Colloids

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Final Project for AP225 Fall 2011, written by Hyerim Hwang

Definition

Colloids are fluids containing particles suspended in a liquid. A representative example is milk which is an emulsified colloid of liquid butterfat globules dispersed within a water-based solution. In this case, colloidal particles give special physical properties of fluids. The light is scattered by particles in the colloid and other colloids may be opaque or have a slight color. These properties can be used in many applications. Paint is also a kind of colloidal dispersions. The colloidal particles produce the special properties in the solid when the solvent dries.

Figure 1. Light shining through a colloidal dispersion.

Types of Colloids

As referred, colloids are common in everydaylife. Whipped cream, mayonnaise, butter, gelatin, jelly, muddy water, colored glass,, and paper are the examples. They consists of colloidal particles and the dispersion medium in which the colloidal particles are dispersed. Any colloid consisting of a solid dispersed in a gas is called smoke, and a liquid dispersed in a gas is referred to as a fog.

Figure 2. Stabilized Colloid System.

Properties

Colloids exhibit Brownian movement. Brownian motion is the random motion of particles that we can easily see under a microscope. This movement is caused by the collision of molecules with colloidal particles in the dispersion medium. Additionally, colloids display the Tyndall effect as referred above. When a strong light is shone through a colloidal dispersion, the light beam becomes visible, like a column of light. A common example of this effect can be seen when a spotlight is turned on during a foggy night. We can see the spotlight beam because of the fuzzy trace it makes in the fog which is a colloid.

Stability and Phase Behavior

The interaction energy of colloidal particles is important to decide the behavior of colloids. Small changes in the solvent can be a huge effect on the interaction energy between two colloidal particles. That is from a hard-core repulsion to an attraction which is greater than thermal energy. Colloidal particles can be stabilized mainly by the electrostatic stabilization and steric stabilization. With such an attraction the particles stick together and there can be aggregation and sedimentation which hinders the stability. If attractive forces get stronger than repulsive interaction, particles aggregate in clusters.

References

1. Israelachvili, Jacob N. (2011). Intermolecular and Surface Forces. Academic Press. ISBN 9780123919274.

2. "Colloid". Wikipedia.

Additional Readings

1. Mayers, Drew. (2002). Surfaces, Interfaces, and Colloids: Principles and Applications, Second Edition. ISBN 9780471330608.

2. Caruso, Frank. (2004). Colloids and colloid assemblies: synthesis, modification, organization and utilization of colloid particles. Academic Press. ISBN 3527306609.

3. See also: Colloids in What is soft matter in the General Introduction from Lectures for AP225.

Keyword in References

Biofilms as complex fluids

Bulk Synthesis of Polymer-Inorganic Colloidal Clusters

Dispersion of Silica Fines in Water-Ethanol Suspensions

Diffusion through colloidal shells under stress

Phase diagrams of colloidal spheres with a constant zeta-potential

Stress Enhancement in the Delayed Yielding of Colloidal Gels

The Role of Polymer Polydispersity in Phase Separation and Gelation in Colloid−Polymer Mixtures