12.1: The Colloidal State

The formation of a colloidal system is exemplified by an aqueous solution containing Cl⁻ ions is introduced to another containing Ag⁺ ions, resulting in the precipitation of solid AgCl as extremely tiny crystals. Instead of settling out as a filterable precipitate, these crystals remain suspended in the liquid, showcasing a colloidal system.

A colloidal system involves colloidal particles within the approximate range of 1 to 1000 nm in at least one dimension, dispersed in a medium called the dispersion medium or continuous phase. The term "colloid" can refer to either the entire system, including particles and dispersion medium, or specifically to the colloidal particles.

A specific type of colloidal system is a sol, which consists of particles dispersed in a liquid or gas. When the dispersion medium is a gas, the sol is termed an aerosol. Examples include fog, an aerosol with liquid particles, and smoke, an aerosol with liquid or solid particles. The Earth's atmosphere contains an aerosol resulting from the burning of sulfur-containing fuels and volcanic eruptions, producing aqueous H₂SO₄ and (NH₄)₂SO₄ droplets. This sulfate aerosol contributes to acid rain and reflects sunlight, cooling the Earth.

A sol can also consist of solid particles suspended in a liquid, forming a colloidal suspension. Gold nanoparticle sols, which have been studied for applications like drug delivery, represent another example. Foams, colloidal systems where gas bubbles are dispersed in a liquid or solid, are familiar in soap, beer, and at the beach. Pumice stone, a foam with air bubbles dispersed in volcanic rock, illustrates the diversity of colloidal systems.

Lyophilic colloidal systems, thermodynamically stable, form spontaneously from the dry bulk material of colloidal particles in the dispersion medium. The formation of lyophilic colloids also depends on factors such as solubility, concentration, temperature, pH, and the nature of the solvent. These colloids form when the dispersed phase, typically macromolecules like starch or gelatin, has a strong affinity for the dispersion medium, often due to hydrogen bonding, van der Waals forces, or electrostatic interactions. A sufficient concentration of the substance is required to exceed the threshold needed for colloid formation. Temperature influences both solubility and molecular mobility, aiding dispersion, while pH and ionic strength affect ionization and electrostatic stability. The solvent’s polarity and dielectric constant also play crucial roles, with polar solvents like water promoting the formation of stable lyophilic colloids.

Lyophobic colloids, on the other hand, are thermodynamically unstable and may require stabilization through factors such as adsorbed ions or the presence of polymers. Lyophobic colloids can be prepared through precipitation reactions or the mechanical breakdown of bulk substances. For instance, emulsions are colloidal suspensions in which the suspension medium and the colloid are immiscible liquids. They can be formed by vigorously shaking two immiscible liquids in the presence of an emulsifying agent.

In non-colloidal suspensions, solid particles will eventually settle down due to gravity, a process known as sedimentation. However, in colloidal suspensions with particles smaller than 10³ Å, thermal convection currents and random molecular collisions prevent sedimentation, although larger particles may still settle over time.