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. Due to complete miscibility, there is no

interface between the gases. The bulk phases that

we come across in surface chemistry may be pure

compounds or solutions. The interface is normally a

few molecules thick but its area depends on the size

of the particles of bulk phases. Many important

phenomena, noticeable amongst these being

corrosion, electrode processes, heterogeneous

catalysis, dissolution and crystallisation occur at

interfaces. The subject of surface chemistry finds

many applications in industry, analytical work and

daily life situations.

To accomplish surface studies meticulously, it

becomes imperative to have a really clean surface.

Under very high vacuum of the order of 10

–8

to 10

–9

pascal, it is now possible to obtain ultra clean surface

of the metals. Solid materials with such clean

surfaces need to be stored in vacuum otherwise these

will be covered by molecules of the major components

of air namely dioxygen and dinitrogen.

In this Unit, you will be studying some important

features of surface chemistry such as adsorption,

catalysis and colloids including emulsions and gels.

Surface Chemistry

After studying this Unit, you will be

able to

• describe interfacial phenomenon

and its significance;

• define adsorption and classify it

into physical and chemical

adsorption;

• explain mechanism of adsorption;

• explain the factors controlling

adsorption from gases and

solutions on solids;

• explain adsorption results on the

basis of Freundlich adsorption

isotherms;

• appreciate the role of catalysts in

industry;

• enumerate the nature of colloidal

state;

• describe preparation, properties

and purification of colloids;

• classify emulsions and describe

their preparation and properties;

• describe the phenomenon of gel

formation;

• list the uses of colloids.

Objectives

Some of the most important chemicals are produced industrially by

means of reactions that occur on the surfaces of solid catalysts.

Unit

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124Chemistry

There are several examples, which reveal that the surface of a solid has the

tendency to attract and retain the molecules of the phase with which it

comes into contact. These molecules remain only at the surface and do

not go deeper into the bulk. The accumulation of molecular species

at the surface rather 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 the

surface of which the adsorption takes place is called adsorbent.

Adsorption is essentially a surface phenomenon. Solids, particularly

in finely divided state, have large surface area and therefore, charcoal,

silica gel, alumina gel, clay, colloids, metals in finely divided state, etc.

act as good adsorbents.

Adsorption in action

(i) If a gas like O

, H

, CO, Cl

, NH

or SO

is taken in a closed vessel

containing powdered charcoal, it is observed that the pressure of the

gas in the enclosed vessel decreases. The gas molecules concentrate

at the surface of the charcoal, i.e., gases are adsorbed at the surface.

(ii) In a solution of an organic dye, say methylene blue, when animal

charcoal is added and the solution is well shaken, it is observed

that the filtrate turns colourless. The molecules of the dye, thus,

accumulate on the surface of charcoal, i.e., are adsorbed.

(iii) Aqueous solution of raw sugar, when passed over beds of animal

charcoal, becomes colourless as the colouring substances are

adsorbed by the charcoal.

(iv) The air becomes dry in the presence of silica gel because the water

molecules get adsorbed on the surface of the gel.

It is clear from the above examples that solid surfaces can hold

the gas or liquid molecules by virtue of adsorption. The process of

removing an adsorbed substance from a surface on which it is

adsorbed is called desorption.

In adsorption, the substance is concentrated only at the surface and

does not penetrate through the surface to the bulk of the adsorbent,

while in absorption, the substance is uniformly distributed throughout

the bulk of the solid. For example, when a chalk stick is dipped in ink,

the surface retains the colour of the ink due to adsorption of coloured

molecules while the solvent of the ink goes deeper into the stick due

to absorption. On breaking the chalk stick, it is found to be white from

inside. A distinction can be made between absorption and adsorption

by taking an example of water vapour. Water vapours are absorbed by

anhydrous calcium chloride but adsorbed by silica gel. In other words,

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.

Both adsorption and absorption can take place simultaneously

also. The term sorption is used to describe both the processes.

Adsorption arises due to the fact that the surface particles of the adsorbent

are not in the same environment as the particles inside the bulk. Inside

the adsorbent all the forces acting between the particles are mutually

5.1.1 Distinction

between

Adsorption

and

Absorption

5.1 Adsorption5.1 Adsorption

5.1 Adsorption

5.1.2 Mechanism

Adsorption

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125 Surface Chemistry

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 particles on its surface.The extent of

adsorption increases with the increase of surface area per unit mass of

the adsorbent at a given temperature and pressure.

Another important factor featuring adsorption is the heat of

adsorption. During adsorption, there is always a decrease in residual

forces of the surface, i.e., there is decrease in surface energy which

appears as heat. Adsorption, therefore, is invariably an exothermic

process. In other words, ∆H of adsorption is always negative. When a

gas is adsorbed, the freedom of movement of its molecules become

restricted. This amounts to decrease in the entropy of the gas after

adsorption, i.e., ∆S is negative. Adsorption is thus accompanied by

decrease in enthalpy as well as decrease in entropy of the system. For

a process to be spontaneous, the thermodynamic requirement is that,

at constant temperature and pressure, ∆G must be negative, i.e., there

is a decrease in Gibbs energy. On the basis of equation, ∆G = ∆H – T∆S,

∆G can be negative if ∆H has sufficiently high negative value as – T∆S

is positive. Thus, in an adsorption process, which is spontaneous, a

combination of these two factors makes ∆G negative. As the adsorption

proceeds, ∆H becomes less and less negative ultimately ∆H becomes

equal to T∆S and ∆G becomes zero. At this state equilibrium is attained.

There are mainly two types of adsorption of gases on solids.

If accumulation of gas on the surface of a solid occurs on account of

weak van der Waals’ forces, the adsorption is termed as physical

adsorption or physisorption. 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 involves a 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, dihydrogen is first adsorbed on nickel by van

der Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen

atoms which are held on the surface by chemisorption.

Some of the important characteristics of both types of adsorption

are described below:

Characteristics of physisorption

(i) Lack of specificity: A given surface of an adsorbent does not show any

preference for a particular gas as the van der Waals’ forces are universal.

(ii) Nature of adsorbate: The amount of gas adsorbed by a solid

depends on the nature of gas. In general, easily liquefiable gases

(i.e., with higher critical temperatures) are readily adsorbed as van

der Waals’ forces are stronger near the critical temperatures. Thus,

1g of activated charcoal adsorbs more sulphur dioxide (critical

temperature 630K), than methane (critical temperature 190K) which

is still more than 4.5 mL of dihydrogen (critical temperature 33K).

5.1.3 Types of

Adsorption

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126Chemistry

(iii) Reversible nature: Physical adsorption of a gas by a solid is

generally reversible. Thus,

Solid + Gas l Gas/Solid + Heat

More of gas is adsorbed when pressure is increased as the

volume of the gas decreases (Le–Chateliers’s principle) and the

gas can be removed by decreasing pressure. Since the adsorption

process is exothermic, the physical adsorption occurs readily at

low temperature and decreases with increasing temperature

(Le-Chatelier’s principle).

(iv) Surface area of adsorbent: The extent of adsorption increases

with the increase of surface area of the adsorbent. Thus, finely

divided metals and porous substances having large surface areas

are good adsorbents.

(v) Enthalpy of adsorption: No doubt, physical adsorption is an

exothermic process but its enthalpy of adsorption is quite low (20–

40 kJ mol

-1

). This is because the attraction between gas molecules

and solid surface is only due to weak van der Waals’ forces.

Characteristics of chemisorption

(i) High specificity: Chemisorption is highly specific and it will

only occur if there is some possibility of chemical bonding

between adsorbent and adsorbate. For example, oxygen is

adsorbed on metals by virtue of oxide formation and hydrogen

is adsorbed by transition metals due to hydride formation.

(ii) Irreversibility: As chemisorption involves compound formation, it

is usually irreversible in nature. Chemisorption is also an

exothermic process but the process is very slow at low

temperatures on account of high energy of activation. Like most

chemical changes, adsorption often increases with rise of

temperature. Physisorption of a gas adsorbed at low temperature

may change into chemisorption at a high temperature. Usually

high pressure is also favourable for chemisorption.

(iii) Surface area: Like physical adsorption, chemisorption also

increases with increase of surface area of the adsorbent.

(iv) Enthalpy of adsorption: Enthalpy of chemisorption is high

(80-240 kJ mol

-1

) as it involves chemical bond formation.

1. It arises because of van der

Waals’ forces.

2. It is not specific in nature.

3. It is reversible in nature.

4. It depends on the nature of

gas. More easily liquefiable

gases are adsorbed readily.

5. Enthalpy of adsorption is low

(20-40 kJ mol

–1

)in this case.

1. It is caused by chemical bond

formation.

2. It is highly specific in nature.

3. It is irreversible.

4. It also depends on the nature

of gas. Gases which can react

with the adsorbent show

chemisorption.

5. Enthalpy of adsorption is high

(80-240 kJ mol

–1

) in this case.

Table 5.1: Comparison of Physisorption and Chemisorption

Physisorption Chemisorption

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127 Surface Chemistry

The variation in the amount of gas adsorbed by the adsorbent with

pressure at constant temperature can be expressed by means of a

curve termed as adsorption isotherm.

Freundlich adsorption isotherm: 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:

= k.p

1/n

(n > 1) ... (5.1)

where x is the mass of the gas adsorbed on mass m of the

adsorbent at pressure P, k and n are constants which depend

on the nature of the adsorbent and the gas at a particular

temperature. The relationship is generally represented in the

form of a curve where mass of the gas adsorbed per gram of

the adsorbent is plotted against pressure (Fig. 5.1). These curves

indicate that at a fixed pressure, there is a decrease in physical

adsorption with increase in temperature. These curves always

seem to approach saturation at high pressure.

Taking logarithm of eq. (5.1)

log

= log k +

log p ... (5.2)

The validity of Freundlich isotherm can be

verified by plotting log

on y-axis (ordinate)

and log p on x-axis (abscissa). If it comes to be

a straight line, the Freundlich isotherm is valid,

otherwise not (Fig. 5.2). The slope of the

straight line gives the value of

. The intercept

on the y-axis gives the value of log k.

Freundlich isotherm explains the behaviour

of adsorption in an approximate manner. The

factor

can have values between 0 and 1

(probable range 0.1 to 0.5). Thus, equation (5.2)

holds good over a limited range of pressure.

5.1.4 Adsorption

Isotherms

Fig. 5.2: Freundlich isotherm

6. Low temperature is favourable

for adsorption. It decreases

with increase of temperature.

7. No appreciable activation

energy is needed.

8. It depends on the surface

area. It increases with an

increase of surface area.

9. It results into multimolecular

layers on adsorbent surface

under high pressure.

6. High temperature is favourable

for adsorption. It increases with

the increase of temperature.

7. High activation energy is

sometimes needed.

8. It also depends on the surface

area. It too increases with an

increase of surface area.

9. It results into unimolecular

layer.

195 K

244 K

273 K

Fig. 5.1: Adsorption isotherm

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128Chemistry

When

= 0,

= constant, the adsorption is independent of pressure.

When

= 1,

= k p, i.e.

∝ p, the adsorption varies directly

with pressure.

Both the conditions are supported by experimental results.

The experimental isotherms always seem to approach saturation at

high pressure. This cannot be explained by Freundlich isotherm. Thus,

it fails at high pressure.

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 precipitate of Mg(OH)

attains blue colour

when precipitated in presence of magneson reagent. The colour is due

to adsorption of magneson. 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 is taken into account, i.e.,

= kC

1/n

...(5.3)

(C is the equilibrium concentration, i.e., when adsorption is complete).

On taking logarithm of the above equation, we have

log

= logk +

logC ...(5.4)

Plotting log

against log C a straight line is obtained which

shows the validity of Freundlich isotherm. This can be tested

experimentally by taking solutions of different concentrations of acetic

acid. Equal volumes of solutions are added to equal amounts of

charcoal in different flasks. The final concentration is determined in

each flask after adsorption. The difference in the initial and final

concentrations give the value of x. Using the above equation, validity

of Freundlich isotherm can be established.

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.

5.1.5 Adsorption

from

Solution

Phase

5.1.6 Applications

Adsorption

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