Properties of Colloids

Colloid dispersions exhibit several properties. Among these are the scattering of a light beam directed through a colloidal dispersion. This is known as the Tyndall effect and its magnitude is due to the size and number of particles present. When observed under ambient light, colloidal dispersions may appear translucent, opalescent or cloudy depending on the type of colloid and the degree of particle concentration and dispersion.

Under a microscope colloidal particles may be seen to "dance" or move at random. This is known as Brownian movement and is due to bombardment of the colloidal particles by molecules of the dispersion medium. Brownian movement is usually observed when particles are below 5µ in size.

The presence of a charge on colloidal particles gives them electrical properties. When exposed to an electrical potential colloids can be forced to migrate toward the electrode of opposite charge. This is known as electrophoresis and may be used to separate a mixture of colloidal substances such as proteins.

Colloids do not pass through a semi-permeable membrane. Thus, when a protein solution such as albumin is placed into a cellophane sac and submerged into water, water molecules will enter the sac to dilute the albumin molecules which cannot diffuse out. This principle explains the role of human serum albumin in maintaining the osmotic pressure of blood. The principle is also operational in the kidney where ions and small molecules are filtered out of the blood across the glomerular membrane but the macromolecular serum proteins are retained. Sterilization of injections is sometimes performed by filtration through a synthetic membrane having a mean pore size of 0.22µ (220 nm). It is important to realize that colloidal injections may not be sterilized by this method unless the particles are smaller than the mean pore size of the membrane.