Dispersed Systems and Colloids

Dispersed systems across scales

A dispersed system contains a dispersed phase distributed through a dispersion medium. Classification names both physical states: aerosols have a liquid or solid in gas; foams have gas in liquid or solid; emulsions have liquid in liquid; sols have solid in liquid; gels contain a liquid held in a solid-like network.

Particle size and observation

True solutions contain molecular or ionic species, commonly below about 1 nm. Colloidal particles are roughly 1–1000 nm; coarse dispersions are larger. Boundaries are approximate and depend on method. Colloids show Brownian motion and often scatter light, producing the Tyndall effect; a true solution has no clearly visible light path under the same conditions.

Stability

Particles may remain separated because of electrostatic repulsion, adsorbed surfactants or steric layers. Changing ionic strength, pH or temperature can reduce stability in some systems and cause flocculation or coagulation. Effects are system-specific: added electrolyte does not destabilize every dispersion equally.

Preparation and separation

Mechanical mixing can disperse phases but does not guarantee long-term stability. Surfactants lower interfacial tension and may form protective layers. Coarse suspensions may be filtered; centrifugation separates some dispersions by density; dialysis lets small solutes cross a semipermeable membrane while retaining colloidal particles. Ordinary filter paper usually does not retain a stable colloid.