Dispersed systems consist of one phase, known as the dispersed phase, distributed throughout a continuous phase or dispersion medium.
Colloidal
dispersions
Introduction
Dispersed
systems consist of one phase, known as the dispersed
phase, distributed throughout a continuous
phase or dispersion medium. The
dispersed systems range in size from particles of atomic and molecular
dimensions to visible particles, which can be up to several millimeters in
diameter. On the basis of the size of the dispersed phase, dispersed systems
are classified into the following types:
1. Molecular
dispersions (<1
nm): Molecular dispersions are true solu-tions of one component in another.
They are visibly homogeneous. The size of dispersed phase of molecular
dispersions is typically less than 1 nm in diameter. True solutions do not
scatter light and are clear or colored.
2. Colloidal
dispersions (1
nm to 0.5 μm): Colloidal dispersions have the dispersed-phase size larger than
the molecular dimensions of the dispersed phase in true solutions, while being
much smaller than the particles that would be visible to the naked eye. The
size of dispersed phase of molecular dispersions is typically 1 nm to 0.5 μm in diameter. Colloidal dispersions
scatter light and appear turbid. Many natural systems, such as suspensions of
microorgan-isms, blood, and isolated cells in culture, are colloids. Some
hydro-philic colloids can be used as blood plasma substitutes to maintain
osmotic pressure.
3. Coarse
dispersions (>0.5 μm): Coarse dispersions have a
particle size significantly larger
than molecular and colloidal dispersions, such that the dispersed phase rapidly
and spontaneously segregates if appropriate stabilization strategies are not
utilized. The size of the dispersed phase of coarse dispersions is typically
greater than 0.5 μm in diameter.
Coarse dispersions scatter light and are visually cloudy/milky. Emulsions and
suspensions are examples of coarse dispersions.
Colloidal
solutions are preferred for pharmaceutical applications, where maximizing the
surface area of the dispersed phase is important. Some examples of colloids
used as pharmaceuticals are as follows:
·
Colloidal kaolin is used for toxin absorption in the
gastrointestinal (GI) tract.
·
Colloidal aluminum hydroxide is used for neutralizing excess
acid in stomach.
·
Colloidal dispersion of amphotericin B and sodium
cholesteryl sulfate (Amphocil®) is used as an antifungal agent.
·
Colloidal silver chloride, silver iodide, and silver protein
are effective germicides. They do not cause irritation, which is characteristic
of ionic silver salts.
·
Colloidal copper has been used in the treatment of cancer.
·
Colloidal gold as a diagnostic agent for paresis and
colloidal mercury for syphilis.
·
Psyllium hydrophilic colloid (Metamucil®) is used
as an oral laxative.
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