methods of preparation of dispersed system

1. Various Methods of Preparation of
Dispersed system
Dispersed System:
A kinetically stable mixture of one phase in another largely immiscible
phase. Usually at least one length scale is in the colloidal range.
Emulsions:
An emulsion is a dispersion in which the dispersed phase is composed
of small globules of a liquid distributed throughout a vehicle in which it
is immiscible.
General Types of Pharmaceutical Emulsions:
1. Lotions
2. Liniments
3. Creams
4. Ointments
5. Vitamin drops
Methods of Preparation of Emulsions:
These are the methods used for the Preparation of Emulsions:
1. Continental or Dry Gum Method
2. English or wet Gum Method
3. Bottle or Forbes Bottle Method
Continental or Dry Gum Method

2. i. The continental method is also referred to as the 4:2:1 method
because for every 4 parts by volume of oil, 2 parts of water and 1
part of gum are added in preparing the initial or primary
emulsion.
ii. For instance, if 40 mL of oil is to be emulsified, 20 mL of water and
10 g of gum would be employed
iii. In the primary emulsion, with any additional water or other
formulation ingredients added afterward.
iv. In this method, the acacia or other o/w emulsifier is triturated
with the oil in a perfectly dry Wedgwood or porcelain mortar until
thoroughly mixed. A mortar with a rough rather than smooth
inner surface must be used to ensure proper grinding action and
reduction of the globule size.
v. A glass mortar is too smooth to produce the proper reduction of
the internal phase.
vi. Solid substances such as preservatives, stabilizers, colorants, and
any flavoring material are usually dissolved in a suitable volume of
water (assuming water is the external phase) and added as a
solution to the primary emulsion
vii. Rather than using a mortar and pestle, the pharmacist can
generally prepare an excellent emulsion using the dry gum
method and an electric mixer or blender.
English or Wet Gum Method:
i. By this method, the same proportions of oil, water, and gum are
used as in the continental or dry gum method
ii. but the order of mixing is different, and the proportion of
ingredients may be varied during the preparation of the primary
emulsion as is deemed necessary by the operator.

3. iii. Generally, a mucilage of the gum is prepared by triturating in a
mortar granular acacia with twice its weight of water. The oil is
then added slowly in portions, and the mixture is triturated to
emulsify the oil.
iv. Should the mixture become too thick, additional water may be
blended into the mixture before another portion of oil is added
for several minutes
v. After all of the oil has been added, the mixture is thoroughly
mixed
vi. Then, as with the continental or dry gum method, the other
formulative materials are added, and the emulsion is transferred
to a graduate and brought to volume with water.
Bottle or Forbes Bottle Method:
i. The bottle method is useful for the extemporaneous preparation
of emulsions from volatile oils or oleaginous substances of low
viscosities.
ii. Powdered acacia is placed in a dry bottle, two parts of oil are
added, and the mixture is thoroughly shaken in the capped
container.
iii. When all of the water has been added, the primary emulsion thus
formed may be diluted to the proper volume with water or an
aqueous solution of other formulative agents.
iv. This method is not suited for viscous oils because they cannot be
thoroughly agitated in the bottle when mixed with the emulsifying
agent.
Suspensions:
Suspensions, micro heterogeneous dispersion systems consisting of

4. solid medicinal substances in the suspended state, which are in the
liquid dispersion medium (water, non-aqueous solvents).
Classes Of Suspensions:
These are:
1. Lotions
2. Gels
3. Magmas and Milk
4. Mixtures
Methods Of Preparation Of Suspensions:
The methods which are used for Preparation of Suspensions are:
1. Dispersion Methods
2. Precipitation Methods
Dispersion Methods:
i. In this method large particles of the substances are broken, into
particles of colloidal dimensions in presence of dispersion
medium.
ii. When the dispersion method is utilized for suspension
preparation, the vehicle must be formulated so that the solid
phase is easily wetted and dispersed.
iii. The use of surfactants is desirable to ensure uniform wetting of
hydrophobic Solids.
iv. The use of suspending agents, Such as the synthetic polymeric
polyelectrolytes, natural gums, or clay, may be indicated,
depending un the specific application.

5. v. The actual method of dispersing the solid is one of the more
important considerations because Particle size reduction may or
may not result from the dispersion Process.
vi. If Particle size reduction occurs, the particles obtained may have
different solubilities if a metastable state is involved, and this may
lead to transient Supersaturation of the System.
Precipitation Method:
i. Precipitations methods involved organic solvent precipitation,
Precipitation effected by changing the pH of the medium and
double decomposition.
ii. Water insoluble drugs Can be precipitated by dissolving them in
water miscible organic solvents and then adding adding the
organic phase to distilled water under standard conditions.
iii. several important Considerations are involved when this method
is used, Perhaps the most important factor next to Particle size
control is that the correct polymeric form.
iv. Formulation of Suspensions by double decomposition only
involves the simple chemistry
v. Some Physical factors also Come into play.
vi. Insulin Suspensions also may be Prepared by PH change method
,Insulin has an isoelectric point of approximately pH 5.when it is
mixed with basic protein such as Protamine, it is readily
precipitated when the pH is between the isoelectric points of two
components, i.e. ,pH 6.9 to 7.3.
Colloids:
By definition, dispersions in which the size of the dispersed Particles is
within the range of 1nm to 0.5µm are termed as Colloid
Methods of Preparations:

6. Lyophilic sols may be prepared by simply warming the solid with liquid
dispersion medium. E.g. Starch with water. On the other hand,
lyophobic sols have to be prepared by special methods. These methods
fall into two categories;
1. Condensation or Aggregation Methods.
2. Dispersion Methods.
Condensation or Aggregation Methods:
i. These methods consists of chemical reactions or change of
solvent whereby the atoms or molecules of the dispersed phase
appearing first, aggregate to form colloidal particles.
ii. The important chemical methods for preparing lyophobic sols are
as follows;
a. Double Decomposition:
i. An Arsenic Sulphide (AS2S3) sol is prepared by passing a slow
stream of hydrogen sulphide gas through cold solution of
arsenious oxide (AS2O3). This is continued till a yellow colour of
sol attains maximum intensity.
AS2O3 + 3 H2S → AS2S3(Yellow Sol) + 3H2O
b. Oxidation:
i. A Colloidal sol of sulphur can be obtained by passing hydrogen
Sulphide into solution of sulphur dioxide in water or through a
solution of an oxidizing agent (Bromine water, nitric acid).
c. Reduction:

7. i. A colloidal solution of a metal like silver, gold and platinum can be
prepared by reducing their salt solutions with suitable reducing
agents, such as stannous chloride, formaldehyde, hydrazine,
tannic acid etc.
2AuCl3 + 3SnCl2 → 2Au(Gold sol.)+ 3 SnCl4
d. Hydrolysis:
i. The method is used to prepare hydroxides and oxides of weakly
electropositive metals like Fe, Al or Sn.
ii. A red sol of ferric hydroxide, is obtained by adding few drops of
30% ferric chloride solution to a large volume of almost boiling
water and stirred with a glass rod.
Dispersion Methods:
In this method large particles of the substances are broken, into
particles of colloidal dimensions in presence of dispersion medium.
1. Mechanical Dispersion
2. Bredig’s Arc Method
3. Peptisation
a. Mechanical Dispersion:
i. In this method, the coarse particles along with dispersion medium
is brought into colloidal state by grinding it in colloidal mill, ball
mill or ultrasonic disintegrator. The solid particles along the
dispersion medium are fed into the colloidal mill.
ii. The solid particles are torn off to colloidal size and are then
dispersed in liquid to give the sol

8. b. Bredig’s Arc Method:
i. This process involves dispersion as well as aggregation. Colloidal
solutions of metals such as gold, silver, platinum etc. can be
prepared by this method.

9. c. Peptisation:
i. Peptisation may be defined as the process of converting a
precipitate into colloidal form by shaking it with dispersion
medium in the presence of small amount of electrolyte
ii. During peptisation, the precipitate adsorbs one of the ion of the
electrolyte on its surface. The adsorbed ion is generally common
with those of the precipitate