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
to 10
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
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
explain mechanism of adsorption;
explain the factors controlling
adsorption from gases and
solutions on solids;
explain adsorption results on the
basis of Freundlich adsorption
appreciate the role of catalysts in
enumerate the nature of colloidal
describe preparation, properties
and purification of colloids;
classify emulsions and describe
their preparation and properties;
describe the phenomenon of gel
list the uses of colloids.
Some of the most important chemicals are produced industrially by
means of reactions that occur on the surfaces of solid catalysts.
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
5.1 Adsorption5.1 Adsorption
5.1 Adsorption5.1 Adsorption
5.1 Adsorption
5.1.2 Mechanism
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 – TS,
G can be negative if H has sufficiently high negative value as – TS
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 TS 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
(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
). 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
) 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
)in this case.
1. It is caused by chemical bond
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
5. Enthalpy of adsorption is high
(80-240 kJ mol
) in this case.
Table 5.1: Comparison of Physisorption and Chemisorption
Physisorption Chemisorption
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
(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 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
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
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
195 K
244 K
273 K
Fig. 5.1: Adsorption isotherm
= 0,
= constant, the adsorption is independent of pressure.
= 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
(C is the equilibrium concentration, i.e., when adsorption is complete).
On taking logarithm of the above equation, we have
= 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
5.1.6 Applications