L’État colloïdal

A colloidal system consists of microscopic particles, typically 1 to 1000 nanometers in size, dispersed in a medium called the dispersion medium. Unlike filterable precipitates that settle quickly, colloidal particles remain suspended due to their small size. For example, when silver nitrate and sodium chloride solutions mix, silver chloride forms tiny crystals that create a colloidal system rather than settling as a visible precipitate.

Colloidal systems are classified by their dispersion medium. Sols contain particles in liquid or solid media; aerosols have gas media and include fog and smoke. Foams contain gas bubbles in liquid or solid media, like soap bubbles or pumice stone. Emulsions consist of immiscible liquids, while colloidal suspensions have solid particles in liquid. Each type exhibits distinct properties based on its composition.

Lyophilic colloids are thermodynamically stable because the dispersed phase, often macromolecules like starch or gelatin, has strong affinity for the dispersion medium through hydrogen bonding, van der Waals forces, or electrostatic interactions. They form spontaneously from bulk material. Lyophobic colloids lack these favorable interactions and are unstable, requiring stabilization through adsorbed ions or polymers to prevent aggregation.

Lyophilic colloid formation depends on solubility, concentration, temperature, pH, and solvent nature. A sufficient concentration must exceed the threshold for colloid formation. Temperature affects molecular mobility and solubility, aiding dispersion. pH and ionic strength influence ionization and electrostatic stability. Polar solvents like water, with high dielectric constants, promote stable lyophilic colloid formation through favorable solvent-solute interactions.

Colloidal particles smaller than 1000 angstroms remain suspended due to thermal convection currents and random molecular collisions, which counteract gravitational settling. This process prevents sedimentation, the gradual settling of particles due to gravity. Larger particles may eventually settle over time, but true colloidal particles maintain suspension through continuous random motion and thermal energy interactions within the dispersion medium.

Emulsions are colloidal suspensions where two immiscible liquids form a stable dispersion. They are created by vigorously shaking the immiscible liquids in the presence of an emulsifying agent, which stabilizes the system. The emulsifying agent reduces interfacial tension and prevents the liquids from separating. This allows one liquid to remain dispersed as tiny droplets throughout the other, creating a stable colloidal emulsion.

Aerosols are colloidal systems with particles dispersed in gas, including those from burning sulfur-containing fuels and volcanic eruptions. These produce aqueous sulfuric acid and ammonium sulfate droplets in Earth's atmosphere. Sulfate aerosols contribute to acid rain formation and reflect sunlight, cooling the Earth. Understanding aerosol behavior is essential for studying atmospheric chemistry and climate effects.