INTRODUCTION:
When two or more ingredients are not mutually miscible and
therefore not capable of forming a homogeneous mixture, they form a
multi-phasic dispersion when mixed together in the development of a
pharmaceutical dosage form.
When one component is distributed more or less uniformly
throughout the second, the first component is called the disperse phase or
dispersed phase, while the second is called dispersion medium or continuous
phase.
The dispersed material varies in size. Actually the size
of the disperse phase is the basis of classification of the
dispersed systems:
True solutions are called Molecular
dispersions (the size of a molecule is less than one nanometer.
When the mean diameter of the
disperse phase ranges between 1mm – 0.5 mm the system is called colloidal
dispersion.
Beyond 0.5 mm, it
is a coarse dispersion.
The size limits are however arbitrary. And there is no
clear-cut distinction between either molecular and colloidal dispersions or
colloidal and coarse dispersions. Proteins and other polymers are colloidal
dispersions but may also be classifies as molecular dispersions. Suspensions
and emulsions are coarse dispersions but they may have particles in the
colloidal range. Some colloidal dispersions under proper conditions of
concentration and temperature set to a solid or semi-solid in which state they
are known as gels. The latter shall be discussed separately.
COLLOIDAL
DISPERSIONS
The
word colloid is derived from the Greek word “kola”, meaning glue. It was
coined from the impression that colloidal substances were amorphous glue like
rather than crystalline forms of matter. Thomas Graham, a British scientist,
while studying diffusion in 1850, found that polypeptides such as gelatin
acacia and starch did not crystallize and diffused slowly when dispersed or
dissolved in water. These substances could be separated from sugar and salts
because the latter diffused through the fine pores of dialysis membranes made
from animal gut, while the polypeptides were retained. Because salt and sugar
are crystalline, Graham called them crystalloids and the polymeric slow
diffusing materials were termed colloids. Subsequent works by Wolfgang
Ostwalds and his publication “The World of neglected dimensions” in 1906 was
the hallmark of colloidal science.
Colloidal
particles are aggregates of many molecules (103-109
individual atoms) in a dispersion medium. The components of dispersion may
exist in any of the three states of matter namely solid, liquid, or gas, and a
component in any of these states may be dispersed in a medium of any state
(except gas-gas dispersions are true solutions).
DISPERSION
MEDIUM
DIPSERSED
PHASE
|
SOLID
|
LIQUID
|
GAS
|
SOLID
|
Solid
suspension e.g. pigmented plastics, colloidal gold in glass, ruby glass
|
Sol,
Suspensions e.g. silver iodide sol, Aluminium hydroxide suspension.
|
=solid
aerosols e.g. smoke, solid aerosols
|
LIQUID
|
Gel,
solid emulsion e.g liquid dispersed in soft paraffin, opals, pearls
|
Emulsions
e.g milk, pharmaceutical emulsions
|
=
liquid aerosols. E.g. Mist, fogs, aerosols.
|
GAS
|
Solid
foam e.g expanded polystyrene
|
Foam
e.g. foam on surfactant solutions, Carbonated beverages
|
=
Not dispersion but a true solution
|
Dispersions
of solid in liquid, solid, gases are termed sols
The
so-called colloidal solutions are sols of solids in liquids. Prefixes are used
t designate the dispersion medium e.g. hydrosol, alcosol, benzosol and
aerosol for water, alcohol, benzene and air respectively. Emulsions
are dispersions of liquid drugs in liquid medium. Suspensions are dispersion
of solid drugs in liquid medium.
