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