Lyophobic dispersions
The most common medium for lyophobic dispersion is water.
Insoluble organic and inorganic compounds usually with a low degree of
hydration are dispersed in aqueous medium and are intrinsically unstable. The
particles tend to coalesce or aggregate to reduce the surface area and hence
surface energy. Thus special means must be utilized to stabilize these systems
–preventing the otherwise spontaneous coalescence or coagulation of the
disperse phase after it has been finely dispersed. There are two methods of
preparing lyophobic sols, namely Condensation methods (aggregation of
small molecules or ions until particles of colloidal dimensions are obtained)
and dispersion methods (reducing coarse particles to colloidal
dimensions through comminution or peptization).
Condensation Method:
This method involves aggregation of small molecules or
ions until particles of colloidal dimensions result.
To illustrate this method, the preparation of sulphur
hydrosol is exemplified. Whereas sulphur is insoluble in water, it soluble in
alcohol. Alcohol and water are miscible. Sulphur
is dissolved in alcohol, and the solution is mixed with water to produce a
bluish-white colloidal dispersion. The dispersion must be stabilized to avoid precipitation
or agglomeration. The same method can be used to prepare hydrosol of stearic
acid, mastic acid and other polymers.
Suphur vapor may also be stremed in water to produce a
colloidal dispersion. This is a less common method.
Condensation may also be produced chemically. For example
bubbling hydrogen sulphide (H2S) gas into a solution of sulphur
dioxide (SO2) :
2H2S + SO2 ===> 3S + H2O
or mixing solutions of sodium thiosulphate and sulfuric acid:
===>H2SO4 + 3NaS2O3 4S
+NaSO4 + H2O
Aluminium hydrxide sol is produced by hydrolysis of
Aluminium chloride.
AlCl3 + 3 H2O ===> Al(OH)3
+ 3HCl
** See preparation of White
lotion (precipitated ZnS and S)
Dispersion method:
The methods involve reducing coarse particles to colloidal
dimensions through comminution or peptization.
Mechanical disintegration:
Using equipment such as
micronizers, colloid mills, homogenizers, or even simple mortar and pestle,
solids and liquids are sheared or attrited into fine size for dispersion.
Ultrasonic generators produce waves that break soft materials e.g. sulphur,
talcum and graphite into very small particles or droplets. Thus method is used
to produce very fine emulsions for I.V. use.
The dispersion must be stabilized
to avoid re-crystallization, coagulation or calescence.
Peptization:
This involves breaking up
aggregates into smaller particles. It is synonymous to defflocculation. It can
be brought by removing flocculating agent (eg electrolyte) or addition of a
deffloculating agents or peptizing agents (eg surfactants, ions or water
soluble polymers). Eg activated charcoal produces a grey dispersion, Addition
of 0.1% Na laurylsulphate or octoxynol disintegrates the particles into fine
ones, results in fine deep black dispersion.
Purification
of colloidal dispersions
Hydrosols may contain low molecular weight water-soluble
impurities including salts formed by reaction producing the dispersion. The
salts tend to coagulate the dispersion and hence must be removed. Blood ( a colloidal dispersion of plasma proteins)
of patients with renal insufficiency has high concentration of urea and other
metabolites. These as well must be removed to acceptable levels. Substances in
true solutions may be separated from those in colloidal dispersion by means of
dialysis or ultrafiltration.
Dialysis:
This is based upon the fact that colloidal particles do
not diffuse (or diffuse very slowly) through membrane of parchment, cellophane,
collodion or certain animal tissues, while particles of molecular or ionic
dimensions diffuse relatively rapidly. Thus if
the low molecular weight
impurities are to be removed from a colloidal dispersion the latter is placed
inside a sac made of one of the above mentioned membranes and dipped in water.
The small solutes will diffuse out while pure colloidal materials are retained.
The rate of
dialysis is increased increasing the area of membrane (e.g. using numerous
hollow fibres, stirring and maintaining a high concentration gradient (dialysis
fluid must be replenished continuously). If the impurities are electrolytes,
the dialysis process can be speeded up by applying an electric potential to the
sol. The process is then known as electrodialysis, and the equipment
used is electrodialyser. Application of pressure in a dialytic process
also speeds up the process. This is known as ultrafiltration.
Application
of dialysis.
