A colloid is a substance microscopically
dispersed throughout another substance.
The word colloid comes from a Greek word
kolla, which means glue thus colloidal
particles are glue like substances. These
particles pass through a filter paper but not
through a semipermeable membrane.
Colloids can be made settle by the process
The colloidal system consist of two
A dispersed phase ( A discontinuous
A dispersion medium ( A continuous
The dispersed-phase particles have a diameter of
between approximately 1nm – 0.5um .
Such particles are normally invisible in an
optical , though their presence can be confirmed
with the use of an ultramicroscope or an electron
Homogenous mixtures with a dispersed phase in
this size range may be called colloidal
aerosols, colloidal emulsions, colloidal
foams, colloidal dispersions, or hydrosols.
If the dispersed phase is solid and the dispersion
medium is liquid then it is called colloidal
suspension but if the dispersed phase and
dispersion medium both are in liquid state then it
is known as colloidal emulsion.
The dispersed-phase particles or droplets are
affected largely by the surface chemistry present
in the colloid.
Some colloids are translucent because of
the Tyndall effect, which is the scattering of
light by particles in the colloid. Other colloids
may be opaque or have a slight color.
Colloidal solutions (also called colloidal
suspensions) are the subject of interface and
This field of study was introduced in 1861
by Scottish scientist Thomas Graham.
. Examples of colloids are silver
solutions, milk, synthetic polymers and blood
INTERACTION BETWEEN PARTICLES : The
following forces play an important role in the
interaction of colloid particles:
1) EXCLUDED VOLUME REPULSION:
IN LIQUID THEORY STATE :
In liquid state theory, the 'excluded volume' of a
molecule is the volume that is inaccessible to other
molecules in the system as a result of the presence
of the first molecule. The excluded volume of a hard
sphere is eight times its volume—however, for a
two-molecule system, this volume is distributed
among the two particles, giving the conventional
result of four times the volume.
2) ELECTROSTATIC INTERACTION :
Colloidal particles often carry an electrical charge and
therefore attract or repel each other. The charges of
both the continuous and the dispersed phase, as well as
the mobility of the phases are factors affecting this
3) VAN DER WAALS FORCE : van der Waals force (or van
der Waals interaction), is the sum of the attractive or
repulsive forces between molecules other than those
due to covalent bonds, the hydrogen bonds, or the
electrostatic interaction of ions with one another or with
neutral molecules or charged molecules
5) STERIC FORCES :
Steric effects arise from the fact that
each atom within a molecule occupies a
certain amount of space. If atoms are
brought too close together, there is an
associated cost in energy due to
overlapping electron clouds (Pauli or
Born repulsion), and this may affect the
molecule's preferred shape
(conformation) and reactivity.
TYPES OF COLLOIDS:
Classification of Colloids based
on Physical State of Dispersed
Phase and Dispersion Medium:
Classification of Colloids based
on Nature of Interaction
Between Dispersed Phase and
• Lyophilic colloids are liquid loving colloids (Lyo means
solvent and philic means loving).
Ease of Preparation:
• As these colloids are liquid loving, their solutions are
easy to prepare and can be prepared directly by mixing
colloid with liquid.
• Lyophilic sols are relatively stable as strong forces of
interaction exist between colloidal particles and liquid.
If large quantity of liquid is added to
precipitations or the colloidal
solution is stirred properly lyophilic
sols can regain their original state.
This shows that lyophilic sols are
also reversible in nature.
Lyophobic colloids are liquid hating colloids (Lyo
means solvent and phobic means hating).
Ease of Preparation:
As these colloids are water hating, their solutions are
easy to prepare and cannot be prepared directly by
mixing colloid with liquid. Special methods are
employed to prepare lyophobic solutions.
Lyophobic sols are less stable as weak forces of
interaction exist between colloidal particles and liquid.
Lyophobic colloids do not regain
their original state as coagulated
mass cannot be dispersed into
colloidal form. This shows
that lyophobic solutions are also
irreversible in nature.
• These are the colloids which behave as normal strong
electrolytes at low concentrations but exhibit colloidal
properties at higher concentrations due to the
formation of aggregated particles. The aggregated
particles thus formed are called micelles.
The associated colloids are usually formed by
surfactants (surface active agents) like soaps and
synthetic detergents. These agents form micelles when
present in solution at a concentration greater than
critical micellar concentration (CMC).
PREPARATION OF COLLOIDS
Lyophilic and lyophobic colloidal
solutions (or sols) are generally
prepared by different types of
methods. Some of the common methods
are as follows.
Dispersion methods :
In these methods, larger particles of
a substance (suspensions) are broken
into smaller particles. The following
methods are employed.
A) Mechanical dispersion:
In this method, the substance is first ground to
• It is then mixed with the dispersion medium to
get a suspension.
• The suspension is then grinded in colloidal mill.
• It consists of two metallic discs nearly touching
each other and rotating in opposite directions at a
very high speed about 7000 revolution per minute.
• The space between the discs of the mill is so
adjusted that coarse suspension is subjected to
great shearing force giving rise to particles of
• Colloidal solutions of black
ink, paints, varnishes, dyes etc. are obtained by
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. The mill consists of two steel plates nearly
touching each other and rotating in opposite directions with
high speed (7000 rev/min).
The solid particles are torn off to colloidal size and
are then dispersed in liquid to give the sol
colloidal graphite (lubricant) and printing inks are
made by this method. Recently, a mercury sol has
been prepared by disintegrating a layer of
mercury into sol particles in water, by means of
(B) By electrical dispersion or
Bredig’s arc method:
oThis method is used to prepare sols of
platinum, silver, copper or gold.
o The metal whose sol is to be prepared is made as
two electrodes which immerge in dispersion
medium such as water etc.
oThe dispersion medium is kept cooled by ice.
oAn electric arc is struck between the electrodes.
oThe tremendous heat generate by this method and
give colloidal solution.
oThe colloidal solution prepared is stabilised by
adding a small amount of KOH to it.
This process involves dispersion as well as
Colloidal solutions of metals such as
gold, silver, platinum etc. can be prepared by this
method. In this method electric arc is struck
between electrodes of metal immersed in the
(C) By peptisation:
The process of converting a freshly prepared
precipitate into colloidal form by the addition of
suitable electrolyte is called peptization. The
electrolyte is used for this purpose is called peptizing
agent or stabilizing agent. Cause of peptisation is the
adsorption of the ions of the electrolyte by the
particles of the precipitate. Important peptizing
agents are sugar, gum, gelatin and electrolytes.
Freshly prepared ferric hydroxide can be converted
into colloidal state by shaking it with water
containing Fe3+ or OH– ions, viz. FeCl3 or NH4OH
Fe(OH)3 + FeCl
→ *Fe(OH)3Fe]3+ + 3Cl–
Precipitate electrolyte Colloidal sol
Preparation of colloids by condensation
In condensation method, the smaller
particles of the dispersed phase are
aggregated to form larger particlesof colloidal
dimensions. Some important condensation
methods are described below:1) By change of physical state:
Solutions of substances like mercury and sulphur
are prepared by passing their vapours through a
cold water containing a suitable stabilizer such as
ammonium salt or citrate.
2) By excessive cooling:
A colloidal solution of ice in an organic
solvent like ether or chloroform can be
prepared by freezing a solution of water in
the solvent. The molecules of water which
can no longer be held in solution,separately
combine to form particles of colloidal size.
3) By exchange of solvent:
Colloidal solution of certain substances such as
sulphur,phosphorus which are soluble in alcohol but
insoluble in water can be prepared by pouring their
alcoholic solution in excess of water. For example
alcoholic solution of sulphur on pouring into water
gives milkey colloidal solution of sulphur.
4) Chemical methods:
Some chemical reactions used to aggregate
smaller particles of atomic or ionic size to
form large particles of colloidal dimensions.
Colloids can be prepared by following
Addition of oxygen and removal of hydrogen is called oxidation.
For example: Colloidal solution of sulphur can be prepared by
oxidizing an aqueous solution of H2S with a suitable oxidizing
agent such as bromine water.
H2S + Br2
→ 2HBr + S
2H2S + SO2
Addition of hydregen and removal of oxygen
is called reduction. For example: Gold sol can
be obtained by reducing a dilute aqueous
solution of gold with stannous chloride.
2AuCl3 + 3SnCl2 → 3SnCl4 + 2Au
It is the break down of water. Sols of ferric
hydroxide and aluminium hydroxide can be
prepared by boiling the aqueous solution of the
corresponding chlorides. for example.
FeCl3 + 3H2S → Fe(OH)3 + 3HCl
d) Double decomposition:
The sols of inorganic insoluble salts such as
arsenous sulphide, silver halide etc may be
prepared by using double decomposition
reaction. For example: Arsenous sulphide sol
can be prepared by passing H2S gas through a
dilute aqueous solution of arsenous oxide.
As2O3 + 3H2S → As2S3(OH)3 + 3H2O
Purification of colloids
There are three common methods
used for purification of colloids:
The removal of ions or molecules from colloidal
dispersion by process of diffusion across the
The process of dialysis is very slow. The process is
speeded up by application of electrical potential.
This is called electro dialysis.
Application of electro dialysis:
Artifical kidney machine make use of electro
Ultra filtration is a process of high pressure
filtration through a semi permeable membrane in
which colloidal particles are retained while the
small sized solutes and the solvent are forced to
move across the membrane by hydrostatic
pressure forces. The process makes use of the
filter paper made up of nitrocellulose or
Application of ultra filtration:
Ultra filtration is a vital process that takes place
in the kidneys.
Related to the motion of particles
with respect to the dispersion
Zigzag movement of colloidal particles
continuously and randomly.
Movement of particles from a region of higher
concentration to one of lower concentration.
Settling down of particles under the
influence of gravity at a very slow
Depends on the presence of a
charge on the surface of a particle
Motion of charged particles related to the fluid
under the influence of an applied electric field.
Motion of liquid induced by an applied potential.
Process of precipitating a colloidal solution.
Properties of a substance relative
Faraday Tyndall Effect:
The phenomenon in which light is scattered by
particles of matter in its path. It enables a beam of
light to become visible by illuminating dust
Used to give information about particle size and
shape and for determination of molecular weight of
ADVANTAGES OF COLLOIDS
Colloids allow the dispersion of normally insoluble
materials, such as metallic gold or fats. These can
then be used more easily, or absorbed more easily.
Colloidal gold, for example, can be used in medicine to
carry drugs and antibiotics, because it is highly nonreactive and non-toxic, unlike silver.
Pharmaceutical industry makes use of colloidal solution
preparation in many medicines. A wide variety of
medicines are emulsions. An example is Cod Liver Oil.
Paint industry also uses colloids in the preparation of
Asphalt emulsified in water is used for building roads.
In milk, the colloidal suspension of the fats
prevents the milk from being thick, and
allows for easy absorption of the nutrients.
Sewage water contains particles of
dirt, mud etc. which are colloidal in
nature and carry some electrical charge.
These particles may be removed by using
the phenomenon of electrophoresis.
The sky is the empty space around
earth and as such has no colour. It
appears blue due to the scattering of
light by the colloidal dust particles
present in air (Tyndall effect).
The sugar present in milk produces
lactic acid on fermentation. Ions
produced by acid, destroy the charge
on the colloidal particles present in
milk, which then coagulate and
separate as curd.
Soap solution is colloidal in nature. It
removes the dirt particles either by
adsorption or by emulsifying the greasy
matter sticking to the cloth.
DISADVANTAGES OF COLLOIDS
Colloids are frequently hard
to extract or purify, which
can lead to large losses in
analysis or extraction having
a large impact on the
economics of the process.
PHARMACEUTICAL APPLICATION OF COLLOIDS:
• Some important application of colloids are
• Colloidal material are used for a variety of
pharmaceutical application including therapeutic &
diagnostic agents, drug delivery system.
• Example: copper colloids ( anti cancer) & mercury
colloids ( anti syphilis).
• Drug substances may also be prepared as
colloidal sized particles to improve
bioavailability or therapeutic activity
.Example: colloidal sulphur etc.
• Colloidal dispersion of gelatin is used in coating over
tablets & granules which upon drying leaves uniformly
dry film over them & protect them from adverse
condition of the atmosphere.
• Most of the medicines are colloidal such as Calcium &
Gold are administered by injections to raise the vitality
of human system.
• Large numbers of food particles which we use
in our daily life are colloidal in nature.
• Example: Milk, butter, & ice cream etc
Colloidal dispersion containing radioactive
isotopes are being used as diagnostic &
therapeutic agents in nuclear medicines.
Example: Colloidal gold is made by reducing a
solution of AgCl either by treatment with