COLLOIDS

PARTICLE SIZE :
Particle size < 1nm diameter 1 - 1000nm diameter > 1000nm diameter
True solution Colloids Suspension

PHASE OF COLLOIDS :
A colloidal system is heterogeneous in character. It consists of two phases, namely a dispersed phase and a
dispersion medium.
(a) Dispersed Phase: It is the component present in small proportion and is just like a solute in a true solution. For
example, in the colloidal state of sulphur in water, the former acts as a dispersed phase.
(b) Dispersion Medium: It is normally the component present in excess and is just like a solvent in a solution.
The particles of the dispersed phase are scattered in the dispersion medium in a colloidal system.
CLASSIFICATION OF COLLOIDS:
Colloids can be classified in a number of ways based upon some of their important characteristics.

(1) Physical state of Dispersed Phase & Dispersion Medium:

Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gaseous, eight
types of colloidal system are possible. A gas mixed with another gas forms a homogeneous mixture and cannot
form a colloidal system. Typical examples of various type along with their characteristic names are given in the
following table.
Common Colloidal Systems

Dispersed Phase Dispersion medium Colloidal system Examples

Gas Liquid Foam or froth Soap sols, lemonade froth,

Whipped cream.

Gas Solid Solid foam Pumice stone, styrene, foam,

Foam rubber.

Liquid Gas Aerosols of Fog, clouds, fine insecticide

Liquids sprays.

Liquid Liquid Emulsions Milk

Liquid Solid Gels Cheese, butter, boot polish,

Table jellies, curd.

Solid Gas Aerosols of Smoke, dust

Solid

Solid Liquid Sols Mist, paint, starch dispersed in

water, gold sol, muddy water, inks.

Solid Solid Solid sols Ruby glass, some gem stones.

* A colloidal system in which the dispersion medium is a liquid or gas are called sols. They are called hydrosols or
aqua sols, if the dispersion medium is water. When the dispersion medium is alcohol or benzene, they are
accordingly called alcosols or benzosol.
* Colloidal systems in which the dispersion medium is a gas are called aerosols.
* Colloids in which the dispersion medium is a solid are called gels, e.g. cheese etc. They have a more rigid
structure. Some colloids, such as gelatin, can behave both as a sol and a gel. At high temperature and low
concentration of gelatin, the colloid is a hydrosol. But at low temperature and high gelatin concentration, the
hydrosol can change into a gel.

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(2) Based on interaction or affinity of phases: On the basis of the affinity or interaction between the dispersed
phase and the dispersion medium, the colloids may be classified into two types:

(i) Lyophilic Colloids: The colloidal system in which the particle of dispersed phase have great affinity for the
dispersion medium, are called lyophilic (solvent-loving) colloids. In such colloids, the dispersed phase does not
get easily precipitated and the sols are more stable. Such colloidal systems, even if precipitated (say by
evaporation), may be reconverted to the colloidal state by simply agitating them with the dispersion medium.
Hence lyophilic colloids are reversible. When the dispersion medium is water, these are called hydrophilic
colloids. Some common examples of lyophilic colloids are gum, gelatin, starch, rubber, proteins, etc.

(ii) Lyophobic colloids: The colloidal system in which the dispersed phase have no affinity for the dispersion
medium are called lyophobic (solvent hating) colloids. They are easily precipitated (or coagulated) on the addition
of small amounts of the electrolyte, by heating or by shaking. They are less stable and irreversible. When the
dispersion medium is water, these are known as hydrophobic colloids. Examples of lyophobic colloids include
sols of metals and their insoluble compounds like sulphides and oxides. Upon the addition of electrolytes,
lyophilic sols undergo coagulation and precipitation.
(3) Based on type of particles of the dispersed phase: Depending upon the molecular size, the colloidal system has
been classified into three classes:
(i) Multimolecular colloids : The multimolecular colloidal particles consists of aggregate of atoms of small
molecules with diameters less than 109 m or 1 nm. For example, a sol of gold contains particles of various sizes
having several atoms. A sol of sulphur consists of particles containing a thousand or so S8 molecules. These particles
are held together by van der Waal's forces. These are usually lyophobic sols.
(ii) Macromolecular colloids: The macromolecular colloidal particles themselves are large molecules. They have
very high molecular weights varying from thousands to millions. These substances are generally polymers. Naturally
occurring macromolecules are such as starch, cellulose and proteins. Artificial macromolecules are such as
polyethylene, nylon, polystyrene, dacron, synthetic rubber, plastics, etc. The size of these molecules are comparable to
those of colloidal particles and therefore, their dispersions are known as macromolecular colloids. Their dispersion
also resemble true solutions in some respects.
(iii) Associated colloids or Micelles: These colloids behave as normal electrolytes at low concentrations
but as colloids at higher concentrations. This is because at higher concentrations, they form aggregated
(associated) particles called micelles. Soap and synthetic detergents are examples of associated colloids.
They furnish ions which may have colloidal dimensions.
The micelles are formed by the association of dispersed particles above a certain concentration and certain
minimum concentration is required for the process of aggregation to take place. The minimum concentration required
for micelle formation is called critical micelle concentration (CMC) and its value depends upon the nature of the
dispersed phase. The formation of micelles also require a minimum temperature known as Kraft temperature (TK).
Mechanism of Micelle Formation:
Micelles are formed by surface active molecules called surfactants such as soaps and detergents. These
molecules have lyophilic group at one end and a lyphobic group at the other end. Let us take the
example of a soap (say sodium oleate, C17H33COO-Na+) in water. The long hydrocarbon part of oleate
-
radical (C17H33) is lyophobic end while COO part is lyophilic end. When the concentration of the
solution is below its CMC, sodium oleate behaves as a normal electrolyte and ionizes to give
-
C17H33COO and Na+ ions. When the concentration exceeds CMC, the lyophobic part starts receding
-
away from the solvent and tends to approach each other. However, the polar COO ends tends to interact
with the solvent (water). This finally leads to the formation of bigger molecules having the dimensions
of colloidal particles. Thus 100 or more oleate ions are grouped together in a spherical way keeping their
-
hydrocarbon parts inside and the COO part remains projected in water.

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 LIST OF SURFACTANTS AND THEIR CRITICAL MICELLE CONCENTRATION (CMC)

Surfactant CMC (g/1) Temp.(C)

Anionic
CH3(CH2)6COONa 6.5 10 1 20
CH3(CH2)10COONa 5.6 2070
CH3(CH2)7OSO3Na 3.0 101 2550
CH3(CH2)11OSO3Na 2.6 2560
CH3(CH2)5C6H4SO3Na 9.8 75
CH3(CH2)11C6H4SO3Na 4.0 101 5075

Cationic
CH3(CH2)9NH3Cl 8.5 25
CH3(CH2)11NH3Cl 2.7 3050
CH3(CH2)7N(CH3)3Br 7.8 10 1 25
CH3(CH2)11N(CH3)3Br 5.4 25
Non-ionic
CH3(CH2)7C6H11O6 7.3 25
C12H20O9(C16H31O2)2 1.1. 102 20
CH3(CH2)10COOC12H21O10 7.1 10 3 50

PREPARATION OF COLLOIDAL SOLUTIONS

(1) Preparation of lyophilic sols: The colloidal solutions of lyophilic colloids like starch, glue, gelatin etc., can be
readily prepared by dissolving these substances in water either in clod or on warming. Solutions of colloidal
electrolytes such as soaps and dye stuffs can also be prepared similarly.

(2) Preparation of lyophobic sols: To get a substance in colloidal form either the substance in bulk is broken down
into fine particles of colloidal dimension (1 to 103 ) or increasing the size of molecular particles as to form
larger aggregates. In some cases, a third substance is usually added to increase the stability of the sol. These
substances are called stabilizers. Thus, there are two ways by which the lyophobic sols can be prepared:
(i) Dispersion methods: By splitting coarse aggregates of a substance into colloidal size.
(ii) Condensation methods: By aggregating very small particles (atoms, ions or molecules) into colloidal size.
S. No. Dispersion methods Condensation methods
1. Mechanical dispersion 1. Exchange of solvents
2. Electrodispersion 2. Change of physical state
3. Ultrasonic dispersion 3. Chemical methods:
(i) Double decomposition
(ii) Oxidation
(iii) Reduction
(iv) Hydrolysis

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 (i) Dispersion Methods

(1) Mechanical dispersion: Solid material is first finely ground by usual methods. It is then mixed with dispersion
medium which gives a coarse suspension. The suspension is now introduced into the colloid mill. The simplest
form of colloid mill consists of two metal discs held at a small distance apart from one another and capable of
revolving at a very high speed (about 7000 revolutions per minute) in opposite directions. The particles are
ground down to colloidal size and are then dispersed in the liquid. A stabilizer is often added to stabilize the
colloidal solution. Colloidal graphite (a lubricant) and printing ink are made by this method. Tannin is used as a
stabilizer in the preparation of colloidal graphite and gum Arabic in lampblack colloidal solution (Indian ink).
(2) Electro-dispersion (Bredig's arc method): This method is suitable for the preparation of colloidal
solutions of metals like gold, silver, platinum, etc. An arc is struck between the metal electrodes under
the surface of water containing some stabilizing agents such as a track of KOH. The water is cooled by
immersing the container in an ice bath. The intense heat of the arc vaporizes some of the metal which
condenses under cold water.

(3) Peptization: The dispersion of a freshly precipitated material into colloidal solution by the action of an
electrolyte in solution is termed peptization. The electrolyte used is called a peptizing agent.
A few examples of sols obtained by peptization are:
(i) Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride solution at once
forms a dark reddish brown solution. Ferric chloride acts as a peptizing agent.
(ii) Freshly prepared stannic oxide on treatment with a small amount of dilute hydrochloric acid forms a
stable colloidal solution of stannic oxide.
(iii) Freshly precipitated silver chloride can be converted into a colloidal solution by a small amount of
hydrochloric acid.
(iv) Cadmium sulphide can be peptized with the help of hydrogen sulphide.
(v) Cellulose is peptised by the addition of organic solvents, such as ethanol-ether mixture, leading to
familiar collodion sol. In this example, the solvent itself is peptizing agent.
The process of peptization thus involves the adsorption of suitable ions (supplied by the
electrolyte addedparticularly a common ion) and electrically charged particles then split from the
precipitate as colloidal particles.

(ii) Condensation methods:

(1) Chemical methods: The chemical methods involve chemical reactions in a medium in which the dispersed phase
is sparingly soluble. A condition of supersaturation is produced but the actual precipitation is avoided. Some
familiar reactions used are:
(a) Double decomposition: (i) Arsenious sulphide sol: A 1% solution of arsenious oxide is prepared in hot water.
The solution is cooled, filtered and is then gradually in hot water saturated with hydrogen sulphide. This is
continued till an intense yellow-colored solution is obtained. Excess of H2S is removed by bubbling hydrogen
through the solution.
As2O3 + 3H2S As2S3 + 3H2O
Yellow sol
(ii) Antimony sulphide sol: A 0.5% solution of potassium antimonyl tartarate is added drop by drop to water
saturated with H2 S, whilst H2S is being passed through the solution. Orange colored solution of antimony
sulphide is obtained.
CH(OH)COOK CH(OH)COOK
2 + 3H2S 2
CH(OH)COO(sbO) CH(OH)COOH + Sb2S3 + 2H2O
Orange sol

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 (b) Oxidation: A colloidal solution of sulphur is obtained by passing H2O into a solution of sulphur dioxide.
2H 2S+SO 2
2H 2 O+3S (sol)
Sulphur sol can also be obtained when H2S is bubbled through an oxidising agent (bromine water or nitric acid).
(c) Reduction: Colloidal solutions of metals like gold, silver, platinum, lead, etc., can be obtained when their salts
solutions are acted upon by reducing agents.
2AuCl3 + 3SnCl2 3SnCl4 + 2Au (sol)
Organic reducing agents such as formaldehyde, phenyl hydrazine, tannic acid, etc., can also be used.
AgNO3 + tannic acid Silver sol
AuCl3 + tannic acid Gold sol
(d) Hydrolysis: Colloidal solutions of some salts can be prepared by hydrolysis. A colloidal solution of ferric
hydroxide is obtained by boiling a dilute solution of ferric chloride.
FeCl3 +3H 2 O
Fe(OH)3 (red coloured sol) +3HCl

Charge on colloidal particles: Majority of the colloidal particles are found to be charged which is proved by the
migration of colloidal particles during electrophoresis.
Origin of charge: Various reasons have been given regarding the origin of charge on the colloidal particles.

These are given below:

(i) Frictional electrification: The friction due to the rubbing of the dispersed phase particles with medium
molecules may be responsible for the charge.
(ii) Dissociation of the surface molecules: This happens in lyophilic colloids where charges often result
largely from the ionization of the material constituting the dispersed phase. It leads to electric charge on
colloidal particles. For example, an aqueous solution of a soap (sodium palmitate) dissociates into ions.

C15H31COONa C15H31COO + Na+
sod. palmitate
The Na+ ions pass into the solution while C15H31COO- ions have a tendency to form aggregates due to
weak attractive forces present in the hydrocarbon chains. Thus, the anions which are of colloidal size
bear negative charge.
(iii)Preferential adsorption of ions from solution (this is believed to be the most probable reason): The
charge on the colloidal particles is generally acquired by preferentially adsorbing positive or negative
ions from the electrolyte.

E M RAO 5
 Examples:
1. When silver nitrate solution is added to KI solution, the precipitated AgI adsorbs iodide ions
from the dispersion medium and the colloidal particles become negatively charged. However,
when KI is added to AgNO3 solution, the precipitated AgI adsorbs Ag+ ions from the
dispersion medium and the colloidal particles become positively charged.
AgI/I- AgI/Ag+
2. When FeCl3 is added to hot water Fe2O3.xH2O/Fe+3
When FeCl3 is added to hot aqueous NaOH Fe2O3.xH2O/OH-

(iv) Capture of electron: It is from air during preparation of sol by Bredig's arc method.
(v) Dissociation of molecular electrolytes on the surface of particles: H2S molecules get adsorbed on
sulphides during precipitation. By dissociation of H2S, H+ ions are lost and colloidal particles become
negatively charged.

Electrically charged sols.

Positively charged sols Negatively charged sols

1. Ferric hydroxide, aluminium hydroxide Metals such as Pt, Au, Ag etc. Metallic sulphides,
Fe2O3.xH2O, Al 2O3.xH2O e.g. arsenius sulphide, Sb2S3, CdS sols.
2. Basic dyes such as methylene blue Starch, clay, gelatin, silicic acid etc.
3. Haemoglobin Acid dyes, such as eosin.
4. Oxides, e.g., TiO2 sol

PURIFICATION OF COLLOIDAL SOLUTIONS

Colloidal solutions prepared by above methods generally contain excessive amount of electrolytes and some other
soluble impurities. The presence of traces of electrolyte is essential for the stability of the colloidal solution but
larger quantities coagulate it. It is, therefore, necessary to reduce the concentration of these soluble impurities to a
requisite minimum. The process used for reducing of these impurities to a requisite minimum is known as
purification of colloidal solution. The purification of colloidal solution is carried out by the following methods.
(i) Dialysis : It is a process of removing a dissolved substance from a colloidal solution by means of diffusion
through suitable membrane. Since, particles in true solution (ions or smaller molecules) can pass through animal
membranes (bladder) or parchment paper or cellophane sheet but colloidal particles do not, the above can be used
for dialysis.
(ii) Electro-dialysis : Ordinarily, the process of dialysis is quite slow. It can be made faster by applying an
electric field if the dissolved substance in the impure colloidal solution is only electrolyte. The process is then
named electro-dialysis. The colloidal solution is placed between two electrodes while pure water is taken in a
compartment on each side. Electrodes are fitted in the compartment as shown in figure the ions present in the
colloidal solution migrate out to the oppositely charged electrodes.
(iii) Ultrafiltration : Ultrafiltration is the process of separating the colloidal particles from the solvent and
soluble solutes present in the colloidal solution by especially prepared filters, which are permeable to all
substances except the colloidal particles.
Colloidal particles can pass through ordinary filter paper because the pores are too large. However, the
pores of filter paper can be reduced in size by impregnating with collodion solution and subsequently hardened by
soaking in formaldehyde. The usual colloidion is a 4% solution of nitro-cellulose in a mixture of alcohol and
ether.

E M RAO 6
PROPERTIES OF COLLOIDAL SOLUTIONS:

(1) Mechanical Properties:

(a) Brownian movement: Colloids particles exhibit a ceaseless random and swarming motion. This kinetic activity
of particles suspended in the liquid is called Brownina movement.
Robert Brown first observed this motion with pollen grains suspended in water.
Cause of movement: Brownian movement is due to bombardment of the dispersed particles by molecules of the
medium. The Brownian movement (figure) depends upon the size of sol. particles. With the increase in the size of
the particle, the chance of unequal bombardment decrease, and the Brownial movement too disappears. It is due to
the fact that the suspension fails to exhibit this phe-nomenon.
It should be noted that Brownian movement does not change with time but changes with temperatures.

(b) Sedimentation: Heavier sol. particle tend to settle down very slowly under the influence of gravity. This
phenomenon is called sedimentation.

(2) Optical Properties (Tyndal Effect):

When light passes through a medium that contains no particles larger than 10-9m in diameter, the path of the light
cannot be detected and the medium is said to be optically clear. When colloidal particles are present, however,
some of the light is scattered, and the incident beam passes through with diminished intensity.
When a strong and converging beam of light is passed through a colloidal solution, its path becomes
visible (bluish light) when viewed at right angles to the beam of light (figure). This effect is called Tyndall effect.
The light is observed as a bluish cone which is called Tyndall cone.
The Tyndall effect is due to scattering of light by the colloidal particles. The scattering of light
cannot be due to simple reflection, because the size of the particles is smaller than the wave, length of the visible
light and they are unable to reflect light waves. In fact, colloidal particles first absorb light and then a part of the
absorbed light is scattered from the surface of the colloidal particles as a light of shorter wavelength. Since
maximum scattering of light takes place at right angles to the place of incident light, it becomes visible when seen
from that direction.
The Tyndall effect is observed under the following conditions:
(i) The diameter of the dispersed particles must not be much smaller than the wavelength of light employed.
(ii) The refractive indices of the dispersed phase and the dispersion medium must differ widely. This condition is
fulfilled by lyophobic colloids.
It is important to note that Tyndall effect is not shown by true solutions as their particles are too
small to cause scattering. Tyndall effect has been used in devising ultramicroscope and in determining the number
of colloidal particles in a colloidal solution.

(3) Electrical Properties:

The two electrical properties of colloidal solutions are:
(a) Electrophoresis or Cataphoresis and (b) Electro-osmosis

(a) Electrophoresis or Cataphoresis: In a colloidal solution, the colloidal particles are electrically charged and the
dispersion medium has equal but opposite charge. Thus colloidal solution on the whole is electrically neutral.
When an electric current is passed through a colloidal solution, the charged particles move towards the oppositely
charged electrode and coagulate due to loss of charge.
The phenomenon involving the migration of colloidal particles under the influence of electric
field towards the oppositely charged electrodes, is called electrophoresis or cataphoresis. Both lyophilic and
lyophobic colloids show electrophoresis. This phenomenon is used to determine the charge on the colloidal
particles. For example, when a sol of ferric hydroxide is taken in a U-tube and subjected to electric field, the ferric
hydroxide (sol) particles get accumulated near the cathode. This shows that ferric hydroxide sol. particles are
positively charged. The sol particles of metals and their sulphides are found to be negatively charged while those
of metal hydroxides are positively charged. Basic dyes such as methylene blue, hemoglobin are positively charged
while acid dyes like are negatively charged.

E M RAO 7
 (b) Electro-osmosis: The phenomenon involving the migration of the dispersion medium and not the colloidal
particles under the influence of an electric field is electro-osmosis.
Take the pure solvent (dispersion medium) in two limbs of U-tube. In the lower middle portion of
U-tube, a porous diaphragm containing the colloidal system is present which divides the U-tube in two sections.
In each section of U-tube, an electrode is present, as shown in figure. When the electrode potential is applied to
the electrodes, the solid phase of sol. (colloidal system) cannot move but the solvent (dispersion medium) moves
through the porous diaphragm towards one of the electrodes. The direction of migration of dispersion medium due
to electro-osmosis determines the charge on sol. particles e.g., if the dispersion medium moves towards the
cathode (negative electrode), the sol. particles are also negatively charged because the dispersion medium is
positively charged as on the whole colloidal solution is neutral.

(c) Coagulation/flocculation: the colloidal sols are stable due to the presence of electric charges on the colloidal
particles. Because of the electrical repulsion, the particles do not come close to one another to form precipitates.
The removal of charge by any means will lead to the aggregation of particles and hence precipitation will occur
immediately.
The process by means of which the particles of the dispersed phase in a sol. are pecipitated is known as
coagulation.
If the coagulated particles instead of settling at the bottom of the container, float on the surface of the dispersion
medium, the coagulation is called flocculation.
Most of the sols are coagulated by adding an electrolyte of opposite sign. This is due to the fact that the colloidal
particles take up the ions of electrolyte whose charges are opposite to that on colloidal particles with the result
that charge on the colloidal particles is neutralized. Thus coagulation takes place. For example, arsenius sulphide
sol (negatively charged) is precipitated by adding barium chloride solution. It is due to the fact that the negatively
charged particles of the sol adsorp barium ions and get neutralized which lower the stability. As a result
precipitation takes place.
It is observed that different amounts of different electrolytes is required to bring coagulation of a particular
solution. The minimum amount of an electrolyte required to cause precipitation of one litre of a colloidal solution
is called coagulation value or flocculation value of the electrolyte for the sol. The reciprocal of coagulation
value is regarded as the coagulating power.
For example, the coagulation values of NaCl, BaCl2 and AlCl3 for arsenic sulphide sol. are 51, 0.69 and 0.093
millimoles/litre respectively. Thus their coagulating powers are, and i.e., 0.0196, 1.449 and 10.75 respectively.
The coagulation values of a few electrolytes for negatively charged arsenic sulphide and positively charged ferric
hydroxide sol. are given in table given below. The valency of the coagulation ion (the ion whose charge is
opposite to that of the colloidal particles) is also give.
Coagulation values of different electrolytes

Arsenic sulphide sol. Ferric hydroxide sol.

Electrolyte Valency of coagulation Electrolyte Valency of coagulation

coagulating value coagulating value
cation (millimoles/litre) anion (millimoles/litre)

K2 SO4 1 63 KBr 1 138

NaCl 1 51 KNO3 1 132
KNO3 1 50 KCl 1 103
MgSO4 2 0.81 K2 CrO4 2 0.320
BaCl2 2 0.69 K2 SO4 2 0.215
AlCl3 3 0.093 K3 Fe(CN)6 3 0.096

From the above table, it is clear that the coagulating power of Al3+ ions in precipitating the arsenic sulphide sol.
is approximately 550 times more than that of sodium (Na+) or potassium (K+) ions. Again, it is observed that the
negatively charged arsenic sulphide sol. is coagulated by cations while positively charged ferric hydroxide sol. is
coagulated by anions.

E M RAO 8
 Hardy-Schulz rules: H. Schulze (1882) and W.B. Hardy (1900) suggested the following rules to discuss the
effect of electrolytes of the coagulation of the sol.

(1) Only the ions carrying charge opposite to the one present on the sol. particles are effective to cause coagulation,
e.g., the negative charged sol. is best coagulated by cations and a positive sol. is coagulated by anions.

(2) The charge on coagulating ion influences the coagulation of sol.

In general, the coagulating power of the active ion increases with the valency of the active ion.
After observing the regularities concerning the sign and valency of the active ion, a law was proposed by Hardy
and Schulz which is termed as Hardy-Schulze law which is stated as follows:
"Higher is the valency of the active ion, greater will be its power to precipitate the sol."
Thus, coagulating power of cations is in the order of Al3+ > Ba2+ or Mg2+ > Na+ or K+.
Similarly, to coagulating the positively charged sol. the coagulating power of anion is in the order of [Fe(CN)6 ]4
> PO43 > SO42 > Cl
Added electrolytes influence the stability of the colloidal particles by affecting their charges. Again the behavior
is different for lyophobic and lyophilic sols. Usually lyophobic colloids are easily coagulated by the addition of
electrolytes. For example, gold sol is precipitated by the addition of sodium chloride. This is because the sodium
ions reduce the charge on gold sol.
Lyophilic colloids require much larger amounts of electrolytes for coagulation. The reason is that lyophilic
colloids are to some extent protected by a layer of bound water molecules. Higher concentrations of elecrolytes
do, however, exert a salting-out effect; this results from the tendency of the ions to bind water molecules which
are thus less available to the substance being precipitated. The high sensitivity of the lyophobic colloids to
precipitation by electrolytes can sometimes be reduced by the addition of a lyophilic substance, which is said to
be a protective colloid and to exert a protective action.
Na+ Na+
Au NaCl Au Aggregation and precipitation
Na+
Na+

bound water layer

... ... .. .. Na+ Na+
... ... . .
.. . . bound water layer
..
gelatin . .
.. gelatin NaCl .. gelatin . .
gelatin
. . No precipitation
.. .. .. ..
....... .......
Na+ Na+
bound water layer

Gelatin film Na+ Na+

bound water layer

Au NaCl
Au No precipitation

Na+
Na+

Gelatin film

Gold number of a protective colloid is a minimum weight of it in milligrams which must be added to 10
ml of a standard red gold sol so that no coagulation of the gold sol (i.e. change of color from red to blue)
takes place when 1 ml of 10 % sodium chloride solution is rapidly added to it. Smaller the gold number
of a protective colloid, the greater is the protective action.

E M RAO 9
 Some other methods of coagulation:
Apart from the addition of electrolyte, coagulation can also be carried out by following methods:

(i) By persistent dialysis: It has been observed that traces of electrolytes are associated with the solution due to
which it is stable. If the solution is subjected to prolonged dialysis, the traces of electrolytes are removed and
coagulation takes place.
(ii) By mutual coagulation of colloids: When two sols of oppositively charges are mixed together in a suitable
proportion, the coagulation takes place. The charge of one is neutralized by the other. For example, when
negatively charged arsenic sulphide sol. is added to positively charged ferric hydroxide sol., the precipitation of
both occurs simultaneously.
(iii) By electrical method: If the electrical charge of lyophobic sol. is removed by applying any electric field such as
in electrophoresis, they also precipitate out.
(iv) By excessive cooling or by excessive heating.

(4) Colligative properties: Colloidal solutions too exhibit colligative properties such as osmotic pressure, lowering
of vapour pressure, depression in freezing point and elevation in boiling point. But the effect of colloidal particles
on colligative properties except osmotic pressure is very small. This is due to the large size of colloidal particles.
The number of colloidal particles produced by a given mass of colloid is much less than the number produced in a
molecular solution, containing the same mass of solute. Hence the colligative effect in colloidal solutions is too
less.
EMULSIONS

An emulsion is a colloidal solution of a liquid. It may be defined as a heterogeneous system consisting of more
than one immiscible liquids dispersed in one another in the form of droplets whose diameter, in general, exceeds
0.1 m.
For example, milk is an emulsion in which small drops of liquid fat are dispersed in aqueous medium. Cod liver
oil is an emulsion in which the water drops are dispersed in the oil. This means in most of the emulsions one of
the liquid is water and the other liquid is oil. Here the term 'oil' is used to represent all organic substances which
are soluble in water.
The emulsion are classified as :
(1) Oil in water type emulsion (O/W): In this emulsion, oil is the dispersed phase and water is the dispersion
medium. It is denoted by O/W. For example, milk (liquid fat dispersed in water), vanishing cream, etc.
(2) Water in oil typ : In this emulsion, water is the dispersed phase and oil is the dispersion medium. It is denoted by
W/O. For example, butter, cod liver oil, cold cream, etc.
The type of emulsion obtained by agitating two immiscible liquids depends upon the relative amounts of two
components liquids. The liquid that is in excess forms the dispersion medium. Thus, the two types of emulsions
can be interconverted into each other by changing the concentration of one of the liquids.
Preparation of emulsion (Emulsification): Emulsification is the process which involves the preparation of
emulsion. Generally, an emulsion is prepared by subjecting a mixture of the immiscible liquid to a distinct layers
upon standing. The oil globules rise to form an upper layer while aqueous medium forms lower layers. To prevent
the separation of layers and to get the stable emulsion, a small quantity of the third substance is added. This
substance which stabilizes the emulsion is called emulsifier or emulsifying agent. The commonly used
emulsifying agents are soaps, detergents and lyophilic colloids. Casein, a lyophilic colloid present in milk, acts as
an emulsifier as it forms a protective layer around fat molecules dispersed in water. Hence milk is a fairly stable
emulsion.
Function of emulsifier: The main function of emulsifier or emulsifying agents is to lower the interfacial tension
between oil and water and thus helps the intermixing of two liquids. For example, a molecule of a soap or
detergent (emulsifier) gets concentrated at the interface between oil and water. The polar end of the emulsifier is
in water and non-polar end is in oil as shown in figure.
In a soap, RCOONa, R is the non-polar end, whereas COO Na+ is the polar end.
Properties of emulsion:
(i) The size of particles of the dispersed phase of an emulsion is usually larger than in sols.
(ii) Like colloidal particles, emulsions exhibit properties such as Tyndall effect, Brownian movement (provided the
particles are not too large), electrophoresis, coagulation, etc.

E M RAO 10
 Demulsification: The process which involves the breaking of an emulsion into two separate liquid layers is called
demulsification. The following methods may be used to bring demulsification:
(1) Chemical Methods: An emulsion may be demulsified by adding a chemical substance whose action on the
dispersed phase and the dispersion medium is opposite to that of the original emulsifying agent used to produce
the stable emulsion.
(2) Centrifugation: Cream is separated from milk by the centrifugal method.
(3) Cooling: Fat can be removed from milk by keeping it in a refrigerator for a few hours.

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 ADSORPTION

INTRODUCTION
The tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) is
termed adsorption. The molecular species or substance which concentrates or accumulates at the surface
is termed adsorbate and the material on whose surface the adsorption takes place is called adsorbent.
DISTINCTION BETWEEN ADSORPTION AND ABSORPTION
In adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in
absorption the concentration is uniform throughout the bulk of the solid.
Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the
particles inside the bulk. Inside the adsorbent all the force acting between the particles are mutually
balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all
sides and hence they possess unbalanced or residual attractive forces. These forces of the adsorbent are
responsible for attracting the adsorbate particle on its surface.
THERMODYNAMICS OF ADSOPTION
Adsoption is always accompanied by decrease in entropy. As a result, to be spontaneous, adsorption is
to be sufficiently exothermic.
TYPES OF ADSORPTION
There are two main types of adsorption of gases on solids.
If accumulation of gas on the surface of a solid occurs on account of weak van der Walls force, the
adsorption is termed as physical adsorption or physi-sorption. When the gas molecules or atoms are
held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or
chemisorption. The chemical bonds may be covalent or ionic in nature. Chemisorption has a rather high
energy of activation and is, therefore, often referred to as activated adsorption. Sometimes these two
processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical
adsorption at low temperature may pass into chemisorption as the temperature is increased. For example,
hydrogen is first adsorbed on nickel by van der Walls force. Molecules of hydrogen then dissociate and
hydrogen atoms are held on the surface by chemisorption.

Comparison between physisorption and chemisorptions:

Physical adsorption Chemical adsoption
1. It is caused by weak van der Waals forces 1. It is caused by chemical bonds
2. It is not specific. More easily liquefiable 2. It is highly specific
(high critical temperature) gases adsorb
readily.
3. It is reversible 3. It is irreversible
4. Enthalpy of adsorption is low (less than 20 4. Enthalpy of adsorption is high (100 - 500 kJ)
kJ)
5. Low temperatures are favorable. It decreases 5. It increases with increase in temperature until
with increase in temperature. it reaches an optimum value. Then it
amount of adsorbate

decreases. This is because chemisorption

requires activation energy.
amount of adsorbate

Temperature

Temperature
6. It increases with increase in pressure 6. It also increases with increase in pressure
7. It is multilayered 7. It is monolayered

E M RAO 12
 8. It increases with increase in surface area 8. It also increases with increase in surface area

The relationship between physical and chemical adsorption is explained by the following potential
energy diagram. It relates to the adsorption of hydrogen on a surface such as that of a metal.
Physisorption occurs first, the intact molecules being held loosely by the van der Waals forces. The
diagram shoes that initial physisorption is an important feature of the chemisorption process. If there
were no physisorption, there would be much higher activation energy for the chemisorption; the process
via the physisorbed stage is therefore always favoured.

ADSORPTION ISOTHERMS:
An equation that relates the amount of a substance attached to a surface to its concentration in the gas
phase or in solution, at a fixed temperature, is known as an adsorption isotherm.
LONGMUIRS ADSORPTION ISOTHERM
Longmuirs isotherm depends upon the following assumptions.
1. the surface area of the solid contains a fixed number of adsorption sites.
2. each site can adsorb only one gas molecule and adsorption is monolayered.
3. the heat of adsorption is identical to all sites.
4. the adsorbed molecules/atoms do not interact with each other.
Based on the above assumptions,

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 rate of desorption = k d k d - rate constant for desorption, -fraction of surface covered
rate of adsorption = k a p(1-) k a - rate constant for adsorption ([A]can be written in the place of p)
at equilibrium, rate of adsorption = rate of desorption
k d = k a p(1-)
(k a /k d )p
=
1+(k a /k d )p
k1p
= where k1is the equilibrium constant.
1+k1p

[A] 1
[A]
FREUNDLICH ADSORPTION ISOTHERM
Systems that obey Longmuirs isotherm are often referred to as showing ideal adsorptions. Systems
frequently deviate from Longmuir;s isotherm. For non-ideal systems, Freundlich, in 1909, gave an
empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure
at a particular temperature. The relationship can be expressed by the following equation.
Amount of gas adsorbed per

78C
gm of adsorbent (a)

29C
0C

Adsorption isotherm

1 1
x n
x
=k.p or =k.c n ....(i)
m m
where x is the mass of the gas adsorbed on a mass m of the adsorbent at a pressure P. k and n are
constants which depend on the nature of the adsorbent and the gas at a particular temperature. The value
of n is always greater than one.
Y
1
b Slope =
n
a
Log m
x

log k (Intercept)

O X
Log P
Freundlich isotherm

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 Freundlichs adsorption isotherm is also valid for the adsorption of a solution on a solid surface. The
only difference is that the pressure term is replaced by concentration term.
1
x
=k.C n
m

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