colloids are translucent due to the Tyndall effect, which is the scattering of light by the colloid particles. Other colloids may be opaque or have a slight color.

Colloidal suspensions are the subject of interface and colloid science. This field of study was introduced in 1845 by the Italian chemist Francesco Selmi and further investigated since 1861 by the Scottish scientist Thomas Graham.