Colloids

Synopsis

Colloidal dispersions or colloids are a state somewhere between a solution and a suspension. A solution is a homogeneous mixture such as sugar and water in which no solute particles are visible. An example of a suspension is shaken mixture of sand in water. You still see the sand in the water and soon the sand will begin to settle to the bottom of the container. Milk, fog, and mayonnaise are colloids. Probably you have observed that light is scattered by the colloid so that the colloid appears cloudy. The scattering of light, the Tyndall effect, occurs because of the large size of the colloid particles. Thomas Graham coined the word colloid and the words "sol" and "gel". A sol is a dispersion of a solid substance in a fluid medium and a gel is a dispersion that has a structure that prevents it from being mobile. Jello is a sol when it is first prepared and it becomes a gel when the cooking is finished. The particles in the colloids have a very high surface area so that the properties of colloids depend largely upon surface effects.

Colloids with water as the medium are characterized as either hydrophobic (water fearing) or hydrophilic (water loving). Hydrophobic colloids do not form strong bonding with the water molecules and are stabilized by the presence of surface effects..

Forming strong intermolecular bonds with water molecules stabilizes hydrophilic colloids.

An emulsion is a colloid of one liquid in another, such as oil or fat in water. But everyone has seen oil float on water, so how does this combination become an emulsion? An emulsifying agent such as soap or a protein will stabilize such combinations into a colloid. Mixing egg yolks with oil and vinegar will stabilize the colloidal dispersion because the egg yolks contain a protein called lecithin. Soaps and detergents are molecules with both polar and non-polar ends.

The soaps and detergents function by the non-polar end forming an attraction with non-polar substances such as grease and the polar end forming an attraction with water so that the grease is removed. Substances that affect the surface properties of substances are called surfactants, and a surfactant used for cleaning is called a detergent. Substances that are surface absorbed lower the surface tension of the medium, and substances that are bulk absorbed will raise the surface tension of the medium. Salt is bulk absorbed and raises the surface tension of water. A soap will be surface absorbed on water and will lower the surface tension of water.

Review Questions and suggested sites

  1. Carry out a home chemistry experiment by floating a needle on a container of water. Now take another container of water and dissolve some NaCl in the liquid and try floating the needle again. Now take another container of water and put some soap on the water and try floating the needle again. Correlate your results with the video shown on the CD
  2. Here is an excellent site for studying Colloids.
  3. Here is a comprehensive site on colloids


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Web Author: Dr. Leon L. Combs
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