Colloids

When salt is added to water, it dissolves to form a solution. Conversely, when sand is added to water and stirred, the sand particles are spread throughout the liquid to form a suspension, and finally settle at the bottom.

However, when flour is added to water, the water becomes cloudy. This is because flour and water form a colloidal dispersion or a colloid.

A colloid is a mixture in which particles of a solute-like substance are finely dispersed in a solvent-like medium.

The dispersed particles and the dispersing medium can be any combination of solid and liquid, such as opal, liquid and liquid, such as milk, liquid and gas, such as whipped cream, except that they cannot both be gases.

Properties of a colloid are intermediate between those of suspensions and solutions.

Colloids are heterogeneous mixtures like suspensions, in contrast to solutions, which are homogeneous.

With a size of 5 to 1000 nanometers, colloidal particles are much larger than the usual solute molecules, which are around 1 nanometer or less, and smaller than suspended particles, which are 10,000 nanometers or larger.

A laser beam passing through a solution is invisible but can be easily seen in a colloidal dispersion. This is because the colloid particles are large enough to scatter the light while solute particles are too small to do so.

This scattering of light by colloidal particles is called the Tyndall effect.

Colloidal particles can remain stably dispersed throughout the medium by colliding with other molecules and moving constantly in a random path. This movement is called Brownian motion.

If a suspension, a solution, and a colloid are centrifuged, only the suspension separates.

Water-based colloids can be hydrophilic, water-loving, or hydrophobic, water-fearing.

For example, when agar, a seaweed extract, is added to hot water, it forms a hydrophilic colloid.

Hydrophobic colloids, such as oil and vinegar, are unstable in water and tend to separate.

Such colloidal dispersions can be stabilized by adding other substances that attach to the surfaces of the colloidal particles. These additives can be ions that repel other ions on neighboring colloidal particles to remain dispersed.

Other additives can cover the colloid particle surface with hydrophilic groups. For example, sodium stearate, a type of soap, has a sodium ion along with a polar head and a long nonpolar tail of fatty acid.

In water, the soap molecules aggregate in spheres so that their hydrophobic tails point inwards while their charged hydrophilic heads are outside. These spherical structures are called micelles.

The hydrophobic tails trap the non-polar oil inside the micelles while the hydrophilic outside interacts with water. This explains the phenomena seen when soap is washed with water and oil is removed.