colloids

Colloids are mixtures in which one substance consists of microscopic particles dispersed within another substance. The dispersed phase typically contains particles ranging from about 1 nanometer to 1000 nanometers, distributed throughout the continuous phase. Colloids can be categorized by the state of the dispersed material, producing sols (solid in liquid), gels (a solid network with liquid), emulsions (liquid droplets in another liquid), foams (gas in liquids or solids), and aerosols (gas containing liquid or solid particles).

Key properties include the inability of the dispersed particles to rapidly sediment under gravity, largely due to Brownian motion. Colloids often exhibit the Tyndall effect, scattering light as it passes through the mixture. Stability depends on interparticle forces; electrostatic stabilization via surface charges or steric stabilization by protective agents can prevent coagulation and flocculation. The zeta potential is commonly used to assess stability.

Colloids differ from true solutions, in which solute particles are at molecular scales and do not scatter light, and from suspensions, where larger particles settle over time. Preparation methods include dispersion (mechanical or ultrasonic mixing of a larger material) and condensation (formation of particles in situ from molecular precursors). Stability considerations are central, with lyophilic (solvent-loving) colloids generally more compatible with the medium than lyophobic types.

Applications span food and beverages, medicine and drug delivery, cosmetics, paints and inks, catalysts, and advanced materials. The term colloid was introduced by Thomas Graham in 1861 to describe gluelike particles that do not behave as true solutions, with light scattering studied by John Tyndall giving rise to the Tyndall effect.