Summary
Highlights
Lyophilic colloids, common in living systems, are stable due to a strong attraction between the dispersed phase and the dispersing medium (e.g., egg white protein molecules attracting water). Lyophobic colloids, however, are less stable and require a third substance (an emulsifying agent) for stabilization, such as egg yolk protein in mayonnaise.
Colloids are special heterogeneous mixtures that appear homogeneous to the naked eye. They are composed of a dispersed phase (single large molecules or clusters of small molecules) evenly distributed in a continuous phase (solid, liquid, or gas). Their particles range from 1 to 1000 nanometers. Examples include milk, gelatin, creams, and even clouds.
Colloids exhibit the Tyndall effect, scattering light due to the size of their particles, which approach the wavelengths of white light (400 to 700 nm). This effect explains why the sky is blue and the vibrant colors seen during sunsets, as colloidal dust and atmospheric gases scatter blue light more effectively.
The Tyndall effect is also responsible for diverse phenomena like visible car headlights in fog, the color of human eyes (due to colloidal particles in the iris), and the brilliant colors of gems and stained glass. In laboratories, light scattering helps determine the size of polymer molecules.
Due to their fine division, colloids have a large surface area to volume ratio, enabling them to adsorb substances on their surfaces. This property is useful in cleaning up oil spills (dispersing oil for faster evaporation) and in catalysis, where colloidal metals like sponge nickel or palladium speed up chemical reactions due to their adsorptive capabilities.
Activated carbon, an excellent adsorbent, is used in gas masks and as a decolorizing agent in various industrial processes. Alum is used to purify water by adsorbing suspended particles and impurities, causing them to settle out.
Colloids are mixtures with particle sizes between 1 and 1000 nm, exhibiting the Tyndall effect and a high surface-to-volume ratio, facilitating adsorption. They can be lyophilic (stable due to affinity with the medium) or lyophobic (less stable). Common forms include sols (solid in liquid), foams (gas in liquid), solid foams (gas in solid), emulsions (liquid in liquid), solid aerosols (solid in gas), and liquid aerosols (liquid in gas).
Brownian movement, the erratic motion of dispersed particles in lyophobic colloids, contributes to their stability by keeping them suspended. This phenomenon provides visual evidence of the constant, random collisions of molecules as described by the kinetic molecular theory.
Detergents act as emulsifying agents, forming micelles around oil and dirt particles, allowing them to be washed away with water. Emulsification is also vital in the human body, where bile emulsifies food to aid digestion by increasing the surface area for digestive juices.