Surface Chemistry

Introduction
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. The interface or surface is
represented by separating the bulk phases by a hyphen or a slash. For example, the interface between a solid and a
gas may be represented by solid-gas or solid/gas.
Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous
catalysis, dissolution and crystallisation occur at interfaces.

Adsorption
The tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) is termed as
adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed as
adsorbate and the material on whose surface the adsorption has taken place is called adsorbent.

Example of Adsorbents :
(a) Silica gel : It acts as a good adsorbent and is prepared by mixing sodium silicate with 10% hydrochloric acid at
50° C.
(b) Metals : Metals act as good adsorbents and are being used for contact catalysis. These are prepared by the
reduction of their oxides or of the salts under suitable experimental conditions. Examples are Ni, Cu, Ag, Pt and Pd.
(c) Colloids : As colloids possess high surface per unit mass due to their small size, they act as good adsorbents.
Examples of Adsorbates: There are various gases (He, Ne, O2, N2, SO2, NH3 etc.) and substances in solution (NaCl,
KCl) which can be adsorbed by suitable adsorbents.

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.
Sorption : The process in which both adsorption and absorption take place simultaneously is generally termed as
sorption.

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Adsorption Absorption Sorption
(i) In adsorption, the composition of final sample at surface & bulk will differ largely, while in absorption, the
composition will be nearly same.
(ii) The rate of adsorption decreases with time but absorption occurs at nearly constant rate.
(iii) Adsorption is always exothermic but absorption may be endothermic or exothermic.
 Examples for adsorption and absorption :
(i) Water vapour is absorbed by anhydrous calcium chloride while it is adsorbed by silica gel.
(ii) Ammonia is adsorbed by charcoal while it is absorbed by water to form ammonium hydroxide.
NH3 + H2O → NH4OH
(iii) Decolourisation of sugar solution by activated charcoal is another example of adsorption. In this example,
charcoal adsorbs the colouring material and thus decolourises the solution.
(iv) The colour of the lake test for aluminium ions is due to adsorption of dye (litmus) on the freshly precipitated
aluminium hydroxide.
(v) Case of adsorption of water vapour on the surface of a crucible.
(vi) When sponge is put into water, it takes up water. It is example of absorption.

Mechanism of Adsorption :
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 force. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface.

Characteristics of adsorption :
The various characteristics of adsorption are as follows :
(i) Adsorption is a spontaneous process and takes place in no time.
(ii) The phenomenon of adsorption can occur at all surfaces and five types of interfaces can exist : gas-solid, liquid-
solid, liquid-liquid, solid-solid and gas-liquid. The gas-solid interface has probably received the most attention
in the literature and is best understood. The liquid-solid interface is now receiving much attention because of
its importance in many electrochemical and biological systems.
(iii) It is accompanied by a decrease in the free energy of the system, i.e. G. The adsorption will continue to such
an extent that G continues to be negative.
(iv) As the process of adsorption involves loss of degree of freedom of the gas in passing from the free gas to the
adsorbed film there is a decrease in the entropy of the system.
It follows from the Gibbs-Helmholtz equation
G = H – TS
Where G is the change in free energy, H is the change in heat content, S is the change in entropy, and T is
the temperature of the system. As the entropy and free energy decrease in adsorption, the value of H decreases.
This decrease in heat content (H) appears as heat. Hence the adsorption process must always be exothermic.

Illustration 1
A vessel of capacity 8.21 L contains NH3 gas at 1.5 atm and 27°C. Now, 5 gm charcoal is added in the vessel and left for
sufficient time. After sufficient time, the pressure of gas decreased to 1.2 atm. Calculate the mass of NH 3 gas adsorbed
per gram of charcoal. Neglect the volume of charcoal.
Solution:
P.V.M. 0.3  8.21  17
Mass of NH3 gas adsorbed = = = 1.7 gm
RT 0.0821  300
1.7
 Mass of NH3 gas adsorbed per gm of charcoal = = 0.34 gm
5

Types of Adsorption
There are two main types of adsorption of gases on solids.
(i) Physical adsorption or physisorption :
In this adsorption, accumulation of gas on the surface of a solid occurs on account of weak Vander Waals’ forces.
(ii) Chemical adsorption or chemisorption :
The gas molecules or atoms are held to the solid surface by chemical bonds (covalent or ionic) in nature.
Chemisorption has 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.
 Note :
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 Waals’ force. Molecules of hydrogen then dissociate and hydrogen
atoms are held on the surface by chemisorption.

Comparison of Characteristics of Physisorption and Chemisorption

PHYSICAL ADSORPTION CHEMICAL ADSORPTION
1 It is caused by weak forces. It is caused by chemical bond formation.
2 It is not specific. It is highly specific (Bond formation is necessary).
3 It is reversible. It is irreversible.
4 It depends on the nature of gas. More easily It depends on the nature of gas. Gases which from
liquefiable gases are adsorbed readily. compounds with the adsorbent exhibit chemisorption.
5 Enthalpy of adsorption is low (20-40 kJ mol ).
–1
Enthalpy of adsorption is high (80-240 kJ mol–1).
6 Low temperature is favourable. It decreases High temperature is favourable. It increases with the
with increase of temperature. increase in temperature upto certain limit.
7 No appreciable activation energy is involved. High activation energy is involved.
8 High pressure is favourable. Decrease of High pressure is favourable. Decrease of pressure
pressure causes desorption. does not cause desorption.
9 It forms multilayers on adsorbent surface under It forms unimolecular layer.
high pressure.

Illustration 2
The heat of physisorption lie in the range of
(A) 1 – 10 kJ mol–1 (B) 20 – 40 kJ mol–1
(C) 40 – 200 kJ mol–1 (D) 200 – 400 kJ mol–1
Solution:
(B)
Enthalpy of adsorption in physisorption is low and lie in range of 20 – 40 kJ mol–1.

Illustration 3
The nature of bonding forces in chemisorption
(A) purely physical such as Vander Waal’s forces
(B) purely chemical
(C) both chemical and physical simultaneously.
(D) none of these
Solution:
(B)
In chemisorption adsorbate and adsorbent are linked through pure chemical bonds.

Illustration 4
Which statements is/are correct ?
(A) Physical adsorption is multilayer, non-directional and non-specific.
(B) Chemical adsorption is generally monolayer and specific in nature.
(C) Chemical adsorption is due to free valence electrons of atoms.
(D) Chemical adsorption is stronger than physical adsorption.
Solution:
(A,B,D)
Factors affecting adsorption of gases on solids :
(i) Nature of gas : Easily liquefiable gases (having higher critical temperature) gets adsorbed to the greater extent.
(ii) Nature of adsorbate: Charcoal, silica gel, alumina gel, colloids are good adsorbents.
(iii) Surface area : Adsorption increases on increasing the surface area.
(iv) Pressure : Adsorption of gas at solid surface (increases due to) decrease in volume of system & hence extent of
adsorption increases on increasing pressure.

(v) Temperature : T Physisorption Chemisorption

Illustration 5
Which gas will be adsorbed on a solid to greater extent.
(A) A gas having non-polar molecule.
(B) A gas having highest critical temperature (TC).
(C) A gas having lowest critical temperature.
(D) A gas having highest critical pressure.
Solution:
(B)
Gas having higher critical temperature (TC) are adsorbed to greater extent on solid surfaces.

Freundlich adsorption isotherm:

Freundlich, in 1909, gave an empirical relationship between the quantity of gas adsorbed (x) by unit mass of solid
adsorbent (m) and pressure at a particular temperature. The relationship can be expressed by the following equation:
x
= k.p1/n (n > 1)
m
where k & n are constant, depending on nature of gas, solid & temperature.
For any combination of gas, solid & temperature, the values of k & n may be determined graphically.
x 1
log   = log k + logP
m n

Freundlich isotherm is straight line. Freundlich isotherm fails at high pressure.

Langmuir’s adsorption isotherm:
He derived the condition of equilibrium theoretically assuming that adsorption is
(a) monolayer
(b) uniform at solid surface
(c) adsorbate particles do not interact each other at the surface.
adsorption
M(s) + X(g) desorption
M – X(s)

At any instant :

Rate of adsorption, ra  P
 (1 – )
 ra = KaP(1 – )
Where,  = Fraction of surface area already occupied by adsorbate particles and
Rate of desorption, rd    rd = Kd.
At equilibrium ra = rd

Ka
P K eq P
KaP Kd
= = =
K d + K a P 1 + K a P 1 + K eq P
Kd
x
Now, 
m
x K eq P aP
= K = K = (where a, b = constants)
m 1 + K eq P 1 + b.P

Illustration 6
x
Graph between log   vs log P is provided for adsorption of NH3 gas on metal surface. Calculate weight of NH3 gas
m
adsorbed by 50 gm of metal surface at 2 atm pressure.

Solution:
1x
From Freundlich’s adsorption isotherm = kP1/n
1m
x 1
Taking log on both sides : log = log P + log k (y = mx + c form)
m n
 1
Slope = tan 45° = n=1
n
Intercept = log k = 0.3 = log 2
k=2
So, equation will be :
x
= 2p1
m
Given, m = 50 g & P = 2 atm
x
 = 2x2
m
x = 200 g
So, amount of NH3 adsorbed on surface is 200 g.

Adsorption from solution phase:

Solids can adsorb solutes from solutions also. When a solution of acetic acid in water is shaken with charcoal, a part
of the acid is adsorbed by the charcoal and the concentration of the acid decreases in the solution. Similarly, the
litmus solution when shaken with charcoal becomes colourless. The following observations have been made in the
case of adsorption from solution phase:
(i) The extent of adsorption decreases with an increase in temperature.
(ii) The extent of adsorption increases with an increase of surface area of the adsorbent.
(iii) The extent of adsorption depends on the concentration of the solute in solution.
(iv) The extent of adsorption depends on the nature of the adsorbent and the adsorbate.
The precise mechanism of adsorption from solution is not known. Freundlich’s equation approximately describes
the behaviour of adsorption from solution with a difference that instead of pressure, concentration of the solution
x
is taken into account, i.e., = kC1/n
m
(C is the equilibrium concentration, i.e., when adsorption is complete).

Applications of Adsorption :
The phenomenon of adsorption finds a number of applications. Important ones are listed here:
(i) Production of high vacuum:
The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a
very high vacuum.

(ii) Gas masks:

Gas mask (a device which consists of activated charcoal or mixture of adsorbents) is usually used for breathing
in coal mines to adsorb poisonous gases.

(iii) Control of humidity:

Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.

(iv) Removal of colouring matter from solutions:

Animal charcoal removes colours of solutions by adsorbing coloured impurities.

(v) Heterogeneous catalysis:

Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. There are many
gaseous reactions of industrial importance involving solid catalysts. Manufacture of ammonia using iron as a
catalyst, manufacture of H2SO4 by contact process and use of finely divided nickel in the hydrogenation of oils
are excellent examples of heterogeneous catalysis.
 (vi) Separation of inert gases:
Due to the difference in degree of adsorption of gases by charcoal, a mixture of noble gases can be separated
by adsorption on coconut charcoal at different temperatures.

(vii) In curing diseases:

A number of drugs are used to kill germs by getting adsorbed on them.

(viii) Froth floatation process:

A low-grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method
using pine oil and frothing agent.

(ix) Adsorption indicators:

Surfaces of certain precipitates such as silver halides have the property of adsorbing some dyes like eosin,
fluorescein, etc. and thereby producing a characteristic colour at the end point.

(x) Chromatographic analysis:

Chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical
and industrial fields.

Berzelius in 1835 used the word "catalyst" for the first time for some substance, which alter rate of chemical reaction and
themselves remain chemically and quantitatively unchanged after the reaction and the phenomenon is known as catalysis.
Example : Potassium chlorate when heated at 653K to 873K, it gives O2, When MnO2 is used in this reaction, the O2 is
formed quickly at the low temperature hence MnO2 is a catalyst.
2KClO3 → 2KCl + 3O2

Types of catalysts :
On the basis of phases of catalyst and reactants.
(A) Homogeneous Catalysis :
When catalysts and reactants are in same phase then the process is said to be homogeneous catalysis and
Example : (i) 2SO2(g) + O2(g) ⎯⎯⎯
NO(g)
→ 2SO3(g)
(ii) CH3COOCH3() + H2O() ⎯⎯⎯
HCl( )
→ CH3COOH(aq) + CH3OH(aq.)
(iii) C12H22O11(aq.) + H2O() ⎯⎯⎯⎯
H2SO4 ( )
→ C6H12O6(aq.) + C6H12O6(aq.)
Glucose Fructose
(B) Heterogeneous Catalysis :
When catalysts and reactants are in different phases, then process is known as heterogeneous catalysis and
catalyst is called heterogeneous catalyst.
Example:
(i) 2SO2(g) + O2(g) ⎯⎯⎯
Pt(s)
→ 2SO3(g)
(ii) N2(g) + 3H2(g) ⎯⎯⎯
Fe(s)
→ 2NH3(g)
(iii) 4NH3(g) + 5O2(g) ⎯⎯⎯
Pt(s)
→ 4NO(g) + 6H2O(g)
(iv) Vegetable oils () + H2(g) ⎯⎯⎯
Ni(s)
→ Vegetable ghee (s).
Types of Catalysis :
On the basis of the ways, they alter the rate of chemical reaction.
(A) Positive Catalysis :
A substance which increase the rate of chemical reaction is called positive catalyst and this process is called
positive catalysis.
(i) 2SO2(g) + O2(g) ⎯⎯⎯⎯⎯⎯ Pt(s)
(positive catalyst )
→ 2SO3(g)
(ii) N2 (g) + 3H2 (g) ⎯⎯⎯⎯⎯⎯ Fe(s)
(positive catalyst )
→ 2NH3 (g)
 (B) Negative Catalysis :
A substance which decrease the rate of chemical reaction is called negative catalyst and this process is called
negative catalysis.
Example:
H3PO4 or glycerol
(i) 2H2O2 ⎯⎯⎯⎯⎯⎯
(negative catalyst )
→2H2O + O2

(ii) Rate of decomposition of chloroform decreases in the presence of 1% ethyl alcohol.

1
CHCl3 + O2 ⎯⎯⎯⎯⎯⎯
1% ethyl alcohol
(negative catalyst )
→ COCl2 + HCl
2
(iii) T.E.L. is used as negative catalyst in petrol which reduces knocking.
(iv) 2H2SO3 + O2 ⎯⎯⎯⎯⎯⎯phenol
(negative catalyst )
→2H2SO4

(v) 2C6H5CHO + O2 ⎯⎯⎯⎯⎯⎯ quinol

→2C6H5COOH
(negative catalyst )

(C) Auto-catalysis :
When one of the reaction products behave as catalyst for that reaction and increase the rate of reaction then
the phenomenon is called auto-catalysis.
Auto-catalytic reactions are slow in the beginning but become increasingly rapid as the reaction proceeds.
Example :
(i) CH3COOC2H5 + H2O → CH3COOH (Auto-catalyst) + C2H5OH

(ii) 2KMnO4 + 5COOH + 3H2SO4 → K2SO4 + 2MnSO4 (Auto-catalyst) + 8H2O + 10CO2

COOH

(iv) 2 AsH3 → 2As (Auto-catalyst) +3H2O

(D) Induced Catalyst :

When one reaction catalyse another reaction than the phenomenon is called induced catalysis and that reaction
is called induced catalyst. eg :
(i) Sodium sulphite (Na2SO3) is oxidised to Na2SO4 in atmosphere but sodium arsenite (Na3AsO3) does not
oxidises to Na3AsO4 in air. When both are kept together both are oxidised in air. Here oxidation of Na2SO3
catalyse the oxidation of Na3AsO3. Hence oxidation reaction of Na2SO3 act as an induced catalyst for
oxidation reaction of Na3AsO3.
(ii) Similarly, reduction of HgCl2 to Hg2Cl2 by oxalic acid is very slow while that of KMnO4 is fast but mixture of
HgCl2 and KMnO4 is reduced rapidly by oxalic acid.

Promoters/Activators :
Substance which themselves are not catalyst but its presence can increase the catalytic activity of catalyst. A promoter
increases the number of active sites on the surface.
E.g. :
(i) N2 + 3H2 ⎯⎯⎯⎯⎯
Fe (catalyst )
Mo (promoter)
→2NH3

(ii) Vegetable Oil + H2 ⎯⎯⎯⎯⎯

Ni (catalyst )
Cu (promoter)
→ Vegetable ghee.

(iii) CO + 2H2 ⎯⎯⎯⎯⎯→

ZnO (catalyst )
Cr2O2 (promoter)
CH3OH
Catalytic Poisons/ Anti-catalysts/ Catalyst Inhibitor :
Substance which themselves are not catalyst but whose presence decrease the activity of the catalyst. Poisoning is
due to preferential adsorption of poison on the surface of the catalyst.
(i) N2 + 3H2 ⎯⎯⎯⎯⎯⎯⎯⎯
Fe(catalyst )
CO/H2S(catalytic poisons)
→2NH3

(ii) 2SO2 + O2 ⎯⎯⎯⎯⎯⎯⎯⎯

Platinised asbestos (catalyst )
As S (catalytic poisons)
→2SO3
2 3

(iii) Rosemond's Reactions : RCOCl + H2 ⎯⎯⎯⎯⎯⎯⎯

Pd(catalyst )
BaSO (poisons catalyst )
→RCHO + HCI
4

(iv) C2H4 + H2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Ni(catalyst )

CO or Ag vapours(poisons catalyst )
→C2H6

1
(v) 2H2O2 ⎯⎯⎯⎯⎯⎯⎯
colloidal Pt (catalyst )
HCN(poisons catalyst )
→ 2H2O + O2
2

Characteristics of Catalysis :
(i) A Catalyst remains unchanged in mass and chemical compositions at the end of reactions. However, its physical
state can be changed.
Example : Granular MnO2 during decomposition of KClO3 is left as powder at the end of the reaction.
(ii) Finely divided state of catalyst is more efficient for the reactions because surface area increases and more
adsorption take place.
(iii) A small amount of catalyst is generally sufficient to catalyse almost unlimited reaction but in some cases the
rate of reaction depends on amount of catalyst.
Example :
(A) In Friedel Craft reaction more amount of catalyst is required.
(B) In hydrolysis of ester in acidic and alkaline medium, rate of reaction is proportional to concentration of H+
or OH– ions.
(iv) A catalyst cannot initiate the reaction. But sometimes the activation energy is so large that practically a reaction
may not start until a catalyst lowers the activation energy significantly. For example, mixture of hydrogen and
oxygen do not react at room temperature but the reaction occurs very rapid in presence of Pt black.
H2 + O2 ⎯⎯⎯⎯⎯⎯
room temperature
→No reaction
1
H2 + O2 ⎯⎯⎯ Pt black
→ H2O
2
(v) Catalysts are generally specific in nature. A substance which acts as a catalyst in a particular reaction, fails to
catalyse other reaction.
(vi) Catalyst cannot change equilibrium state but it helps to attain equilibrium quickly.
(vii) A catalyst does not change the enthalpy, entropy and free energy of a reaction.
(viii) Optimum temperature : There is a particular temperature at which the efficiency of a catalyst is maximum this
temperature is known as optimum temperature.

Adsorption Theory of Heterogeneous Catalyst :

This theory explains the mechanism of heterogeneous catalyst. This theory is combination of two theories,
intermediate compound formation theory and the old adsorption theory, the catalytic activity is localised on the
surface on the catalyst. The mechanism involves 5 steps.
(i) Diffusion of reactant to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Formation of activated intermediate.
(iv) Formation of product on the catalyst surface.
(v) Diffusion of product from the catalyst surface or desorption.
 Examples :
Let us consider addition of H2 gas to ethylene in presence of Ni catalyst, the reaction takes place as follows.

diffusion of H2 diffusion of H2
+
catalyst
chemical adsorption
H C H strain
C CH2 = CH2
H H
H H
chemical adsorption diffusion of C2H4

activated intermediate
formation

CH3 – CH3
diffusion of product
or desorption
+ CH3 – CH3
catalyst product

Note :
The surface of catalyst unlike the inner part of bulk, has free valency which provides the sheet for chemical forces of
attraction.

Factors Supporting Theory :

(i) This theory explains the role of active centre, more free valency which provide the more space for the more
adsorption and concentration increases as a result increase in rate of reaction.
(ii) Rough surface has more active pores, there will be more free valency so more will be rate of reaction.
(iii) The theory explains centre action of promoters which occupied inertial void as a result surface area for the
adsorption increases and therefore rate of reaction increases.

(v) The theory explains of function of poisons or inhibitors. In poisoning, preferential adsorption of poisons take
place on the catalyst, surface area for the adsorption on the catalyst decreases hence rate of reaction decreases.

Enzyme catalysis
Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are
actually protein molecules of high molecular mass and form colloidal solutions in water. They are very effective
catalysts which catalyse numerous reactions, especially those connected with natural processes. The enzymes are,
thus, termed as biochemical catalysts and the phenomenon is known as biochemical catalysis.

Some Enzymatic Reactions

Enzyme Source Enzymatic reaction
Invertase Yeast Sucrose → Glucose and fructose
Zymase Yeast Glucose → Ethyl alcohol and carbon dioxide
Diastase Malt Starch → Maltose
Maltase Yeast Maltose → Glucose
Urease Soyabean Urea → Ammonia and carbon dioxide
Pepsin Stomach Proteins → Amino acids
Characteristics of enzyme catalysis :
(1) Most highly efficient : One molecule of an enzyme may transform one million molecules of the reactant per
minute.
(2) Highly specific nature : Each enzyme is specific for a given reaction, ie., one catalyst cannot catalyse more than
one reaction. For example, the enzyme urease catalyses the hydrolysis of urea only. It does not catalyse
hydrolysis of any other amide.
(3) Highly active under optimum temperature : The optimum temperature range for enzymatic activity is 298-
310 K. Human body temperature being 310 K is suited for enzyme-catalysed reactions.
(4) Highly active under optimum pH: The rate of an enzyme-catalysed reaction is maximum at a particular pH
called optimum pH, which is between pH value 5-7.
(5) Increasing activity in presence of activators and co-enzymes: It has been observed that when a small non-
protein (vitamin) is present along an enzyme, the catalytic activity is enhanced considerably.
Activators are generally metal ions such as Na+, Co2+, Cu2+, etc. These metal ions, when weakly bonded to
enzyme molecules, increase their catalytic activity.
(6) Influence of inhibitors and poisons: Like ordinary catalysts, enzymes are also inhibited or poisoned by the
presence of certain substances.

Mechanism of enzyme catalysis :

The enzyme catalysed reactions may be considered to proceed in two steps.
Step 1: Binding of enzyme to substrate to form an activated complex.
E + S ⎯⎯
→ES#
Step 2: Decomposition of the activated complex to form product.
ES# ⎯⎯
→E + P
Where, E = Enzyme
S = Substrate,
ES# = Enzyme-substrate Complex, (Disclaimer : # is a symbol used to show an activated complex)

P = product

Illustration 7
Which of the following is incorrect for a catalyst?
(A) Bio-chemical reactions are mostly catalysed by enzymes
(B) Catalyst does not start a reaction
(C) Catalyst changes the equilibrium constant of a reaction
(D) Co-enzymes increase the activity of an enzyme
Solution:
(C)

Illustration 8
What is the effect of enzymes on the rate of biochemical reactions?
(A) The rate increases (B) It does not change
(C) The rate decreases (D) either (B) or (C)
Solution:
(A)
Illustration 9
In heterogenous catalysis of a gaseous reactants over solid catalyst
(A) Reaction occurs on the surface of catalyst
(B) Reaction occurs when gases diffuse towards the surface of catalyst
(C) Reaction starts when gases are about to desorb from the catalyst surface
(D) Reaction occurs before adsorption of reactants
Solution:
(A)
The reactant gases get adsorbed on the surface of the catalyst and reaction occurs on the surface itself.

Illustration 10
Which of the following is the best example of shape selective catalysis?
(A) finely divided nickel
(B) zeolites
(C) palladium
(D) platinum
Solution:
(B)
The catalytic reaction that depends upon the structure of pores of the catalyst and the size of the reactant and product
molecules is called shape selective catalysis. Zeolites are good shape selective catalysts because of their honey-comb
structure.

Illustration 11
Why are enzymes highly specific?
(A) They are nitrogenous material
(B) They have active site on their surface
(C) They are biological catalyst
(D) They are very active
Solution:
(B)
Enzymes are highly selective catalysts, meaning that each enzyme only speeds up a specific reaction. The molecules that
an enzyme works with are called substrates. The substrates bind to a region on the enzyme called the active site.

Thomas Graham classified the soluble substances into two categories depending upon the rate of their diffusion through
animal and vegetable membranes or parchment paper.

Crystalloids :
They diffuse rapidly in solution and can rapidly pass through animal or vegetable membranes, e.g. urea, sugar, salts
and other crystalline substances.

Colloids :
They diffuse very slowly in solution and cannot pass through animal or vegetable membranes, e.g. starch, gelatin,
silicic acid, proteins etc. Since this class of substances generally exist in amorphous or gelatinous condition and
hence the name colloid meaning "glue form".

Note :
Actually, every substance irrespective of its nature can be crystalloid or colloid under suitable conditions. For
example:
(i) NaCl, though a crystalloid in water behaves like a colloid in benzene.
(ii) Soap is a colloid in water, while it behaves like a crystalloid in benzene.
 Therefore, "A substance is said to be in the colloidal state, when it is dispersed in another medium in the form of
very small particles having diameter between 10–4 cm to 10–7cm".

S.No. Property Suspension Colloidal solution True solution

1 Particle size >10–4 cm or 104 Å 10–7 cm to 10–4 cm or 10 Å <10–7 cm or 10 Å
to 104 Å
2 Visibility Visible with naked eye. Visible with ultra Not visible with any
microscope. of the optical means.
3 Diffusion Does not diffuse Diffuse very slowly Diffuse rapidly
4 Settling Settles under gravity Does not settle but it may Does not settle
settle under centrifuge
5 Nature Heterogeneous Heterogeneous Homogeneous
6 Appearance Opaque Generally clear Clear

Colloidal solutions :
They are considered as a heterogeneous system consisting of the following three components:
1. Dispersed phase (discontinuous or inner phase) : It consists of discrete particles significantly larger than
ordinary molecules and this small particles of solute is diffused in solvent.
2. Dispersion medium (continuous phase Or outer phase) : It is the medium in which dispersed phase is present.
This consists of continuously interlinked molecules.
3. Stabilising agent: This is a substance which tends to keep the colloidal particles apart. Some colloids are self
stabilizers.
Dispersed phase + Dispersion medium = Dispersion system (Colloidal solution)
Each of the two phases constituting a colloidal system may be a gas, a liquid or a solid. For example, in milk, the fat
globules are dispersed in water. Hence fat globules form a dispersed phase and water is the dispersion medium.

CLASSIFICATION OF COLLOIDS
1. Depending upon the physical state of the dispersed phase and that of dispersion
medium :
The colloidal solutions are divided into the following eight categories:
1 Solid Solid Solid sol Some coloured glasses, gem stone
2 Solid Liquid Sol Paints, inks, white of eggs
3 Solid Gas Aerosol Smoke, dust
4 Liquid Solid Gel Curds, pudding cheese, jellies
5 Liquid Liquid Emulsion Milk, cream, oil in water
6 Liquid Gas Liquid Aerosol Clouds, mist, fog (water in air)
7 Gas Solid Liquid foam Cake, bread, lava, pumice stone
8 Gas Liquid Solid foam Soap lather, froth, whipped cream

Note :
Since the two gases are completely miscible in each other, they always form a true solution.

2. Depending upon the appearance of colloids :

On this basis, colloids are divided into the following two main categories :
(A) Sol : When a colloidal solution appears as fluids, it is termed as Sol. Sols are named after dispersion medium.
For example, when dispersion medium is water, they are called hydrosols when the dispersion medium is
alcohol they are called alcosols and so on.
(B) Gels : When a colloid has a solid-like appearance, it is termed as gel. The rigidity of gel varies from substance
to substance.
3. Depending upon the interaction of the two phases (i.e. dispersed phase and dispersion
medium)
According to Perrin and Freundlich, colloids may be classified into lyophobic and lyophilic
(A) Lyphobic or solvent-hating : When the dispersed phase has less affinity for the dispersion medium, the
colloids are termed as lyophobic. But when the dispersion medium is water, they are given the name
hydrophobic. These sols are readily precipitated (or coagulated) on the addition of small amounts of
electrolytes or by heating or by shaking & hence are not stable.
(B) Lyophilic or solvent loving: When dispersed phase has a greater affinity for the dispersion medium, the
colloids are termed as lyophilic and when the dispersion medium is water, they are given the name
hydrophilic. "they are also called natural colloids, substances like proteins, starch and rubber etc. are
grouped under this category

S.No. Property Lyophilic Lyophobic (Extrinsic Sol)

1 Preparation They are easy to prepare. Only They are difficult to prepare.
contact with the dispersion Special methods are used.
medium is needed to Stabilise. Addition of stabilizers is
essential for their stability.
2 Size of particles The particles are just bigger The particles are aggregates
molecules of thousands of molecules
3 Nature Reversible; once precipitated Irreversible, once precipitated
easily pass back into the does not easily pass into
colloidal state by contact with colloidal state
dispersion medium
4 Conductivity With lyophilic salts high Owing to their sensitivity in
conductivities can generally be electrolytes the conductivity
measured of lyophobic sol can rarely be
ensured over a considerable
range of concentration
5 Tyndall effect Less distinct More distinct
6 Viscosity Higher than that of water Almost same as that of water
7 Surface Tension Lower than that of water almost same as that of water
8 Hydration Particles are heavily hydrated Particles are poorly hydrated
9 Stability Very stable, coagulated with Less stable, coagulated easily
difficulty
10 Charge Depends on the pH of the Have characteristic charge
medium. It can be even zero. (Positive or negative)
11 Concentration of Higher concentrations of Only low concentrations of the
the dispersed dispersed phase are possible dispersed phase are possible
phase
12 Example Albumin, Glycogen, Rubber, Au, Ag, some emulsions etc.
Silicic acid etc.

4. Depending upon the electrical charge on the dispersed phase:

On this basis the colloids may be divided into the following two main categories :
(A) Positive Colloids : The dispersed phase carries the positive charge. The particles of sol in Water are
positively charged. Examples of this type are methylene blue and TiO2 sols.
(B) Negative Colloids: This dispersed phase carries the negative charge. For example, the particles of As2S3 sol
in water are negatively charged. The other examples are copper or gold sol and certain dye-stuffs like eosin.
Congo red etc.
 5. Depending on the structure of colloid particles :
According to Lumie and others, colloids can also be classified into molecular and micellar colloids. The particles
of molecular colloids are single macromolecules and their Structure is similar to that of small molecules. Particles
of micellar colloids are aggregates of many molecules or groups of atoms which are held together by cohesive
or Vander Waal's forces. The examples of molecular colloids are albumin, silicons, rubber etc. while that of
micellar colloids or Sulphur, gold, soap detergents etc.
(A) Multimolecular colloids : The multimolecular colloidal particles consists of aggregate of atoms of small
molecules with diameter less than 10–10 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 hold together by Vander Waal's forces. These are usually lyophobic sols.
(B) Macromolecular colloids : The macromolecular colloidal particles themselves are large molecules. They
have very high molecular weights varying from thousand to millions. These substances are generally
polymers. Naturally occurring macromolecules such as starch, cellulose and proteins. Artificial
macromolecules such as polyethylene, nylon, polystyrene, Dacron, synthetic rubber, plastics, etc. The size of
these molecules is comparable to those of colloidal particles and therefore, their dispersion known as
macromolecular colloids. Their dispersion also resembles true solutions in some respect.
(C) Associated colloids or micelles : these colloids behave as normal electrolytes at low concentrations but
colloids at higher concentrations. This is because at higher concentrations, they form aggregated
(associated) particles called miscellas. Soap and synthetic detergents are examples of associated colloids.
They furnish ions which may have colloidal dimensions.
RCOONa → RCOO− + Na+
Sodium Stearate soap (R = C17H35)
The long-chain RCOO– ions associate or aggregate at higher concentrations and for micelles and behave as
colloids. They may contain 100 or more molecules.
Sodium stearate C17H35COONa is an example of an associated colloid. It gives Na+ and stearate ions. These
ions associate to form micelles of colloidal size.
In general, lyophilic sols are more stable than lyophobic sols. The additional stability is due to the presence
of an envelope of the solvent (say water) around the colloidal particle. The process is known as hydration.
To coagulate a hydrophilic sol, we have to add a dehydrating agent in addition to an electrolyte.
Note :
Sometimes the names Emulsoids and Suspensoids are also used for hydrophilic and hydrophobic colloids
respectively.

PREPARATION OF SOLS
Preparation of lyophilic Sols : Preparation of lyophilic Sols : Many organic substances like gelatine, starch,
agar-agar, egg albumin, glycogen etc. dissolve readily in water either in cold or on warming to give colloidal
solutions directly. These are the lyophilic colloids. For example, sots of egg albumin or glycogen can be prepared by
dissolving 1-2 g of the finely divided substance in 100 mL of distilled water and then allowing it to stand for two
hours after constant stirring. After two hours, the solutions are filtered.
Preparation of lyophohic Sols: Such Sols can be prepared by the two general way :
(i) By dispersion of coarse particles (Dispersion method). Here we start with bigger particles and break them down
to the colloidal size.
(ii) By inducing molecular particles to form large aggregates (condensation method). Here we start with particles
of molecular dimensions and condense them to the colloidal dimensions.
Dispersion methods :
(A) Mechanical dispersion : Here the substance is first finely powdered and a coarse suspension is made by
shaking the powdered substance with the dispersion medium. This suspension is then passed through a colloid
mill consisting of two discs, moving in opposite directions at a very high speed (Fig. I) The particles of the
suspension are subjected to a great shearing force and break down to the colloidal dimension. The space
between the two discs controls the size of the colloidal particles to be obtained. Rubber, ink, paints and varnishes
are prepared by this method.

Fig-I
(B) Electrical Dispersion- Bredig's arc methods :
This is commonly used method for preparing the colloidal solutions of metals. An electric arc is struck between
two metallic rods kept under the liquid (dispersion medium). A current of 10 amperes and a voltage of 100 to
300 volts is generally employed. The liquid is kept cooled by surrounding it with a cooling mixture. Tiny particles
of the metal break away from the rods and disperse in the liquid. Gold, platinum, silver, copper and such other
metals can thus be obtained in the colloidal form. (Fig.II)
Solvent
+protective
colloid

Ice

Fig-II
(C) Peptization : The process of bringing a precipitated substance back into the colloidal state is known as
peptization. It is carried out by the addition of an electrolyte. The electrolyte added is termed as peptizing or
dispersing agent. It involves the adsorption of a suitable ion supplied by the electrolyte added by the particles
of the precipitate. Peptization may be carried out by the following ways :
(i) By electrolyte : Freshly prepared precipitate of Fe(OH)3 can be changed into colloidal state when precipitate
is treated with a small amount of FeCl3 solution. The sol obtained is positively charged due to the preferential
adsorption of Fe3+ ions (from FeCl3) on sol particles of Fe(OH)3 as [Fe(OH)3]Fe3+. It should be noted that only
freshly prepared precipitates can be peptized.
(ii) By washing a precipitate : Peptization sometimes can be brought about by repeated washings of a
precipitate. For example, if the precipitate of BaSO4, is washed continuously, a state is reached when the
washings carry some of the particles of the substances in the form of colloidal solution.

Chemical methods :
All chemical changes giving rise to insoluble reaction product can be used for the formation of sols, particularly
when suitable stabilizers are also present.
(A) Oxidation : Sols of some non-metals are obtained by oxidation. For example. Sulphur is obtained in colloidal
form by passing H2S gas through bromine water of HNO3 solution.
H2S + Br2 → HBr + S
Colloidal
Similarly, sol iodine is obtained by oxidising hydroiodic acid with iodic acid as
HIO3 + 5HI → 3H2O + 3I2, it can be made stable by adding a small amount of gelatin.
 (B) Reduction : Sols of some metals are obtained by the reduction of their salts. Gold sols are made by reduction
Of HAuCl4 solution with reducing agents like tannic acid, HCHO, H2O2, phosphorus, hydrazine etc. Carey Lee's
silver sol is obtained by reducing solution of AgNO3 containing alkaline dextrin as stabilizer with variety of
reducing agents. Stabilised colloidal suspension of graphite in water is known as aquadag and one in oil is
known as oildag.
(C) Hydrolysis : This method is generally employed for the preparation of sols of number of a hydroxides and
hydrous oxides. Fe(OH)3, Al(OH)3 and sols are obtained by boiling solution of the corresponding chlorides.
FeCl3 + 3H2O → Fe(OH)3 + 3HCl
A beautiful red sol of ferric hydroxide is prepared by boiling ferric acetate in a beaker having distilled water (500
mL).
(CH3COO)3 Fe + 3H2O → Fe(OH)3 + 3CH3COOH
The excess of ferric acetate may be removed by electrodialysis because its presence renders the sol unstable.
Organic esters of silicon, such as ethyl silicate, will hydrolyze in water to form colloidal silicic acid, Si(OH)4 and
alcohol.
(D) Double decomposition : This is the usual way of forming sols of insoluble salts. If the solutions containing
the component ions of an insoluble substance are mixed, a precipitate will result. If the substance has low
solubility, the precipitate will be colloidal. Colloidal sol of Prussian blue may be prepared by mixing very dilute
solutions of FeCl3 and K4[Fe(CN)6].
3K4[Fe(CN)6] + 4FeCl3 → Fe4[Fe(CN)6]3 + 12 KCl
Sols of arsenious sulphide and mercuric sulphide are obtained by passing H2S into the saturated solutions of
corresponding soluble salts of their oxides,
As2O3 + 3H2S → As2S3 + 3H2O
Hg(CN)2 + H2S → HgS + 2HCN
(E) Oxidation-reduction methods : Many colloidal sols are prepared by oxidation- reduction reactions. An
example of this method is the preparation of the colloidal molybdenic oxide which may be prepared by the
reduction of ammonium molybdate with hydrogen sulphide. The sol of hydrated MnO 2 can be prepared by the
reduction of 0.01 M KMnO4 with ammonia. This sol can be made stable by adding a definite quantity of gelatin.

PURIFICATION OF SOLS
Excessive quantities of electrolytes and some other soluble impurities remain in a sol as a result of the method
selected for preparation, particularly in chemical condensation Methods.
1. Dialysis : This method is based on the fact that colloidal particles are retained by animal membrane or a
parchment paper while electrolytes pass through them. The sol is taken in a parchment or cellophane bag, which
itself is placed in running water in a trough. Gradually, the soluble impurities diffuse out leaving a pure sol
behind. Dialysis is a slow process and it takes several hours and sometimes even days for complete purification.
 2. Electrodialysis : Dialysis can be fastened by applying an electric field if the substance in true solution is an
electrolyte. This process is then called electrodialysis. By means of electrodialysis, it is possible to get a colloid
in pure state in short time, although the electric current does not affect non-conducting impurities such as
alcohol, sugar, etc.

3. Ultra filtration : This is a method not only for purification of the sol but also for concentrating the sol. The
pores of the ordinary filter paper are large enough (1030 mg) for the colloidal particles (203 mg) to pass through.
But if the pores are made smaller the colloidal particles may be retained on the filter paper. This is known as
ultra-filtration.

4. Ultra-centrifugation : The colloidal particles share the motion of the molecules of the dispersion medium
and are in a state of continuous zig-zag motion called Brownian movement. Sol particles are prevented from
settling by this continuous haphazard zigzag motion. The sol is kept in a high-speed centrifuging machine
revolving at a very high speed (about 15000 revolutions per minute) so that the colloidal particles settle quickly,
the slime can be suspended in water so as to get a sol.

PROPERTIES OF COLLOIDS
General properties of colloids may be studied under several heads.
1. Heterogeneous nature : Colloidal solutions are heterogeneous in nature consisting of two distinct phases viz.
the dispersed phase and the dispersion medium. Experiments like dialysis, ultrafiltration and ultra-centrifuging, clearly
indicate the heterogeneous character of colloidal system.
2. Non-settling nature: Colloidal solutions are quite stable. The suspended colloidal particles remain
suspended in the dispersion medium indefinitely. In other words, there is no effect of gravity on the colloidal
particles.
3. Filtrability : Colloidal particles readily pass through ordinary filter papers because the size of the pores of
the filter paper is larger than that of the colloidal particles.
4. Size of the particles of the dispersed phase : Colloidal dispersions generally range in particle size
from 1 nm to 1 in diameter. properties which distinguish them from true solution are mainly due to their large
size.
5. Diffusibility : Colloidal suspensions unlike true solutions do not readily diffuse through fine membranes
and have a little power of diffusion. This is due to the large size of the colloidal particles as compared to ordinary
solute particles.
6. Colour : The colour of the sol is not always the colour of the substance in the bulk. The colour of the colloidal
solution changes as the sign and shape of the particles change. It may exhibit different colours when seen by
reflected and transmitted light. For example, diluted milk gives a blue colour in reflected light and red colour in
transmitted light.
7. Shape of the colloidal particles : Different sol particles have different shapes, for example, red gold
sol, silver sol, platinum sol. As2S3 sol has particles Fe(OH)3 sol and gold sol have disc or platelet like particles.
Rod-like particles appear in V2O5 sol, tungstic acid sol etc.
8. Visibility : Most of the sols appear to true solutions with a naked eye, but the colloidal particle can be seen
through an ultramicroscopy
 9. Colligative properties : Sol particles because of their free suspensions amongst the molecules of the
medium, share kinetic energy with them in the same manner as molecules of regular solutes do. This gives rise
to colligative properties like osmotic pressure lowering, depression of freezing point and elevation of boiling
point. Of these, the osmotic pressure alone has measurable values and its measurement has been used for
finding average particle weight in colloidal suspensions. The reason for this may be put as follows:
As colloidal particles are not simple molecules and are bigger particles (i.e. physical aggregation of about 1000
molecules) their number in the colloidal solutions are comparatively small. As the magnitude of colligative
properties depends upon the number of solute particles present in the solvent, their values are smaller.

10. Tyndall Effect : Scattering of light by the colloidal particles present in a colloidal solution is known as
Tyndall effect and is mainly caused by the scattering of blue part of light by the colloidal particles. If a strong
beam of light is passed through a colloidal solution placed in a dark place, the path of the beam gets illuminated.
The illuminated path of beam is called Tyndall Cone.

Darkness

Bright Tyndall cone Vessel containing sol

The scattering is caused if the size of particles is of the order of wavelength of light. The same effect is not
observed when the light is passed through a true solution as the size of solution particles is too small to cause
any scattering.
(a) The diameter of the dispersed particles is not much smaller than the wavelength of the light used.
(b) The refractive indices of the dispersed phase and the dispersion medium must differ greatly in magnitude.
This condition is satisfied by lyophobic sols. The lyophilic sols show little or no Tyndall effect as there is very
small difference in the refractive indices of the dispersed phase and dispersion medium.
Application of Tyndall effect : Tyndall effect has been used by Zsigmondy and Siedentop for making an ultra-
microscope. Ultra-microscope is a microscope arranged so that light illuminates the object from the side instead
from below. In ultra-microscope, incident light does not strike the eye of the observer and thus observes the
scattering produced by the sol particle against a dark background. An actual image formation is not obtained
but the presence of particle can be seen.

11.Brownian movement : The colloidal particles of a colloidal solution when viewed through an ultra-
microscope show a constant zig-zag motion. This type of motion was first observed by Robert Brown and hence
known as Brownian movement. It is caused by the uneven impacts of the particles of the dispersion medium on
the colloidal particles. As the size of the particles increases, the probability of uneven impacts decreases and the
Brownian movement becomes slow. When the dispersed particles acquire the dimensions of suspension, no
Brownian movement is observed.
This motion is independent of the nature of the colloid but depends on the size of the particles and the viscosity
of solution. Smaller the size and lesser the viscosity, faster is the motion. The motion becomes intense at high
temperature.

Importance of Brownian motion :

(a) Brownian movement provides a direct demonstration of ceaseless motion of molecules as postulated by kinetic
theory.
(b) It counters the force of gravity acting on colloidal particles and hence helps in providing stability to colloidal
sols by not allowing them to settle down.
ELECTRICAL PROPERTIES OF COLLOIDS
Electric Double Layer Theory or Helm-holtz Electric double layer :
The surface of colloid particles acquires a positive or negative charge by the selective (preferential) adsorptions of
common ions carrying positive or negative charge respectively to form first layer. This layer attracts counter ions
from dispersion medium and form a second layer. The combination of two layers of opposite charge around the
colloidal particle is called Helm-holtz electric double layer. The first layer of ions is firmly held and is termed as fixed
layer while the second layer is mobile which is termed as diffused layer. The charge of opposite ions of fixed and
diffused layer double layer results in a difference in potential between two opposite charge layers is called the
electro-kinetic potential or zeta potential which can be given by
4
Z=
D
Where  = viscosity coefficient, D = dielectric constant of medium.
 = velocity of colloidal particles when an electric field is applied.
Example :
When silver nitrate solution is added to K solution, the precipitation of Ag adsorb iodide ions from the D.M with
the formation of fixed layer and negatively charged colloidal solution form, however when K solution is added to
AgNO3 solution positive charge sol result due to the adsorbs of Ag+ ions from D.M.
AgI/I– AgI/AI+
Negative charged Positively charged.
This fixed layer attracts counter ions from the medium forming a second layer.

Electro-kinetic effects :
Colloid particles carry electric charge. When the sols are placed in the electric field, certain special effects are noticed,
which are termed as electro-kinetic effect. Such effects involve the relationship between the movement of one phase
with respect to another in the exhibition of some electrical properties.
(1) Electrophoresis or catephoresis
(2) Electro osmosis or Electroendosmosis
(1) Electrophoresis or Catephoresis : The colloidal particles carry an electric charge. The colloidal solution
is taken in a U-shaped tube and two platinum electrodes are dipped in the sol as shown in the figure. The current
is then switched on. On closing the circuit, it is found that colloid particles move to the oppositely charged
electrode and on reaching that electrode they get discharged. As soon as the charge of the particles is
neutralized, they aggregate and settle down. This movement of colloid particles in electric field is known as
electrophoresis in case of the true solutions. Since the current in the colloidal solution must be carried by both
positive and negative particles, ions of the diffused layer must be moving in a direction opposite to the direction
of the movement of colloid particles. In a Fe(OH)3, sol which is positively charged, the sol particles move to the
negative electrode where their charge is neutralised and they aggregate and finally precipitate out. Thus, the
entire colloidal matter settles down at the bottom.

©  Digital [20]

 Coagulated
Sol particles Water

migrating
sol particles

Cataptictesis

Importance : This phenomenon of electrophoresis is used in the following ways :

(i) Determining the charge on the colloidal particles : direction of movement of the colloidal particles in the
electric field shows the charge on them.
(ii) It can also be used to determine the rate at which colloidal particles migrate under the influence of an
electric field.
(iii) It is also used in the identification and determination of homogeneity.
(iv) It is of great importance for the preparative separations of the colloidal substances.
(2) Electro osmosis : It is also known as electro-endosmosis, in the above experiment, a partition is made by
animal membrane or parchment paper in between two electrodes, so that only the dispersion medium can move
through it and not the colloidal particles. When potential difference is set up between the electrodes, then the
dispersion medium is seen to move in a direction opposite to the direction of movement of the colloidal
particles. This movement of the dispersion medium relative to the dispersed phase under the influence of the
electric field is known as electro-osomosis. This is indicated by the rise of water level in one limb of the U-tube.

Measurement of electro-osmosis

PROTECTION OF COLLOIDS
Protection :
When certain hydrophilic colloids such as gum, gelatin, agar-agar etc. are added to a hydrophobic colloid, the
stability of the latter is markedly increased. Now the addition of the small amounts of electrolytes does not cause
the precipitation of the hydrophobic colloid. This action of the hydrophilic colloids to prevent precipitation of the
hydrophobic colloid by the electrolytes is called protection and the hydrophilic colloid is called protective colloid. It
is further observed that the protective colloid not only increase the stability of the hydrophobic colloid but the latter
can be evaporated to dryness and the dry mass peptised by simply shaking with water. Thus, the protective colloid
converses an irreversible (hydrophobic) colloid into a reversible colloid.
Protected Particles

Protected Particles
Formation of protective colloids
 Some examples of protective colloids are :
(i) Soluble substance like Ca3(PO4)2 are held as colloids in blood due to protective action of protein in blood.
(ii) To prevent clogging in pens; superior pen inks contain some protective colloids
(iii) Casein in human milk is better protected than in the cow's milk. It is because of this that cow's milk is more
easily coagulated.
(iv) Protargol and Argyrol powders are the protected forms of colloidal silver.

Explanation :
The particles of the protected colloid get adsorbed on the particles of the hydrophobic colloid, thereby forming as
protective layer around it. The protective layer prevents the precipitating ions from coming in contact with the
colloidal particles. According to a recent view the increase in stability of the hydrophobic colloid is due to the mutual
adsorption of the hydrophilic and hydrophobic colloids. It is immaterial which is adsorbed on which. In fact, the
smaller particles whether of the protective colloid or of the hydrophobic colloid are adsorbed on the bigger particles.
Gold Number :
The power of the hydrophilic colloid to prevent the precipitation of a lyophobic colloid by addition of an electrolyte
depends upon the nature of the substance. The protective character of various hydrophilic substances can be
expressed quantitatively by gold number. The gold number according to Zsigmondy may be defined as: "The
number of milligrams of the protective colloid which must be added to 10 cc of a given gold sol so as to just prevent
its precipitation by addition of 1 cc of 10% NaCl solution."
Smaller the gold number, higher the protective power of a colloid. Gold numbers of some protective colloids are
given below :

S.No. Protective Colloid Gold number

1 Gelatin 0.005 - 0.01

2 Hemoglobin 0.03
3 Gum arabic 0.15
4 Egg albumin 0.08 - 0.10
5 Potato starch 25
6 Sodium oleate 0_4
7 Gum tragacanth 2
8 Starch 25 - 50

The protective power was also measured by Ostwald in terms of Congo-Rubin number. It is the amount of a
protective colloid in mg which prevents colour change in 100 mL. of 0.01% Congo-Rubin dye solution to which 0.16
g equivalent of KCl is added when observed after 10-15 minutes.

COAGULATION OR FLOCCULATION
The colloidal sols are stable by the presence of electric charges on the colloidal particles. Because of the electric
repulsion the particles do not come close to one another and coalesce. The removal of charge by any means will
lead to the aggregation of particles and hence precipitation immediately. The process by means of which the
particles of the dispersed phase in a sol are precipitated is known as coagulation or flocculation.
Electric charges on lyophobic particles can be removed by the application of an electric field as is used in
electrophoresis. But a common method of producing precipitation is by the addition of electrolytes.
The precipitate after being coagulated is known as coagulum.
Methods for coagulating a sol :
There are several methods employed for coagulating a sol. Some of them are noted below :
(1) By addition of electrolytes : When excess of an electrolyte is added. The colloidal particles are
precipitated. The reason is that colloidal particles taken up ions carrying charged opposite to that present on
themselves. This causes neutralisation leading to their coagulation. The ion responsible for neutralisation of
charge on the particles is called the flocculating ion.
It has been observed that, generally, the greater the valency of the flocculating ion added, the greater is its
power to cause precipitation. This is known as Hardy-Schulze Rule.
(a) The ions carrying charge opposite to that of sol particles are effective in carrying the coagulation of the sol.
(b) Coagulation power of an electrolyte is directly proportional to the fourth power of the valency of the ions
causing coagulation.
In the coagulation of a negative sol, the flocculating power of Na+, Ba+2 and Al+3 ions is in the order of :
Al+3 > Ba+2 > Na+
Similarly, in the coagulation of a positive sol, the flocculation power of Cl–, SO4–2, PO4–3, [Fe(CN)6]–4 is in the order
of [Fe(CN)6]–4 > PO4–3 > SO4–2 > Cl–
The minimum concentration of an electrolyte in milli-mole per litre required to cause precipitation of a sol in 2 hours
is called flocculation value. The smaller the flocculating value, the higher will be the coagulating power of an ion.

Arsenious sulphide sol Ferric hydroxide sol

S.No. Coagulation value Coagulation
Electrolyte Electrolyte
milli moles/litre value
1 NaCl 52 KCI (milli moles/litre)
132
2 KCl 51 K2CrO3 0225
3 BaCl2 0.69 K2SO4 0.21
4 MgSO4 0.22 K3[Fe(CN )6] 0.096
5 AICI3 0.093 K4[Fe(CN)6] 0.085

(2) Physical methods : The coagulation of some sols can be carried out by (a) mechanical treatment, (b)
heating or cooling, (c) irradiation, (d) vigorous shaking, (e) treatment with electric current etc.
(3) By continuous dialysis : We know that traces of electrolytes are present in the colloidal system which are
necessary for the stability. If the sol is subjected to continuous dialyser the colloidal system becomes unstable.
(4) Salting out : Coagulation of lyophilic sol can be made by the addition of sufficient high concentrations of
certain ions. Thus, salting out of lyophilic colloids is due to the tendency of ions to become solvated, causing
the removal of adsorbed water from the dispersed particles.

EMULSIONS
Emulsion is a colloidal system consisting of immiscible liquids. e.g. milk is an emulsion in which particles of liquid fat
are dispersed in water. In common occurrence, however, one of the liquids is water and the other, and oily substance
insoluble in it. The suspended droplets are larger than the particles of the cols: it is because of the density differences
between the phases being small. Emulsion droplets can be observed under an ordinary microscope and sometimes
even with a magnifying lens.
An emulsion is a heterogeneous system consisting of more than one immiscible liquids dispersed in one another in
form of droplets whose diameter, in general, exceeds 0.1. Such systems possess an extremely small stability which
is made by the addition of surface-active agents, finely divided solids, etc.
Type of Emulsions :
Emulsions are of two types :
(i) Oil in water (o/w) type : In these emulsions oil forms the dispersed phase and water, the dispersion
medium. For example, milk, vanishing cream, etc. These are also called aqueous emulsions.
(ii) Water in oil (w/o) type : In these emulsions water is in the dispersed phase and oil in the dispersion
medium. For example, butter, cold cream etc. are also called oil emulsions.
In addition to above there is one usual type known as multiple emulsion. As the name indicates, a multiple
emulsion is one in which both types of emulsion exists simultaneously. It can be denoted as w/o/w emulsion.

Factors determining the type of emulsions :

When two liquids, say oil in water are shaken to form an emulsion, the type of emulsion formed, (i.e. oil in water or
water in oil) depends upon the following factors :
(A) Relative proportion of the two liquids : As a general rule the liquid present in excess forms the
dispersion medium. For example, to obtain an emulsion of oil in water, water is taken in excess and to obtain
water in oil emulsion, oil is taken in excess.
(B) Surface tension of the two liquids : The liquid with greater tendency to form spherical drops and
hence the dispersed phase. Thus, if the surface tension of an oil in is greater than that of water, it will form an
oil in water type emulsion.
Preparation of emulsions : Emulsions are usually obtained by spraying mixtures of phases through narrow
nozzles or in counter-rotary agitators.
A condensation method given by Summer has been employed in preparation of concentrated o/w emulsions.
Note :
Emulsifying agents or stabilizers are used to form stable emulsions.

Characteristic of emulsions:
(i) Concentration and particles size : In the case of emulsions the amount of one liquid dispersed in
another is relatively much greater as compared to the soles. The maximum amount of one liquid which can
dispersed in another cannot exceed 74% of the total volume available. Emulsions more concentrated than 74%
have also been found. The diameter of droplets in case of emulsions is of the order of 0.001 – 0.05 mm.
(ii) Optical properties : A relationship between optical properties and particle size and also between light
scattering with the properties of suspensions have been reported. The interfacial areas in emulsions by optical
measurements have been determined by Langlois and other's in 1954.
(iii) Viscosity : The property of viscosity (resistance to flow) is quite important both for practical and theoretical
purposes. It provides some information about the structure of emulsions.
(iv) Electrical Conductivity : This property is useful in distinguishing between o/w and w/o type of
emulsions. The emulsion in which water is the dispersion medium possesses high conductivity than the emulsion
having oil as dispersion medium.

Emulsifiers :
In order to prepare stable emulsions, it is important to add a third component known as emulsifier or emulsifying
agent in suitable amounts. Several types of emulsifiers are known.
(i) Long chain compounds with polar groups such as soap, sulphonic acid, sulphates etc.
(ii) Most of the lyophilic colloids also act as emulsifiers such as glue, gelatin etc.
(iii) Certain insoluble powders as clay, lamp, black etc.
(iv) Soluble substances like iodine also act as emulsifiers.
 Role of an emulsifier : An emulsifier may act in two ways :
(1) It may be more soluble in one liquid than in the other: In this case it will form a sort of protective film around
the drops of this liquid in which it is less soluble and thus prevents them from coming together (fig-a)] For
example neutral soaps which are more soluble in water than in olive oil is water type emulsion [Fig.-b]. Acidic
soaps which are more soluble in oils than in water, give water in oil type emulsion [fig-c]
(2) The emulsifiers may be insoluble in both the liquids but not unequally wetted by the two.

Emulsification of oil Emulsification of oil Emulsification of water

and water by an acid soap and water by neutral soap and kerosene by soot particles
[Fig-a] [Fig-b] [Fig-c]

Importance of Emulsions :
Emulsions find manifold applications in various fields.
(i) Medicine - Numerous medicines and pharmaceutical preparations are emulsions. In such forms they have
been found to be more effective. Cod-liver oil, caster oil, petroleum oil are used in medicines and are emulsions.
(ii) Articles of daily use- Milk is an emulsion of fat dispersed in water stabilised by casein. Ice cream is an
emulsion. Butter, coffee, fruit jellies etc., are all emulsions in nature.
(iii) Cosmetics : The skin penetrating vanishing creams o/w type emulsions and hair creams, cold creams are
w/o type emulsions. The lotions, creams and ointments are stabilized by lanoline.
(iv) Industry : The latex obtained form the sap of certain trees is an emulsion of negatively charged rubber
particles dispersed in water.
During the concentration of sulphide ores, froth floatation process is employed. In the process, oil emulsion is added
to the finely divided ore and foam produced by passing ore contains most of the particles of the ore.
Emulsion of oils and fats have been employed in leather industry for making soft leather and water proof. Asphalt
emulsified in water is used for building roads, with the necessity of melting the Asphalt. Emulsions are also employed
in oil and fat industry, paints and varnishes, cellulose and paper industry etc. Furthermore, spraying liquids in the
form of emulsions are used in agriculture

SURFACTANTS
Surfactants are substances which get preferentially adsorbed at the air-water, oil-water and solid-water interfaces,
forming an oriented monolayer where in the hydrophilic groups point towards the aqueous phase and hydrocarbon
chains point towards the air or the oil phase. The surfactants can be cationic, anionic on non-ionogenic.
Sodium salts of higher fatty acids such as sodium palmitate (C15H31COONa), sodium stearate (C17H35COONa) and
sodium oleate (C17H33COONa) are anionic surfactants. The salts of sulphonic acids of high molar mass and general
formula (CnH2n+1 SO3M, alkyl sulphonates) or (CnH2n+1C6H4SO3M, alkyl and aryl sulphonates) where M+ is Na+, K+,
NH4+, are other anionic surfactants.
Cationic surfactants are those which dissociate in water to yield positively charged ions. Some example are octadecyl
ammonium chloride, Cetyl trimethyl ammonium chloride, Cetyl pyridium chloride etc.
Non-ionogenic surfactants are those whose molecules cannot undergo dissociation. When an alcohol having a high
molar mass reacts with several molecules of ethylene oxide, a non-ionogenic surfactant is produced
The hydrophilic nature of hydroxy ethylated surfactants can be controlled during their synthesis by varying not only
the number of carbon atoms in a hydrophobic chain but also the number of hydroxy ethylene groups. These
surfactants are soluble even in hard water. Hydrophobic surfaces become hydrophilic when non-ionogenic
surfactants are adsorbed from aqueous solutions.
MICELLES
When the surfactant molecules in the water-air interface become so packed in the monolayer that no more
molecules can be accommodated with ease, they aggregate in the bulk of the solution leading to the formation of
associated colloids also called micelles.
The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above
a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to
individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the
CMC is 10–4 to 10–3 mol L–1

Na+
Na+
COO–
Na+
Hydrophilic
Na+ Hydrophobic tail Head
of stearate ion
Na+ Na+
Na+
A Spherical micelle

Mechanism of micelle formation : Let us take the example of soap solutions. Soap is sodium or potassium
salt of a higher fatty acid and may be represented as RCOO –Na+. (example: sodium stearate CH3(CH2)16COO–Na+.
When dissolved in water, it dissociates into RCOO– and Na+ ions. The RCOO– ions, however, consist of two parts - a
long hydrocarbon chain R which is hydrophobic and a polar group COO– which is hydrophilic.
The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the hydrocarbon chains
R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the
bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the
centre of the sphere with COO– part remaining outward on the surface of the sphere. An aggregate thus formed is
known as 'ionic micelle'.
(a) Arrangement of stearate ions on the surface of water at low concentrations of soap.
(b) Arrangement of stearate ions inside the bulk of water (ionic micelle) at critical micelle concentrations of soap

Cleansing action of soaps : The cleansing action of soap is due to the fact that soap molecules form micelle
around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic
part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet
surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus, soap helps in
emulsification and washing away of oils and fats. The negativity charged sheath around the globules prevents them
from coming together and forming aggregates.
(a) Grease on cloth
(b) Stearate ions arranging around the grease droplet and
(c) Grease droplet surrounded by stearate ions (micelle formed)

GELS
Colloidal system in which liquids are the dispersed phase and solid act as the dispersion medium are called gels.
The common examples are boot polishes, jelly, gum Arabic, agar agar, processed cheese and silicic acid. When the
gels are allowed to stand for a long time, they give out small quantities of trapped liquids with accumulate on its
surface. This action of gels is known as Syneresis or Weeping. Some gels such as silica, gelatin and ferric hydroxide
liquify on shaking and reset on allowing to stand. This phenomenon of Sol-gel transformation is called thixotropy.
Gels are divided into two categories i.e. elastic gels and non-elastic gels. The two categories differ from their behavior
towards dehydration and rehydration as under
 S.No. Elastic gels Non-elastic gels

They change to solid mass on dehydration They change to solid mass on

which can be changed back to original dehydration which cannot be
1
form by addition of water followed by changed back to original form with
warming. water.

They absorb water placed in it with

2 simultaneous swelling. This phenomenon They do not exhibit imbibition.
is called imbibition.

Consider the following pairs

S.No. Dispersed Phase Dispersing Medium Type
1 Liquid Gas Aerosol
2 Gas Liquid Foam
3 Liquid Liquid Emulsion
4 Liquid Solid Solid Sol

Illustration 12
Which of the following will show Tyndall effect?
(A) Aqueous solution of soap below critical micelle concentration
(B) Aqueous solution of soap above critical micelle concentration
(C) Aqueous solution of Sodium Chloride
(D) Aqueous solution of Sugar
Solution:
(B)
Aqueous solution of soap above critical temperature show Tyndall effect.

Illustration 13
Given below are statements regarding colloids. Identify the correct statement.
(A) Based on the nature of the interaction between the dispersed phase and dispersed medium, the colloids are classified
into multimolecular colloids, macromolecular colloids, and associated colloids.
(B) In multimolecular colloids, the particles have the size in the colloidal range of diameter >1
(C) Starch, cellulose, etc. are examples of naturally occurring macromolecular colloids.
(D) Bredig's arc method is one of the chemical methods to prepare colloids.
Solution:
(C)
Starch, cellulose, etc. are examples of naturally occurring macromolecular colloids

Illustration 14
Identify the INCORRECT statement regarding colloids.
(A) Cheese is an example of Gel in which dispersed phase is liquid and dispersion medium is solid.
(B) A colloid is a homogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in
another substance called dispersion medium.
(C) The lyophobic colloids are also termed as irreversible sols.
(D) The formation of associated colloids takes place above Kraft temperature.
Solution:
(B)
A colloid is a homogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another
substance called dispersion medium

Illustration 15
Which property of colloids is applied in rubber plating & sewage disposal?
(A) Peptization
(B) Brownian movement
(C) Tyndall effect
(D) Electrophoresis
Solution:
(D)
Electrophoresis movement of colloidal particles towards an electrode, when they are subjected to an electrical field.

Illustration 16
Ferric hydroxide is a negative sol, which of the following electrolyte will coagulate it most:
(A) FeCI3
(B) CaCO3
(C) BaSO4
(D) NaCI
Solution:
(A)
If FeCI3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of
Fe3+ ions.
Ferric hydroxide is a negative sol, FeCI3 electrolyte will coagulate the most.