A colloid is a mixture where one substance is finely dispersed as particles throughout another substance. These dispersed particles are much larger than the molecules found in a simple solution, yet they remain too small to be seen with the naked eye. This intermediate size gives colloids distinctive properties, including their remarkable stability. The central question when examining these systems is whether the dispersed matter will eventually succumb to gravity and settle out over time. The answer lies in the complex forces that keep the particles suspended indefinitely.
Differentiating Solutions, Colloids, and Suspensions
Mixtures are classified into three categories based on particle size and their behavior over time. Solutions have the smallest particle size, less than one nanometer (nm). These particles are too small to be affected by gravity or scatter light, meaning the mixture is transparent and never separates.
Suspensions contain large particles, typically greater than 1,000 nm. Due to their significant mass, these particles will visibly settle out of the dispersion medium over time due to gravity. They are unstable mixtures and often appear opaque.
Colloids fall between these two extremes, with particle sizes ranging from 1 nm to 1,000 nm. This intermediate size allows them to appear uniform, but they are large enough to scatter light (the Tyndall effect). Unlike suspensions, colloids are stable, preventing the dispersed phase from settling.
How Colloids Maintain Stability
The long-term stability of a colloid against the continuous pull of gravity is governed by two primary physical mechanisms. The first is kinetic stability, maintained by the constant, random movement of the dispersed particles, known as Brownian motion. This motion is caused by the bombardment of the colloidal particles by the smaller molecules of the surrounding dispersion medium.
The energy transferred during these collisions keeps the colloidal particles in motion. This continuous, erratic movement effectively counteracts the downward force of gravity. The random impulses from the surrounding fluid molecules are sufficient to keep the small particles suspended and prevent settling.
The second mechanism is electrostatic stability, which prevents the individual particles from aggregating into heavier clusters. Colloidal particles often acquire an electric charge on their surface by selectively adsorbing ions from the surrounding medium. Since all dispersed particles carry the same charge, they strongly repel one another when they approach.
This strong repulsive force prevents the particles from getting close enough for attractive van der Waals forces to cause them to stick together. If aggregation occurred, the resulting mass would become large enough for gravity to overcome the kinetic stability provided by Brownian motion.
Methods for Forcing Colloidal Settling
Colloidal stability can be intentionally broken in a process called coagulation or flocculation, allowing the particles to settle. One common method involves adding electrolytes (salts containing charged ions) to neutralize the electrical charge on the surface of the colloidal particles.
Neutralizing the surface charge eliminates the electrostatic repulsion keeping the particles apart. Once the repulsive barrier is removed, van der Waals forces dominate, causing the particles to aggregate into larger clumps, or flocs. Coagulants such as aluminum sulfate or iron salts are frequently used in water treatment.
Another method for inducing settling is increasing the temperature of the colloidal system. Heating the mixture increases the kinetic energy of the particles, leading to more frequent and forceful collisions. This increased energy can overcome repulsive forces, promoting aggregation into a size large enough to settle.
In industrial settings, mechanical force is used to accelerate settling. Centrifugation applies a strong rotational force that mimics a vastly increased gravitational pull. This external force overcomes the stabilizing effects of Brownian motion and electrostatic repulsion, forcing the suspended particles to rapidly sediment.