Competing Forces in Colloidal Dispersions

As we introduced in the preceding section, colloidal systems are stabilized by a delicate balance of competing forces. These forces either act to repel the colloidal particles from each other or mutually attract them into aggregates. Colloidal particles are small, and as a result a colloidal system can have an extremely high surface area-to-volume ratio. Consider a 1-cm cube of gold the surface area-to-volume ratio of the cube is 6 cm /l cm. Now, if we take that same cube of gold but split it into 10-nm wide cubes, the total surface area-to-volume increases to 6 X 10 cm /1 cm. This calculation shows that as particle size decreases, the surface interactions between particles increase in importance and tend to dominate the physics of the system at very small particle sizes. 

In a fluid containing dispersed colloids, there will be competing forces acting on the particles. Gravity promotes separation of the particles from the solution by density variation, and interparticle forces promote either aggregation or dispersion. 

---