Colloids and Interfaces

Colloids are dispersions of particles between nanometres and micrometres in size, whose enormous interfacial area and the forces acting across it determine whether they remain stable or aggregate.

Definition

Colloids are heterogeneous systems in which particles of one phase, intermediate in size between molecules and bulk matter, are dispersed throughout another, and the interfaces between them control the system's stability and properties.

Scope

This topic covers colloidal systems and the interfaces within them: the classification of colloids into sols, emulsions, foams, gels, and aerosols; the optical and transport properties such as the Tyndall effect and Brownian motion; and the forces that govern stability, including van der Waals attraction, electrostatic double-layer repulsion, and steric stabilization. It develops the DLVO theory of colloid stability, the processes of flocculation and coagulation, and the role of zeta potential. Surfactant self-assembly and the detailed structure of charged interfaces are treated in sibling topics.

Core questions

• How are colloids classified, and what gives them their characteristic properties?
• What forces act between colloidal particles across the intervening medium?
• How does DLVO theory explain colloidal stability and flocculation?
• How do electrostatic and steric mechanisms keep dispersions stable?

Key concepts

• Classification of colloids
• Tyndall effect and Brownian motion
• Van der Waals and double-layer forces
• DLVO theory
• Flocculation, coagulation, and zeta potential

Key theories

DLVO theory

The interaction between charged colloidal particles is the sum of attractive van der Waals and repulsive electrical double-layer contributions; the resulting energy barrier determines whether particles aggregate, and adding salt lowers the barrier by screening the repulsion.

Steric and electrostatic stabilization

Dispersions are kept stable either by like charges on particle surfaces that repel one another or by adsorbed polymer layers that resist overlap, the two principal mechanisms exploited to prevent colloidal aggregation.

Clinical relevance

Colloid science governs the stability and formulation of paints, inks, foods, cosmetics, and drug delivery systems, the clarification of water and wastewater, the behaviour of clays and soils, and many biological dispersions, with controlled flocculation central to separation and purification.

History

Graham coined the term colloid in 1861, and the early twentieth century saw Einstein and Perrin establish the molecular reality of colloidal Brownian motion; the DLVO theory, developed independently by Derjaguin and Landau and by Verwey and Overbeek in the 1940s, gave colloid stability a quantitative basis.

Key figures

• Thomas Graham
• Boris Derjaguin
• Jan Theodoor Gerard Overbeek

Seminal works

• israelachvili2011
• adamson1997

Frequently asked questions

Why does adding salt often cause a colloid to clump together?

Dissolved ions screen the electrostatic repulsion between charged particles by compressing the electrical double layer; once the repulsive barrier is lowered enough, attractive van der Waals forces dominate and the particles flocculate and settle out.

What makes a beam of light visible passing through a colloid?

Colloidal particles are large enough to scatter light, producing the Tyndall effect; true solutions, whose solute particles are molecular in size, scatter negligibly, which is one way to distinguish a colloid from a solution.