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.
Key Takeaways: Colloids
- A colloid is a mixture where particles between 1 nanometer and 1 micrometer are dispersed in another medium.
- Colloids do not settle on standing and often exhibit the Tyndall effect, scattering light.
- The dispersed phase is the substance present as small particles; the dispersion medium is the substance that surrounds those particles.
- Common types of colloids include foams, aerosols, gels, emulsions, and sols.
- Colloids differ from solutions (smaller particles) and suspensions (larger particles that settle).
- Real-world examples include milk, fog, whipped cream, butter, paint, and smoke.
- Colloids are essential in food, medicine, cosmetics, environmental science, and industry.
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 solid particles in a liquid 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, polystyrene foam, marshmallow) | gel (gelatin, agar, jelly, butter) | solid sol (cranberry glass, uranium glass, colored gems) |
Difference Between a Sol and a Gel
A sol and a gel are both types of colloids, but they differ in the physical states of the dispersed phase and the dispersion medium.
| Property | Sol | Gel |
|---|---|---|
| Dispersed Phase | Solid | Solid |
| Dispersion Medium | Liquid | Liquid |
| Texture | Fluid-like, pourable | Semi-solid, jelly-like |
| Examples | Ink, paint, muddy water | Gelatin, agar, hair gel |
- Sol: A colloid where solid particles are dispersed in a liquid. It flows easily and resembles a liquid.
- Gel: A colloid where a liquid disperses solid particles in a network structure that resists flow, forming a semi-rigid material.
Properties of Colloids
Colloids exhibit a range of physical and chemical properties that distinguish them from other mixtures:
- Particle Size: Between 1 nanometer (nm) and 1 micrometer (μm).
- Heterogeneous Nature: Microscopically heterogeneous, but often appears uniform to the naked eye.
- Brownian Motion: Particles exhibit random motion due to collisions with molecules of the dispersion medium.
- Tyndall Effect: Colloids scatter light, making a beam of light visible in the mixture.
- Kinetic Stability: Particles do not settle over time due to their small size and continuous motion.
- Cannot Be Filtered Easily: Standard filtration techniques do not remove colloidal particles.
- Electrostatic Stabilization: Many colloids carry surface charges that prevent particle aggregation.
- Adsorption: Colloidal particles readily adsorb ions or molecules onto their surfaces.
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.
Real-World Applications of Colloids
Colloids are present in numerous aspects of daily life, industry, and scientific research. Their unique properties, such as stability, light scattering, and particle size, make them essential in various fields.
Chemistry and Industry
- Paints and inks use colloidal suspensions of pigments to maintain uniformity, improve flow, and prevent settling.
- Lubricants often incorporate colloidal particles to reduce wear and friction between surfaces.
- Detergents and soaps form micelles, which are colloidal structures that help solubilize oils and grease.
Food Science
- Milk is a classic emulsion of fat droplets dispersed in water, stabilized by proteins.
- Mayonnaise is another emulsion, typically stabilized by lecithin in egg yolk.
- Whipped cream and meringue are foams, containing gas bubbles dispersed in a liquid.
- Gelatin desserts are gels formed by solid particles (proteins) dispersed in water.
Medicine and Pharmaceuticals
- Colloidal silver has been used as an antimicrobial agent, though its use is controversial today.
- Nanocolloids are used in targeted drug delivery systems to improve absorption and bioavailability.
- Plasma expanders, such as dextran solutions, are colloids used to maintain blood volume during surgery or trauma.
Environmental Science
- Fog, mist, and smoke are natural aerosols, each representing colloids with liquid or solid particles dispersed in a gas.
- Colloidal clays in soil affect water retention and influence the movement of nutrients and pollutants.
- Wastewater treatment often involves destabilizing colloidal particles to promote settling or filtration.
Cosmetics and Personal Care
- Lotions, creams, and conditioners are emulsions or gels, formulated for consistency, texture, and delivery of active ingredients.
- Toothpaste is a colloidal gel, designed to stay on the brush while delivering abrasive and antibacterial agents.
- Deodorants and sunscreens often contain colloidal suspensions for even distribution and absorption.
Colloids are also central to research in nanotechnology, materials science, and biochemistry, where they serve as models for studying surface phenomena and transport processes.
Frequently Asked Questions (FAQs)
Q: What is a colloid in simple terms?
A colloid is a mixture where tiny particles are evenly spread throughout another substance and don’t settle over time.
Q: Is milk a colloid?
Yes. Milk is an emulsion—fat droplets are dispersed in water with proteins acting as emulsifiers.
Q: How do you tell if something is a colloid?
Shine a flashlight through it. If you see the beam scatter (Tyndall effect), it’s likely a colloid.
Q: Can you filter colloids?
Not with ordinary filters. The particles are too small and pass through standard filtration membranes.
Q: Is blood a colloid?
Yes. Blood contains cells and proteins suspended in plasma, making it a complex colloid.
Q: Are all colloids liquids?
No. Colloids are solids, liquids, or gases as either the dispersed phase or the dispersion medium.
Glossary of Terms
| Term | Definition |
|---|---|
| Colloid | A mixture where particles between 1 nm and 1 μm are dispersed in a medium and do not settle. |
| Dispersed Phase | The substance present as microscopic particles in a colloid. |
| Dispersion Medium | The substance that surrounds and holds the dispersed phase. |
| Sol | A colloid with solid particles in a liquid medium. |
| Gel | A colloid with a liquid medium and a network of solid particles, forming a semi-solid. |
| Emulsion | A colloid formed by mixing two immiscible liquids. |
| Aerosol | A colloid of solid or liquid particles dispersed in a gas. |
| Foam | A colloid where gas is dispersed in a liquid or solid. |
| Tyndall Effect | Scattering of light by colloidal particles. |
| Brownian Motion | Random motion of particles caused by collisions with molecules of the dispersion medium. |
| Lyophilic | A colloid that forms readily due to attraction between particles and medium. |
| Lyophobic | A colloid that does not form easily and needs stabilizers or special conditions. |
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