Colloid

In general, a colloid or colloidal dispersion is a two-phase system of matter; a type of mixture intermediate between homogeneous mixtures and heterogeneous mixtures.

Interaction between colloid particles

Colloids usually are too large to be affected by quantum effects. However, they are light enough to be affected by the thermic motion happening in the suspension.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The following forces play an important role in the interaction of colloid particles:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Hard sphere repulsion: Often colloids are made of hard materials. So two colloids cannot get closer to each other than the sum of their radii.
• Electrostatic interaction: Colloids can be manufactured so that they carry a charge. The Coulomb Potential is proportional to . However, if there are solvent particles with a charge opposite to that of the colloids, they assemble around the colloids and screen the repulsion. The potential is proportional to $e^\{-kappa\; r\}/r$.
• Van der Waals forces: If the index of refraction of the colloids is different from the solvent, they will attract through a $r^\{-6\}$ Van der Waals potential.
• Entropic forces: According to the second law of thermodynamics, a system evolves to a state in which entropy is maximized. This can result in effective forces even between hard spheres.