colloid

Colloid refers to a substance in which microscopically dispersed particles are suspended in a continuous medium. Particle size ranges roughly from 1 nanometer to 1000 nanometers. The dispersed phase consists of substances that do not form a true solution, while the continuous phase is the surrounding medium. Common colloidal systems include sols (solid in liquid), gels (three-dimensional networks that immobilize liquid), emulsions (liquid droplets in another liquid), foams (gas in a liquid or solid), and aerosols (solid or liquid in a gas). Particles are larger than those in true solutions but too small to be seen with the naked eye under normal conditions; they resist rapid settling.

Key properties include the Tyndall effect, the scattering of light by colloidal particles, and Brownian motion, which helps keep particles suspended. Colloids can be stabilized by electrostatic repulsion or by steric hindrance from adsorbed polymers; destabilization can occur by coagulation, flocculation, or depletion under certain conditions.

Preparation methods include dispersion (breaking down aggregates) and condensation (formation of colloidal particles from dissolved species). Common examples: milk (fat droplets in water), gelatin, ink pigments in water, paint suspensions, and smoke or fog.

Historically, the term was coined by Thomas Graham in the 19th century, and the Tyndall effect is named after John Tyndall who studied light scattering by colloids. Colloids are important in food, medicine, cosmetics, materials science, and environmental chemistry.