In chemistry, a colloid is a mixture of tiny particles that are dispersed in another medium. The particles are microscopic in size, ranging from 1 nanometer (nm) to 1 micrometer (μm) in diameter. In contrast, particles in a solution are smaller than this size, while particles in a suspension are larger. As in a solution, the particles in a colloid do not separate upon standing. The particles in a colloid are called the dispersed phase, which is spread throughout the dispersion medium.
Types and Examples of Colloids
Colloids are classified as foams, aerosols, emulsions, gels, or sols, depending on the nature of the dispersed phase and dispersion medium. Familiar examples of colloids include mayonnaise, milk, fog, smoke, and gelatin.
- A gel is a colloid of solid particles in a liquid medium.
- A sol consists of liquid particles in a solid medium.
- An emulsion is a colloid formed by two or more liquids.
- A foam forms by gas particles trapped within a liquid or solid.
- An aerosol is a colloid consisting of liquid or solid particles dispersed in a gas.
- There are no known gas-gas colloids, although it is possible helium or xenon may be insoluble in certain situations.
| Dispersion Medium | Gas Dispersed Phase | Liquid Dispersed Phase | Solid Dispersed Phase |
|---|---|---|---|
| Gas | none known | liquid aerosol (mist, fog, hair spray, steam) | solid aerosol (smoke, ice cloud) |
| Liquid | foam (shaving cream, whipped cream) | emulsion (milk, mayonnaise, hand lotion) | sol (ink, paint, precipitates) |
| Solid | solid foam (aerogel, pumice, styrofoam, marshmallow) | gel (gelatin, agar, jelly, butter) | solid sol (cranberry glass, uranium glass, colored gems) |
The Tyndall Effect
The Tyndall effect is the scattering of light by the particles in a colloid or fine suspension. A good example is the way a glass of skim milk (a colloid) shows a flashlight beam, while a glass of salt water (a solution) does not. It is a quick and easy test that distinguishes a colloid or suspension from a solution.
Not all colloids display the Tyndall effect. Sometimes the dispersion medium is opaque or too dark. For example, you don’t see the Tyndall effect in whipped cream. However, it is evident in gelatin, opal, mist, smoke, milk, and aerogel.
Difference Between a Colloid and a Suspension
The particles in a suspension are larger than in a colloid. So, the particles in a suspension typically settle out of their medium, while those in a colloid remain mixed and appear homogeneous (under a microscope, they are heterogeneous). A good example of a suspension is a mixture of flour and water. The flour particles are suspended after freshly mixing the ingredients, but gravity pulls them to the bottom of the container pretty quickly.
Difference Between a Colloid and a Solution
The particle size in a solution is smaller than in a colloid,. Also, the solute and solvent constitute one phase of matter in a solution. For example, a solution of table salt in water or sugar in water consists solely of the liquid phase. The salt breaks into component ions, while the sugar dissolves into individual molecules. In either case, the particles are in aqueous solution. In contrast, the particles in a sol are not necessarily the same phase as the medium. For example, milk contains solid protein particles dispersed in the liquid.
| Solution | Colloid | Suspension |
|---|---|---|
| homogeneous | visually homogeneous, microscopically heterogeneous | heterogeneous |
| particle size 0.01-1 nm atoms, ions, molecules | particle size 1-1000 nm molecules or aggregates | particle size >1000 nm large particles or aggregates |
| do no separate on standing | do not separate on standing | particles settle out |
| cannot be separated by filtration | cannot be separated by filtration | can be separated by filtration |
| does not scatter light | Tyndall effect or opaque | Tyndall effect or opaque |
How to Prepare a Colloid
There are two methods of preparing colloids:
- Mechanical action, such as shaking, spraying, or milling, disperses particles or droplets into the medium.
- Small molecules aggregate into colloidal particles, via condensation, precipitation, or redox reactions.
References
- Berg, J.C. (2010). An Introduction to Interfaces and Colloids: The Bridge to Nanoscience. World Scientific Publishing Co. ISBN 981-4293-07-5.
- Everett, D. H. (1988). Basic Principles of Colloid Science. London: Royal Society of Chemistry. ISBN 978-1-84755-020-0.
- Hiltner, P.A.; Krieger, I.M. (1969). “Diffraction of light by ordered suspensions”. J. Phys. Chem. 73 (7): 2306. doi:10.1021/j100727a049
- Levine, Ira N. (2001). Physical Chemistry (5th ed.). Boston: McGraw-Hill. ISBN 978-0-07-231808-1.
- Stepto, Robert F. T. (2009). “Dispersity in polymer science (IUPAC Recommendations 2009)”. Pure and Applied Chemistry. 81 (2): 351–353. doi:10.1351/PAC-REC-08-05-02