UNIT 07 SURFACE CHEMISTRY

Syllabus
• Adsorption
• Adsorption of gases on solids
• Adsorption from Solutions
• Catalysis
• Colloids
• Classification of Colloids
• Preparation of Colloidal Sols
• Purification of Colloidal Sols
• Important Properties of Colloidal Sols
• Emulsions
• Application of Colloids

Surface Chemistry is the chemistry at the boundary separating two bulk phases. This boundary which is also known as surface or interface is represented by separating the bulk phases by a hyphen or a slash. For example, the interface between a solid and liquid may be represented by solid-liquid or solid/liquid interface. There is no interface between gases due to their complete miscibility. The bulk phases may be pure compounds or solutions. The interface is usually a few molecules thick but its area depends on the size of the particles of the bulk phases. Dissolution and crystallisation , electrode processes , heterogeneous catalyses, corrosion and many other important phenomena take place at interfaces. Therefore , subject of surface chemistry is of great importance in industry, analytical chemistry and our daily lives.
To carry out surface studies, it is necessary to have a really clean surface. It is now possible to obtain ultra clean surfaces of metals by heating them under very high vaccum (108 to 109 pascal). Such clean solid materials have to be stored in vaccum, otherwise their surface will be covered by molecules of oxygen and nitrogen from air.
ADSORPTION
In the interior of a liquid or a solid, molecules experience attraction on all sides, whereas the molecules on the surface have other neighbouring molecules only below and on the sides. As a result molecules at the surface experience a net attraction downward and move toward the interior to increase attractions. The molecules on the surface , therefore have higher energy than those inside (Fig).

Molecules at the surface of a liquid experience a net attraction downward.

The result is that the surface of a solid or a liquid is in a strain or tension on account of the unbalanced or residual forces. The surface of a solid( or a liquid) , therefore , tends to attract and retain other molecules when it is brought in contact with a gas or a solution. For example, when finely divided active carbon or clay is stirred into a dilute solution of a dye, we observe that the intensity of colour in the solution is decreased. Similarly, when a finely divided solid is exposed to a gas at low pressure, the pressure decreases noticeably. In both these situations , the dye or the gas are adsorbed on the solid surface.
As the molecules remain only at the surface and do not go deeper into the bulk, their concentration is more at the surface than in the bulk of the solid or liquid as the case may be. This phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting into higher concentration of the molecules on the surface is called adsorption. As a result of adsorption, there is a decrease of surface energy. The substance on the surface of which adsorption takes place is called adsorbent and the substance adsorbed is known as adsorbate. The process of removal of an adsorbed substance from the surface on which it is adsorbed is called desorption. It is the reverese of adsorption and can be brought about by heating or by reducing the pressure.
Solids , particularly when finely divided , have a large surface area and therefore, show this property of adsorption to a much larger extent. Charcoal, silica gel, alumina gel, clay etc. are very good adsorbents because they have highly porous structures and hence large surface areas. Colloids, on account of their extremely small dimensions, possess enormous surface area per unit mass and are therefore good adsorbents.
ABSORPTION
The term absorption implies that substance is uniformly distributed throughout the body of a solid or a liquid. In adsorption, there is a higher concentration of the substance on the surface of a solid or a liquid. Thus water vapour is absorbed by anhydrous CaCl2, while it is adsorbed by silica gel. Similarly while ammonia is absorbed in water, it is adsorbed by charcoal. The differences between adsorption and absorption is illustrated in Fig.

Sold 1 has absorbed the gas uniformly
Solid 2 has adsorbed the gas on its surface

Distinction between adsorption and absorption
Adsorption Absorption
1. It is the phenomenon of higher concentration of gas or liquid on the surface than in the bulk of the solid. 1. It is the phenomenon in which the gas or the liquid particles get uniformly distributed throughout the body of the solid.
2. It occurs only at the surface 2. It occurs through out the body of the material
3. It is rapid in the beginning, but becomes slow later. 3. It occurs at uniform rate
SORPTION
In some cases , both absorption and adsorption occur together and not distinguishable. In such cases, the substance gets uniformly distributed into the bulk of the solid but at the same time, its concentration is higher at the surface than in the bulk. Such phenomenon is called sorption.
Forces in adsorption
The forces involved in adsorption are of various types such as van der Waal’s forces and even chemical bond forces.
The forces of adsorption come into existence as soon as a solid is broken to give two surfaces.

Residual forces at the surface of a solid

In a complete piece of a solid, the binding forces are used up to bind the constituent particles. In new surfaces, some forces are left free to act on particles of gases and of solutions to attract them and attach them to their surface. This explains adsorption.
Adsorbent
It is the material upon whose surface the adsorption takes place.
Adsorbate
The substance being adsorbed is called adsorbate.
Desorption
The process of removal of an adsorbed substance on which it is adsorbed is known as desorption.
Types of adsorption
Adsorption is of two types:
i) Positive adsorption (ii) Negative adsorption
Positive adsorption
When the concentration of adsorbate is more on the surface of adsorbent relative to its concentration in the bulk, it is called positive adsorption.
Negative adsorption
When the concentration of the adsorbate is less on the surface relative to its concentration in the bulk, it is called negative adsorption. For example, in the case of some liquid solutions, it is observed that the concentration of the solute is less on the surface than in the bulk of the solution. This type of adsorption is called negative adsorption.


TYPES OF ADSORPTION
Depending upon the nature of forces which hold the molecules of the adsorbate on the surface of adsorbent, the adsorption is classified into two types.
(i) Physical adsorption (ii) Chemical adsorption
Physical Adsorption
When particles of the adsorbate are held to the surface of adsorbent by physical forces such as van der Waal’s forces, adsorption is called physical adsorption or physisorption. The attractive forces are weak and therefore, these can be easily overcome either by increasing or decreasing the pressure . In other words, physical adsorption can easily reversed or decreased.
Characteristics of Physical adsorption
i) The attractive forces between adsorbent and adsorbate molecules are weak and therefore molar heat of adsorption is low and is of the order of 20 – 40 kJ/mol.
ii) The physical adsorption process is reversible and therefore equilibrium is reached rapidly.
iii) If temperature is increased, the kinetic energy of the gas molecules increases and they leave the surface of the adsorbent.
iv) It is not specific in nature and therefore all the gases are adsorbed on all solids to the same extent.
v) The extent of physical adsorption depends upon the ease of liquefaction of the gas. The gases which are easily liquefied can be adsorbed rapidly.
vi) In physical adsorption, the state of adsorbate is same as in the bulk.
Chemical Adsorption
When the molecules of the adsorbate are held to the surface of adsorbent by chemical forces, the adsorption is called chemical adsorption or chemisorption.
In this case , a chemical reaction occurs between the adsorbed molecules and adsorbent on the surface. This type of adsorption is irreversible. Since chemical reaction takes place in this type of adsorption, therefore a state of adsorbed molecules may be different from that in bulk.
Characteristics of Chemical adsorption
i) Attractive forces between adsorbent and adsorbate molecules are strong chemical bonds and therefore , molar heat of adsorption is high and is of the order of 200 – 400 kJ/mol.
ii) Unlike physical adsorption, it is irreversible.
iii) Chemical adsorption first increases with increase in temperature up to a certain extent and then decreases regularly.
iv) Unlike physical adsorption, chemical adsorption involves the formation of chemical linkages between adsorbed molecules and the surface of adsorbent. Therefore, it is highly selective. In other words, chemical adsorption depends upon the adsorbent and adsorbate.
v) Since chemical reaction takes place in this type of adsorption, therefore state of the adsorbed molecules may be different from that in the bulk. For example, oxygen exists as O2 in bulk but on the surface it may exist as O2, O22, O, O2 etc.
Enthalpy of Adsorption
The amount of heat evolved when one mole of an adsorbate (gas or liquid) is adsorbed on the surface of an adsorbent is called enthalpy of adsorption. The enthalpy of adsorption for chemisorption is larger than that for physical adsorption. For example, the enthalpy of adsorption for chemisorption is of the order of 200  400 kJ/mol, while the enthalpy of adsorption for physical adsorption is about 20 to 40 kJ/mol.
Difference between Physical adsorption and chemical adsorption
Physical adsorption Chemical adsorption
1. The forces between the adsorbate molecules and adsorbent are weak van der Wall’s forces. 1. The forces between the adsorbate molecules and the adsorbent are strong chemical forces.
2. Low heats of adsorption of the order of 20 - 40 kJ/mol 2. High heat of adsorption of the order of 200 - 400 kJ/mol
3. Usually occurs at low temperature and decreases with increasing temperature. 3. It occurs at high
temperature

4. It is reversible 4. It is irreversible
5. The extent of adsorption depends upon the ease of liquefaction of the gas 5. There is no correlation between the extent of adsorption and ease of liquefaction of gas.
6. It is not specific in nature i.e., all gases are adsorbed on the surface of solid to the same extent. 6. It is highly specific in nature and occurs only when there is bond formation between adsorbent and adsorbate molecules.
7. The state of adsorbate is same as in the bulk. 7. The state of adsorbate molecules may be different from that in the bulk.
8. It forms multi-molecular layers. 8. It forms monomolecular layer
9. Rate of adsorption increases with increase in pressure of adsorbate. 9. Rate of adsorption usually decreases as the pressure increases.
Adsorption of nitrogen on Iron
The difference between physical adsorption and chemisorption is typified by the behaviour of nitrogen on iron. At 83 K nitrogen is physisorbed on iron surface as nitrogen molecules, N2. The amount of nitrogen adsorbed decreases rapidly as the temperature rises. At room temperature, practically three is no adsorption of nitrogen on iron. At 773 K and above , nitrogen is chemisorbed on the iron surface as nitrogen atoms.
GIBB’S ENERGY CHANGE DURING ADSORPTION
Adsorption is an exothermic process and therefore H is negative for adsorption and favours the process. On the other hand the molecules of adsorbate (gas) are held on the surface of adsorbent and therefore, they have lesser tendency to move about freely. In other words, entropy decreases i.e., S is negative and the entropy factor opposes the process. According to Gibbs Helmholtz equation,
G = H – T S or G = (–) – T (–)
Thus , for the process of adsorption to occur, G must be negative, which is possible only when H > T S in magnitude. This is true in the beginning. However, as the adsorption continues, H keeps on decreasing and TS keeps on increasing and ultimately G becomes zero. This state is called adsorption equilibrium.
ADSORPTION OF GASES ON SOLIDS
Gases are adsorbed on finely divided metals such as Ni, Pd, Pt, Fe etc. The extent of adsorption of a gas on solid surface is affected by the following factors.
i) Nature of the gas ii) Nature of adsorbent
iii) Effect of pressure iv) Effect of temperature
v) Activation of adsorbent
1. Nature of the gas
The adsorption depends upon the nature of the gas adsorbed. The easily liquefiable gases such as HCl, NH3, Cl2 etc are adsorbed more than permanent gases such as H2, N2 and O2.
The ease of liquefaction of a gas depends up on the critical temperature. The critical temperature of the gas is the minimum temperature above which a gas cannot be liquefied however high pressure may be applied. The higher the critical temperature, the more easily a gas is liquefied and hence more readily it will be adsorbed. For example, 1 g of activated charcoal can adsorb the following amounts of gases.
Gas SO2 NH3 CO2 CH4 CO N2 H2
Critical temperature Tc(K)
430
406
304
190
134
126
33
Amount adsorbed
( in ml)
380
180
48
16.2
9.3
8.0
4.5
However, in the case of chemical adsorption, a gas gets adsorbed on the solid only, if it forms chemical bonds.
2. Nature of adsorbent
Activated charcoal can adsorb gases which are easily liquefied. Many poisonous gases get adsorbed by charcoal. Therefore it is used in gas masks for adsorbing these poisonous gases. Gases such as H2, N2 and O2 are generally adsorbed on finely divided transition metals eg. Ni and Co. The extent of adsorption depends on the available surface.
3. Specific area of the adsorbent : Specific area of an adsorbent is the surface area available for adsorption per gram of adsorbent. The greater the specific area of the solid, the greater would be its adsorbing capacity. That is why porous or finely divided forms of adsorbents adsorb larger quantities of adsorbate. However, the pores should be large enough to allow the gas molecules to enter them.
4. Effect of pressure : Adsorption isotherms :
The extent of adsorption of a gas on solid is expressed as x/m where m is the mass of the adsorbent and x is the number of moles of the adsorbate when dynamic equilibrium has been attained between free gas and the adsorbed gas. One can measure experimentally the relation between x/m and pressure (P) of the gas at constant temperature is called adsorption isotherm. Adsorption isotherms of different shapes have been experimentally observed. To account for these isotherms, many different adsorption isotherms have been proposed. We shall confine ourselves only to adsorption isotherms.
Freundlich’s adsorption isotherm
The extent of adsorption of gas per unit mass of adsorbent depends upon the pressure of the gas. The variation of extent of adsorption (expressed as x / m where x is the mass of adsorbate and m is the mass the adsorbent) and the pressure is given in Fig.

It is clear from the figure that adsorption (x/m) increases with increased pressure and becomes maximum corresponding to pressure P0, called equilibrium pressure. At this pressure, the extent of adsorption becomes constant even though the pressure is increased. This state is also called saturation state and P0 is called saturation pressure. The variation of extent of adsorption (x/m) with pressure (P) was given by Freundlich. From the adsorption isotherm , the following observations can be easily made.
(i) At low pressure : At low pressure the graph becomes nearly straight and slopping. This indicates that at these pressures x/m is directly proportional to pressure. This is put as :

where k is a constant.
(ii) At high pressure : At pressures beyond Po the graph becomes constant which means that x/m becomes independent of pressure. This may be expressed as :

(iii) Thus in an intermediate range of pressure, x/m will depend upon the power of pressure which lies between 0 and 1 , i.e., fractional power of pressure. This may be expressed as :

where n may take any whole number value which depends upon the nature of adsorbate and adsorbent. The above relationship is also called Freundlich’s adsorption isotherm. The constant k and n can be determined as explained below:
Taking logarithms on both sides of equation (3) , we get :

Thus if we plot a graph between log (x/m) and log P, a straight line will be observed. The slope of the line (Fig below) is equal to 1/n and intercept is equal to log k.

Plot of log (x/m) vs log P

LANGMUIR ADSORPTION ISOTHERM
One of the drawbacks of the Freundlich adsorption is that it fails at high pressure of the gas. Langmuir derived an adsorption isotherm on theoretical considerations based on kinetic theory of gases. This is named as Langmuir adsorption isotherm. This isotherm is based on the assumption that every adsorption site is equivalent and that the ability of the particle to bind there is independent of whether or not nearby sites are occupied. In his derivation Langmuir considered adsorption to consist of the following opposing processes :
(i) Adsorption of the gas molecules on the surface of the solid.
(ii) Desorption of the adsorbed molecules from the surface of the solid.
Langmuir believed that eventually a dynamic equilibrium is established between the above two opposing processes. He also assumed that the layer of the adsorbed gas was only one molecule thick, i.e., unimolecular. Since such type of adsorption is obtained in the case of chemisorption, Langmuir adsorption isotherm works particularly well for chemisorption.
Langmuir adsorption isotherm is represented by the relation :

where a and b are two Langmuir parameters. At high pressures, the above isotherm acquires the limiting form :

At very low pressures , Eq 1) is reduced to :

In order to determine the parameters a and b , Equ (1) may be written in its inverse form :


A plot of m/x againt 1 / p gives a straight line with slope equal to 1 / a and intercept b / a. Thus both parameters can be determined. (Fig a)

The Langmuir isotherm , in the form of equation (1) is generally more successful in interpreting the data than the Freundlich isotherm when a monolayer is formed. A Plot of x/m versus P is shown in Fig b.

Fig b Langmuir Adsorption isotherm
At low pressures according to Equ (3) x/m increases linearly with p. At high pressure according to equation (2) , x/m becomes constant i.e., the surface is fully covered and change in pressure has no effect and no further adsorption takes place, as is evident from the above Fig (b).
5. Effect of temperature : Adsorption isobar
The process of adsorption is exothermic. According to Le-Chatelier principle, the rate of adsorption will not be favoured by the increase in temperature. A graph drawn between extent of adsorption x/m and temperature at constant pressure is called adsorption isobar. The adsorption isobars for physical adsorption and chemical adsorption is shown in Fig. These isobars are different from each other. While physical adsorption isobar shows a decrease in x/m as temperature rises , the isobar of chemisorption shows an increase in the beginning and then a decrease as temperature increases. The initial increase is due to the fact that like chemical reactions, chemisorption also requires some activation energy.

Physical adsorption Chemical adsorption
6. Activation of adsorbent : Activation of adsorbent means the increasing of adsorbing power of the adsorbent. It is very necessary to increase the rate of adsorption. This can be done by the following methods :
(a) Metallic adsorbents are activated by mechanical rubbing or subjecting it to some chemical reactions.
(b) To increase the adsorbing power of adsorbents, they are sub-divided into smaller pieces. As a result, the surface area increases and therefore, the adsorbing power increases.
(c) Some adsorbents are activated by strong heating in contact with super heated steam. For example, charcoal is activated by subjecting it to the action of superheated steam.
ADSORPTION FROM SOLUTIONS
The process of adsorption can take place from solutions also. It is observed that solid adsorbents adsorb certain solutes from solutions in preference to other solutes and solvents. For example, animal charcoal decolourises impure sugar solution by adsorbing dye in preference to sugar molecules. The Freundlich’s adsorption isotherms obtained for the adsorption of gases on the surface of solid adsorbents have also been found to be applicable to adsorption of solutes from solutions. Here the equilibrium pressures in adsorption of gases has been replaced by equilibrium concentrations (C) of the adsorbates in solution. The adsorption isotherm may be represented as :

Taking logarithms , equation ( 5) becomes :

A graph drawn between log (x/m) vs log C will be a straight line. From the graph, the value of (1/n) and log k can be calculated as slope and intercept respectively.
For Langmuir adsorption isothems

The parameters of Equ (6) and (7) can be obtained by methods described for adsorption of gases on solids.
Applications of adsorption
The phenomenon of adsorption finds a large number of applications. A few examples are mentioned below :
(i) Creation of high vaccum : Adsorption by charcoal cooled in liquid air helps in creating a high vaccum in a vessel which has already been evacuated by a vaccum pump.
(ii) Removal of toxic gases : Adsorption of toxic gases by activated charcoal makes useful in gas masks.
(iii) Removal of moisture : Alumina and silica gel are used as adsorbents for removing moisture and for controlling humidities in rooms.
(iv) Removal of colour : Animal charcoal is used as decolouriser in the manufacture of cane sugar.
(v) In catalysts : Adsorption also plays a significant role in heterogeneous catalysis.
(vi) In chromatography : Adsorption forms the basis of chromatography.
ROLE OF ADSORPTION IN CATALYTIC REACTIONS
Many gaseous reactions proceed rapidly in the presence of suitable solid catalysts. Granular forms of catalysts are more useful due to adsorption of the reactants on the surface of the catalyst. The following types of help is provided by adsorption in catalytic reactions.
(i) After the reactant molecules are adsorbed, the attack by other molecules on it becomes easier. This is because the adsorbed substance is not free to move about and escape the collision of other molecules.
(ii) Adsorbed molecules may expose its own attackable part for reaction with other molecules.
(iii) Adsorbed substances may get dissociated into active atoms or free radicals which are capable of reacting much faster than molecules. For example, hydrogen molecule split up into atoms during adsorption on the surface of nickel and platinum.
(iv) Reaction proceeds rapidly at higher concentration. Thus the simultaneous adsorption of reactants provides increased concentration conditions.
(v) Chemisorption provides the necessary activation energy due to heat of adsorption evolved in the process.