N. Vamsi Krishna
Introduction:
• Colloidal system are the ones in which one of three states (solid, liquid and gas)
is finely dispersed in another.
• Colloids were named first in the early 19th century by the Father of Physical
Chemistry, Thomas Graham (1805-1869). In 1920's and 1930's, the importance of
colloids to industrial processes and biochemistry changed everything making it a
hot field.
• A colloidal system consists of an internal phase (dispersion phase), which is the
material of colloidal dimensions, and an external phase (dispersion medium)
.Similar to the terms solute and solvent used for simple solutions.
• As the particles of a colloid system become smaller and smaller, we go over
imperceptibly from a two-phase colloid to a single-phase solution, and there is no
definite boundary i.e. true solution.
Table 1: Characteristics of Suspension, Colloid and Solution
Properties
Suspension
Colloid
Solution
1.
Particle size
>100nm
1-100nm
<1nm
2.
Separation
1)ordinary filtration
2) ultra filtration
possible
possible
not possible
possible
not possible
not possible
Settles under
gravity
Settles on
Centrifugation
Does not
settle
opaque
Generally clear
clear
Not possible
Diffuses slowly
Diffuses rapidly
3.
Settling
4.
Appearance
5.
Diffusion
6.
Brownian motion
shows
shows
Not observable
7.
Tyndall effect
shows
shows
Not observable
• Total interfacial area (NAp) per 1cm3 of colloid is given by,
NAp = N4πR2 = 4π R2Ф/(4/3π R3 ) = 3Ф/R
Where R = radius, Ф = volume fraction
Number of particles per 1cm3 of colloid
N = Ф/Vp = Ф/ (4/3π R3 )
• The large area emphasizes surface effects relative to volume effects, giving
colloids different properties than those of bulk matter.
• Because of their various properties, colloidal systems have many applications
in various industries like food industry (diary products, chocolate etc),
pharmaceutical and cosmetic industry (gels and emulsions), Photographic
industry (films, carbon paper, ink etc), Electrical and electronic industry (liquid
crystals, isolating materials etc), paint industry and agrochemical industry.
Classification of colloids:
Colloids can be classified mainly by 3 ways;
• Classification Based on the State of the Dispersed Phase and Dispersion Medium
• Classification of Colloids Based on Type of Particles of the Dispersed Phase
• Classification Based on the Nature of Interaction Between Dispersed Phase and
Dispersion Medium
Classification Based on the State of the Dispersed Phase and Dispersion Medium
Table 2: Types of colloids
Dispersion Medium
Dispersed phase Type of colloid
Example
Gas
Liquid
Aerosol
Fog, clouds
Gas
Solid
Aerosol
Smoke
Liquid
Gas
Foam
Whipped cream,
soda water
Liquid
Liquid
Emulsion
Milk, hair cream
Liquid
Solid
Sol
Paints, cell fluids
Solid
Gas
Foam
Pumice, plastic foams
Solid
Liquid
Gel
Jelly, cheese
Solid
Solid
Solid Sol
Ruby glass
Classification of Colloids Based on Type of Particles of the Dispersed Phase
a) Multimolecular colloids usually have lyophobic character.
Ex: gold and sulphur sols
b) Macromolecular colloids resemble true solutions
Ex: proteins, cellulose, starch and polymers such as polyethylene, nylon and
polystyrene
c) Associated colloids type of micelle it forms depends on the nature of solvent
(hydrophilic or hydrophobic)
Kraft Temperature (Tk )
Critical micelle concentration (CMC)
Ex: soaps and synthetic detergents
Surfactant
Micelle
Classification Based on the Nature of Interaction Between Dispersed Phase and Dispersion Medium
Table 3: Distingustion between lyophilic and lyophobic colloids
Property
Lyophilic sols (suspensoid)
Lyophobic sols (Emulsoid)
Surface tension
Lower than that of the medium
Same as that of the medium
Much higher than that of the
medium
Same as that of the medium
Reversibility
Reversible
Irreversible
Stability
More stable
Less stable
Visibility
Particles can’t be detected
even under ultra
microscope
Particles can be detected under
ultra microscope.
Migration
Particles may migrate in either
direction or do not migrate
in an electric field because
do not carry any charge.
Action of electrolyte
Addition of smaller quantity of
electrolyte has little effect
Coagulation takes place
Hydration
Extensive hydration takes
place
No hydration
Examples
Gum, gelatin, starch, proteins,
rubber etc.
Metals like Ag and Au, hydroxides
like Al(OH3), Fe(OH)3 metal
sulphides like AS2S3 etc.
Viscosity
Particles migrate either towards
cathode or anode in an electric
field because they carry
charge.
Preparation of colloids:
1) lyophilic sols
2) lyophobic sols
easy to prepare
unstable and irreversible, difficult to prepare
a) Condensation or aggregation method
b) Dispersion method
a) Condensation or aggregation method
Chemical methods:
•
By Reduction: Metal sols are generally prepared by this method.
HCHO
Ex: HAuCl4¯ (aq)
AgO (aq)
•
H2, 50-60 0C
Ag (sol)
By Oxidation: Sulphur sols are easily obtained by the oxidation.
Ex: 2H2S+SO2
•
Au (sol)
2H2O + 3S (sol)
By double decomposition:
Ex: As2O3 (aq) + 3H2S
As2S3 (sol)+ 3H20
• By hydrolysis: To obtain sols of oxides or hydroxides of weakly electropositive metals.
Ex: FeCl3 + 3H2O
Fe(OH)3 (sol) + 3HCl
Physical methods:
• By excessive cooling: used to prepare a colloidal solution of ice.
• By exchange of solvent:
Ex: S (alc) + H2O
S (sol)
• By change of physical state: Sols of substances like mercury and sulphur are prepared
b) Dispersion method
• Mechanical disintegration: Colloidal solutions of black ink, paints, varnishes, dyes etc.
and food products like concentrated milk, food additives etc.
Figure1: Pictorial view of colloidal mill
• Peptization:
The process of dispersing a precipitate in to a colloidal solution by adding small quantity
of electrolyte is called peptization.
Ex: Fe(OH)3 ↓+ FeCl3
[ Fe(OH)3Fe3+] (sol) + 3Cl¯
• Bredig’s arc method:
used to prepare sols of platinum, silver, copper or gold. Traces of KOH are required to
stabilize the colloidal solution.
Figure 2: Bredig’s arc method
Generally used methods to prepare other colloidal systems are;
− Aerosol is formed by passing gas jet to a liquid spray.
− Emulsions are usually prepared by vigorously shaking the two constituents together,
often with the addition of an emulsifying agent . The phase in which emulsifier is more
soluble forms the outer layer. lyophobic colloids .
− Gels are often formed by cooling lyophilic sols that contain large linear molecules and
have a much greater viscosity than the solvent.
Elastic and Rigid gels.
− Foams are formed when gas and liquid are mixed together in a container and shaken
along with a foaming agent.
Purification of colloidal solutions:
• Colloidal solutions prepared by the above methods contain some soluble impurities and
excess of electrolytes; these have to be removed to obtain pure sols.
•Generally a) Dialysis, b) ultra filtration and c) ultra centrifugation are the techniques used
• Ultra filtration: colloidal sols are filtered through ultra-filtrers. pore size of filter paper is
decreased such that it will restrict the passage of colloidal particles.
• Ultra-centrifugation: Centrifugation is carried out at very high speeds such that the
colloidal particles settle down at the bottom of the tube and the impurities remain in the
solution .
• It is important to note that above methods to purify colloidal solution do not produce
100% solution.
Figure 3: Electro-dialysis
• used for purification of blood in case of kidney failure.
Properties of colloids:
a) Optical properties
Figure 4: Size dependent change of colour in Au sol
Figure 6: Schematic drawing of ultra microscope
Opalescence in colloids is due to scattering of light by particles. This effect was studied
by Tyndall and is generally known as tyndall effect. Tyndall observed that scattered beam
to be polarized and intensity of the same to be dependent on position of observer, nature
of system and wavelength of light used.
b) Colligative Properties
• The magnitudes of these properties for colloidal solutions are much smaller than those
obtained for true solutions. They exhibit measurable osmotic pressures. This property is
used for the determination of the average molecular masses of the colloids.
c) Kinetic properties
liquid undergoes continuous chaotic and random motions. This motion of the particles
is called Brownian motion. This Brownian motion is found to decrease by increase in
particle size or by increase in viscosity of medium.
r = (3bυ/ 4π nd) ⅓
where, b = number of grams of substance per dm3
n = number of particles observed in view
υ = volume dm3
d = density of dry substance
c) Electrical properties
Lyophobic sols carry charge, because of which they get repelled on approaching another
particle avoiding their coagulation.
i) Due to presence of electrolytes
Ex: positive charge on ferric hydroxide sol prepared by hydrolysis of ferric chloride is due
to adsorption of Fe3+ ions on surface.
ii) dissociation of molecular electrolytes adsorbed on the surface of particles.
Ex: H2S molecules get adsorbed on sulphides during precipitation. H2S undergoes
dissociation and the hydrogen ions are lost. The particles become negatively charged due
to (S2-) which are left on the colloidal particles.
iii) by dissociation of surface molecules
Ex: soaps, proteins
Figure 7: Electrical double layer
The influence of net charge decreases with distance and so the number of oppositely
charged ions, equaling the number of ions of both charges prevailing electro neutrality.
The difference in potential between the surface of shear plane and electro neutral region
of solution is called Zeta potential (ζ), which is given by Helmholtz- Smoluchowski
equation.
ζ = 4πηu/εr
where η = viscosity of dispersion medium
εr = relative permittivity of dispersion medium
u = mobility of colloidal particle
• In case of lyophobic colloids;
(i) Coagulation is brought about by ions having opposite charge to that of the sol.
(ii) The efficiency of an ion to cause coagulation depends upon the square of its valency.
Thus efficacy of cations Al3+>Mg2+> Na+ and PO43->SO42->Cl-.
(iii)The minimum concentration of an electrolyte required to cause coagulation of a sol
is called its flocculation value.
The other methods to coagulate sol are by mutual mixing of oppositely charged sols,
persistent dialysis and by mechanical means.
• In case of lyophilic sols,
i) water of solvation (cage of water surrounding) prevents particles from coagulation.
ii) Their coagulation is due to removal of salvation water a s electrolytes binds them.
Thus efficacy of cations Al3+>Mg2+> Ca2+> Ba2+>Na+>K+ and efficacy of anions PO43>SO42->Cl->NO3-> ClO3->I- for coagulation of lyophilic sols and this is known as lyotropic
series or Hofmeister series.
iii) Zsigmondy quantified stabilizing action of protective colloids by gold number, which is
defined as largest number of milligrams of a protective colloid when added to 10 ml of
standard gold sol, just fails to prevent the colour change from red to blue upon addition
of 1 ml of 10% sodium chloride solution
Properties of colloids:
• Purification of water by alum (coagulation)
• In rubber platting
• In tanning
• Artificial rains
• Formation of deltas (coagulation)
• Blood clot formation
• Colloidal medicine: Argyrol and protargyrol are colloidal solution of silver and are used
as eye lotions .Colloidal sulphur is used as disinfectant and colloidal gold, calcium and
iron are used as tonics. A wide variety of medicines are emulsions.
• Coating of Photographic plates
• Sewage disposal
• Metallurgy
• Smoke precipitation (Coagulation)
Figure 8: Schematic diagram of Cottrell smoke precipitator