Colloids represent a unique category of mixture, occupying an intermediate space between a true solution and a coarse suspension. Unlike true solutions, where particles are dissolved at the molecular level, or suspensions, where large particles readily settle out, colloids feature particles that remain stably dispersed. The study of these mixtures is formally known as colloidal chemistry. Colloidal systems are pervasive in nature and industry, forming the basis for many everyday materials and biological processes.
Defining Characteristics and Size
The defining feature of any colloid is the size of the dispersed particles. Colloidal particles typically have a diameter ranging from 1 nanometer (nm) to 1000 nm (1 micrometer). This size range places them above the individual molecules found in a true solution (smaller than 1 nm) and below the particles of a suspension (larger than 1000 nm). This intermediate size prevents the particles from quickly settling out due to gravity.
A colloidal system is always composed of two distinct phases: the dispersed phase and the dispersion medium. The dispersed phase is the substance distributed throughout the mixture, while the dispersion medium is the continuous substance in which the particles are spread. For example, in milk, tiny fat globules constitute the dispersed phase, and the surrounding water serves as the dispersion medium. The large surface area relative to the volume in this size range governs many of the colloid’s unique properties.
How Colloids Are Classified
Colloids are systematically classified based on the physical state—solid, liquid, or gas—of both the dispersed phase and the dispersion medium. Since there are three possible states for each of the two components, eight possible combinations exist. The ninth combination, a gas dispersed in a gas, always results in a homogeneous mixture, which is a true solution, not a colloid.
The classification system assigns specific names to these combinations, indicating the nature of the colloid. The major types include:
- A Sol describes a system where a solid is dispersed in a liquid, such as pigment particles in paint.
- An Emulsion results when a liquid is dispersed in another liquid, exemplified by homogenized salad dressing.
- Aerosols have a gas dispersion medium, with the dispersed phase being either a liquid (fog) or a solid (smoke).
- Foams occur when a gas is dispersed in either a liquid (whipped cream) or a solid (styrofoam).
- A Gel is a unique structure where a liquid is dispersed throughout a solid medium, creating a semi-rigid structure like jelly or gelatin.
Unique Observable Physical Properties
Colloids exhibit distinct physical behaviors that allow them to be identified and distinguished from true solutions. One of the most famous phenomena is the Tyndall Effect, which is the scattering of light as a beam passes through the colloidal system. This occurs because the colloidal particles are large enough to deflect light, making the path of the beam visible, a phenomenon not observed in true solutions.
The second defining behavior is Brownian Motion, the erratic, random movement of the dispersed particles. This movement is a consequence of the constant, unbalanced bombardment by the much smaller molecules of the dispersion medium. This continuous kinetic energy transfer prevents the larger colloidal particles from settling out, contributing directly to the mixture’s stability.
Common Colloids in Everyday Life
Colloids are integral to many familiar products and natural occurrences. Milk is a common example of an emulsion, consisting of tiny droplets of liquid fat dispersed uniformly throughout a water-based medium.
Another easily observed colloid is fog, where microscopic water droplets are dispersed in air. Paint is a sol, where solid pigment particles are held in suspension within a liquid medium. Gelatin and jelly are examples of gels, defined by a liquid phase dispersed throughout a solid matrix, providing their semi-solid texture.