COLLOIDS determining the molecular

mass of the macromolecules.

PROPERTIES OF COLLOIDAL
SOLUTIONS
MECHANICAL PROPERTIES
OPTICAL PROPERTIES: Tyndall Effect
i. Brownian Movement
a) Robert Brown, a botanist i. Tyndal Effect
discovered in 1827 that the - When light passes through a
pollen grains suspended in sol, its path becomes
water do not remain at rest visible because of
but move about scattering of light by
continuously and randomly particles.
in all directions. - This phenomenon was studied
b) Later on, it was observed that for the first time by Tyndall.
the colloidal particles are - Tyndal cone - illuminated
moving at random in a zig – path of the beam.
zag motion. This type of
motion is called Brownian ii. The intensity of the scattered light
movement. depends on the difference
c) The molecules of the between the refractive indices of
dispersion medium are the dispersed phase and the
constantly colloiding with the dispersion medium.
particles of the dispersed
phase. It was stated by iii. In lyophobic colloids, the
Wiener in 1863 that the difference is appreciable and,
impacts of the dispersion therefore, the Tyndall effect is well
medium particles are - defined. But in lyophilic sols, the
unequal, thus causing a zig- difference is very small and the
zag motion of the dispersed Tyndall effect is very weak.
phase particles.
d) The Brownian movement iv. The Tyndall effect confirms the
explains the force of gravity heterogeneous nature of the
acting on colloidal colloidal solution.
particles. This helps in
providing stability to colloidal v. The Tyndall effect has also been
sols by not allowing them to observed by an instrument called
settle down. ultra – microscope.

ii. Diffusion Some examples of Tyndall effect are as

- The sol particles diffuse follows:
from higher concentration
to lower concentration a) Tail of comets is seen as a Tyndall
region. However, due to cone due to the scattering of light
bigger size, they diffuse at a by the tiny solid particles left by the
lesser speed. comet in its path.
b) Due to scattering the sky looks blue.
iii. Sedimentation c) The blue colour of water in the sea is
- The colloidal particles settle due to scattering of blue light by
down under the influence of water molecules.
gravity at a very slow rate. d) Visibility of projector path and circus
This phenomenon is used for light.
 e) Visibility of sharp ray of sunlight b) The ion preferentially
passing through a slit in dark room. adsorbed is held in fixed part
and imparts charge to colloidal
particles.
c) The second part consists of a
diffuse mobile layer of
ELECTRICAL PROPERTIES ions. This second layer
i. Electrophoresis consists of both the type of
a) phenomenon of movement charges. The net charge on
of colloidal particles under the second layer is exactly
an applied electric field is equal to that on the fixed
called electrophoresis. part.
b) If the particles accumulate d) The existence of opposite
near the negative electrode, sign on fixed and diffuse
the charge on the particles is parts of double layer leads
positive. to appearance of a difference
c) if the sol particles accumulate of potential, known as zeta
near the positive electrode, potential or electrokinetic
the charge on the particles is potential. Now when electric
negative. field is employed the particles
d) The apparatus consists of a U- move (electrophoresis)
tube with two Pt-
electrodes in each limb. iii. Electro-osmosis
e) When electrophoresis of a sol a) In it the movement of the
is carried out without stirring, dispersed particles are
the bottom layer gradually prevented from moving by
becomes more concentrated semipermeable membrane.
while the top layer which b) Electro-osmosis is a
contain pure and concentrated phenomenon in which
colloidal solution may be dispersion medium is allowed
decanted. This is called to move under the influence
electro decantation and is of an electrical field, whereas
used for the purification as colloidal particles are not
well as for concentrating the allowed to move.
sol. c) The existence of electro-
f) The reverse of electrophoresis osmosis has suggested that
is called Sedimentation when liquid forced through
potential or Dorn effect. a porous material or a
The sedimentation potential is capillary tube, a potential
setup when a particle is difference is setup
forced to move in a resting between the two sides
liquid. This phenomenon was called as streaming potential.
discovered by Dorn and is So, the reverse of electro-
also called Dorn effect. osmosis is called streaming
potential.

ii. Electrical Double Layer Theory

a) The electrical properties of
colloids can also be explained ORIGIN ON THE CHARGE OF COLLOIDAL
by electrical double layer PARTICLES
theory. According to this  The origin of the charge on the sol
theory a double layer of particles in most cases is due to the
ions appear at the surface preferential adsorption of either
of solid. positive or negative ions on their
surface.
  The sol particles acquire become negatively charged, (at
electrical charge in any one or this stage KI is in excess, and I-
more of the following ways. being common to AgI)

1) Due to the Dissociation of

Surface Molecules
i. Some colloidal particles develope
electrical charge due to the But, when a small quantity of
dissociation / ionisation of the potassium iodide (KI) solution is
surface molecules. added to a large quantity of silver
ii. The charge on the colloidal particles nitrate solution AgNO3 ; the colloidal
is balanced by the oppositely silver iodide particles adsorb  Ag
charged ions in the sol. from the solution to become
iii. For example, an aqueous solution positively charged, (at this stage
of soap (sodium palmitate) which AgNO3 is in excess and  Ag is
dissociates into ions as, common to AgI),

The cations (Na+) pass into the

solution while the anions
(C15H31COO-) have a tendency
to form aggregates due to weak
attractive forces present in the
hydrocarbon chains.

2) Due to Frictional
Electrification
i. It is believed that the frictional
electrification due to the rubbing of
the dispersed phase particles with
iii. Depending upon the nature of
that of dispersion medium results in
charge on the particles of the
some charge on the colloidal
dispersed phase, the colloidal
particles.
solutions are classified into
ii. The dispersion medium must also
positively charged and
get some charge, because of the
negatively charged colloids.
friction. Since it does not carry any
Some typical examples are as
charge, the theory does not seem to
follows:
be correct.
Negatively Charged Colloids
 Metal sulphides (sulfides): As2S3
3) Due to Selective Adsorption
(arsenic trisulfide), CdS (cadmium
of Ions
sulfide)
i. The particles constituting the
 Metal dispersions: Pt, Au, Ag
dispersed phase adsorb only those
 Acid dyes: Eosin, congo red
ions preferentially which are
 Sols of starch, gums, gold, gelatin
common with their own lattice
etc.
ions.
ii. For example, when a small quantity
of silver nitrate (AgNO3) solution is
added to a large quantity of Positively Charged Colloids
potassium iodide (KI) solution, the  Metal Hydroxides: Al(OH)3, Fe(OH)3
colloidal particles of silver iodide  Metal oxides: TiO2
adsorb I- from the solution to  Basic dyes: Methylene blue
  Haemoglobin  Both sols may be partially or
 Sulphur sol completely precipitated
 Mutual coagulation or
meteral coagulation: mixing
STABILITY OF SOLS of ferric hydroxide (+ve sol)
and arsenious sulphide (–ve
 Sols are thermodynamically unstable sol) bring them in precipitated
 The dispersed phase (colloidal form
particles) tend to separate long due
to the Van der Waal’s attractive 3) By boiling
forces  the adsorbed layer is
However, sols tend to exhibit some disturbed due to increased
stability due to: collisions with the molecules
of dispersion medium
1) Stronger repulsive forces  This reduces the charge on the
between similarly charged particles and ultimately, they
particles. settle down to form a
2) Particle-solvent interaction precipitate.
 Due to strong particle-solvent
(dispersion medium)
interactions, the colloidal 4) By persistent dialysis
particles get strongly solvated.  On prolonged dialysis, the
traces of the electrolyte
present in the sol are removed
almost completely and the
colloids become unstable.

COAGULATION OR FOCCULATION OR 5) By addition of electrolytes

PRECIPITATION  When an electrolyte is added
to sol, the colloidal particles
 “The phenomenon of the take up ions carrying opposite
precipitation of a colloidal solution charge from the electrolyte. As
by the addition of the excess of an a result, their charge gets
electrolyte is called coagulation or neutralized and this causes
flocculation.” the uncharged, particles to
The coagulation of the lyophobic come closer and to get
sols can be carried out by following coagulated or precipitated
methods:  For example, if BaCl2 solution
is added to As2S3 sol the  2
1) By Electrophoresis Ba ions are attracted by the
 the colloidal particles move negatively charged sol
towards oppositely charged particles and their charge gets
electrode. neutralized. This lead to
 When in contact with the coagulation.
electrode for long, these are
discharged and 6) Hardy Schulze rule
precipitated  The coagulation capacity of
different electrolytes is
2) By mixing two oppositely different. It depends upon the
charged sols valency of the active ion
 oppositely charged sols mixed called flocculating ion, which
in almost equal proportions, is the ion carrying charge
their charges are opposite to the charge on the
neutralized. colloidal particles.
 “According to Hardy Schulze rule, COAGULATION OF LYOPHILIC SOLS
greater the valency of the
 2 Factors responsible for the stability
active ion or flocculating ion,
of lyophilic sols:
greater will be its coagulating
1) Charge
power” thus, Hardy Schulze law
2) Solvation of the Colloid
state:
Particles
i. The ions carrying the  When these two factors are
charge opposite to that of removed, a lyophilic sol can be
sol particles are effective in coagulated. This is done by
causing coagulation of the 1) Adding electrolyte
sol. 2) Adding suitable solvent

ii. Coagulating power of an  When solvent such as alcohol and

electrolyte is directly acetone are added to hydrophilic sols
proportional to the valency the dehydration of dispersed
of the active ions (ions phase occurs. Under this condition
causing coagulation). a small quantity of electrolyte can
bring about coagulation.
For example: to coagulate negative sol
of As2S3, the coagulation power of
PROTECTION OF COLLOIDS AND GOLD
different cations has been found to
NUMBER
decrease in the order as, Al2+ > Mg2+ >
Na+  Lyophilic sols are more stable
than lyophobic sols.
Similarly, to coagulate a positive sol such
 Lyophobic sols can be easily
as Fe(OH)3, the coagulating power of
coagulated by the addition of small
different anions has been found to
quantity of an electrolyte.
decrease in the order : [Fe(CN)6]4- >
 When a lyophilic sol is added to any
(PO4)3- > (SO4)2- > Cl-
lyophobic sol, it becomes less
sensitive towards electrolytes.
Thus, lyophilic colloids can prevent
7) Coagulation of Flocculation the coagulation of any lyophobic sol.
Value  “The phenomenon of preventing the
 “The minimum concentration coagulation of a lyophobic sol due to
of an electrolyte which is the addition of some lyophilic colloid
required to cause the is called sol protection or
coagulation or flocculation of a protection of colloids.”
sol is known as flocculation  The protecting power of different
value.” or protective (lyophilic) colloids is
 “The number of millimoles of different. The efficiency of any
an electrolyte required to protective colloid is expressed in
bring about the coagulation of terms of gold number.
one litre of a colloidal solution
is called its flocculation
value.” Gold number: Zsigmondy introduced a
term called gold number to describe the
 Coagulation value or protective power of different colloids. This
flocculating value  is defined as, “weight of the dried
1/Coagulating power protective agent in milligrams, which
when added to 10 ml of a standard
gold sol (0.0053 to 0.0058%) is just
sufficient to prevent a colour change
from red to blue on the addition of 1 ml
of 10 % sodium chloride solution, is equal
to the gold number of that protective
colloid.” Thus, smaller is the gold
number, higher is the protective action
of the protective agent.
Protective power  1 / Gold Number

Table : 14.4 Gold numbers of some

hydrophilic substances
iii. The salting out efficiency of an
Hydrophilic Gold Number
electrolyte depends upon the
Substance
tendency of its constituent ions to
Gelatin 0.005 - 0.01
get hydrated i.e, the tendency to
Sodium 0.01
Caseinate squeeze out water initially fed up
Hemoglobin 0.03 - 0.07 with the colloidal particle.
Gum Arabic 0.15 - 0.25
Sodium Oleate 0.4 - 1.0 iv. The cations and the anions can be
Gum 2 arranged in the decreasing order of
Tragacanth the salting out power, such an
Potato Starch 25 arrangement is called lyotropic
series.

Congo Rubin number: Ostwald Cations: Mg2+ > Ca2+ > Li+ >
introduced congo rubin number to account Na+ > K+ > Nh4+ > Rb+ >
for protective nature of colloids. It is Cs+
defined as “the amount of protective
colloid in milligrams which prevents Anions: Citrate3- > (SO4)2- > Cl-
colour change in 100 ml of 0.01 % > NH3- > I- > CNS-
congo rubin dye to which 0.16 g
equivalent of KCl is added.” Ammonium sulphate, due to its
very high solubility in water, is
oftenly used for precipitating
MECHANISMS OF SOL PROTECTION proteins from aqueous solutions.
i. The actual mechanisms of sol
protection is very complex. v. The precipitation of lyophilic colloids
However, it may be due to the can also be affected by the addition
adsorption of the protective colloid of organic solvents of non-
on the lyophobic sol particles, electrolytes.
followed by solvation.
Thus, it stabilizes the sol via For example,
solvation effects  the addition of acetone or
ii. Solvation effects contribute much alcohol to aqueous gelatin
towards the stability of lyophilic solution causes precipitation
systems. For example, gelatin has a of gelatin.
sufficiently strong affinity for water. It  Addition of petroleum ether to
is only because of the solvation a solution of rubber in
effects that even the addition of benzene causes the
electrolytes in small amounts does precipitation of rubber.
not cause any flocculation of
hydrophilic sols. However ,at higher
concentration, precipitation
occurs. This phenomenon is called EMULSION
salting out.
  “The colloidal systems in which fine containing polyvalent metal ions
droplets of one liquid are dispersed indicating the negative charge on the
in another liquid are called globules.
emulsions the two liquids otherwise
being mutually immiscible.” or iii. The size of the dispersed
 “Emulsion are the colloidal particles in emulsions is larger
solutions in which both the dispersed than those in the sols. It ranges
phase and the dispersion medium from 1000 Å to 10,000 Å. However,
are liquids.” the size is smaller than the
 A good example of an emulsion is particles in suspensioins.
milk in which fat globules are
dispersed in water. The size of the iv. Emulsions can be converted into
emulsified globules is generally of two separate liquids by heating,
the order of 6 10 m. Emulsion centrifuging, freezing etc. This
resemble lyophobic sols in some process is also known as
properties. demulsification.
TYPES OF EMULSION
APPLICATIONS OF EMULSIONS
Depending upon the nature of the
dispersed phase, the emulsions are i. Concentration of ores in
classified as; metallurgy
ii. In medicine (Emulsion water-in-oil
1) Oil-in-water emulsions (O/W)
type)
- oil is present as the
iii. Cleansing action of soaps.
dispersed phase and water
iv. Milk, which is an important
as the dispersion medium
constituent of our diet an
(continuous phase)
emulsion of fat in water.
- Milk is an example of the oil-
v. Digestion of fats in intestine is
in-water type of emulsion. In
through emulsification.
milk liquid fat globules are
dispersed in water. Other
examples are, vanishing
GELS
cream etc.
1. “A gel is a colloidal system in which
2) Water-in-oil emulsion (W/O) a liquid is dispersed in a solid.”
- water forms the dispersed
phase, and the oil acts as the 2. The lyophilic sols may be coagulated
dispersion medium to give a semisolid jelly like mass,
- These emulsions are also which encloses all the liquid present
termed oil emulsions. in the sol.
- Butter and cold cream are
typical examples of this types Gelation - The process of gel
of emulsions. Other examples formation
are cod liver oil etc. Gel - the colloidal system formed

3. Thixotropy - some gels are known

PROPERTIES OF EMULSION
to liquify on shaking and reset on
i. Emulsions show all the being allowed to stand. This
characteristic properties of reversible sol-gel transformation
colloidal solution such as Brownian is called thixotropy.
movement, Tyndall effect,
electrophoresis etc. 4. The common examples of gel:
- gum arabic,
ii. These are coagulated by the - gelatin,
addition of electrolytes - processed cheese,
 - silicic acid,
- ferric hydroxide etc.

5. Synereises or weeping - Gels may

shrink by losing some liquid help
them.

6. Gels may be classified into two

types:
 Elastic Gels
- gels which possess the
property of elasticity.
- They readily change their
shape on applying force and
return to original shape when
the applied force is removed.
- Common examples are
gelatin, agar-agar, starch
etc.

 Non- elastic Gels

- These are the gels which are
rigid and do not have the
property of elasticity.
- For example, silica gel.