Dispersion in chemistry refers to a type of mixture where small particles of one substance are scattered uniformly throughout another substance. This system is heterogeneous, meaning the two substances do not fully mix at a molecular level, unlike a true solution such as sugar dissolved in water. Understanding how substances disperse is fundamental to many common materials encountered daily, from paint to fog. The study of dispersions allows for the controlled creation of products like mayonnaise, cosmetic creams, and certain industrial inks.
Defining the Components of a Dispersion System
Every dispersion system is composed of two distinct parts: the dispersed phase and the continuous medium. The dispersed phase is the substance broken up into tiny particles or droplets and spread throughout the mixture. This phase is comparable to the solute in a true solution, but its particles are much larger and do not fully dissolve.
The continuous medium surrounds and contains the dispersed phase, analogous to the solvent. For example, in a dispersion of mud in water, the mud particles are the dispersed phase, while the water serves as the continuous medium.
The two phases do not chemically react with each other; instead, mechanical or chemical forces are used to achieve a uniform distribution. The state of matter (solid, liquid, or gas) of both the dispersed phase and the medium is used to classify the specific type of dispersion.
Categorizing Dispersions by Particle Size
The primary factor in classifying a dispersion is the size of the particles in the dispersed phase, which creates a continuum of mixtures.
True Solutions
True solutions have particle sizes less than 1 nanometer (nm), consisting of individual molecules or ions that are invisible and do not separate. A simple saltwater solution is an example, remaining completely transparent.
Colloidal Dispersions (Colloids)
Colloids have particle sizes ranging from 1 nm up to 1000 nm. These particles are too small to be seen but are large enough to scatter light, a phenomenon known as the Tyndall effect. This scattering makes a beam of light visible when passing through fog or milky water, distinguishing colloids from true solutions. Colloidal systems are stable because the particles are too small to settle out under gravity.
Suspensions
Suspensions are coarse dispersions where the particle size is greater than 1000 nm. These mixtures are visibly heterogeneous and often opaque, such as sand mixed in water. The large size of the dispersed particles means they are susceptible to gravity, causing them to settle out over time in a process called sedimentation. Unlike colloids, suspensions can usually be separated using simple filtration methods.
Specific Types of Colloidal Dispersions
Colloidal dispersions are further classified based on the physical state of the dispersed phase and the continuous medium.
When both the dispersed phase and the continuous medium are liquids, the dispersion is called an emulsion. Common examples include milk, where tiny droplets of fat are dispersed in water, and mayonnaise. A sol is a type of colloid where solid particles are dispersed throughout a liquid medium, such as paint and ink.
Conversely, if a gas is dispersed in a liquid, the result is a foam, with whipped cream being a popular example. Aerosols are colloidal systems where either a liquid or a solid is dispersed in a gas. Liquid aerosols include fog and mist, while smoke is a type of solid aerosol.
Stabilization and Destabilization of Dispersions
Dispersions, particularly colloids, are inherently unstable from a thermodynamic perspective, meaning they will eventually separate without intervention. However, they can be kinetically stable, retaining their uniform distribution over a long period through various mechanisms.
Stabilization Mechanisms
One stabilizing force is Brownian motion, the random movement of particles caused by collisions with the molecules of the continuous medium, which helps keep the dispersed phase suspended. Another factor is electrostatic repulsion, where particles acquire a similar surface charge. This charge causes the particles to repel each other, preventing them from aggregating. The addition of stabilizing agents, such as emulsifiers or polymers, can also prevent aggregation by creating a physical barrier around the particles, a process known as steric stabilization.
Destabilization Processes
Destabilization leads to phase separation through processes like flocculation and coagulation, where particles clump together. External forces can accelerate this breakdown. For instance, adding electrolytes or changing the pH of the medium can neutralize surface charges, removing electrostatic repulsion and causing aggregation. Heating a dispersion can also increase the frequency of particle collisions, potentially leading to faster coalescence or separation.