Chapter 5

surface chemistry

Introduction :

Surface chemistry is that branch of chemistry which deals with study of the phenomena occuring at the surface or interface, i.e. at the boundary separating two bulk phases. In this chapter our main emphasis will be on three important topics related to surface chemistry, viz., adsorption, colloids and emulsions.

Adsorption

The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid resulting into a higher concentration of the molecules on the surface is called adsorption. As a result of adsorption, there is a decrease of surface energy. The process of removal of an adsorbed substance from the surface on which it is adsorbed is called desorption. It is the reverse of adsorption and can be brought about by heating or by reducing the pressure.

Adsorbent and adsorbate :The substance on the surface of which adsorption occurs is known as adsorbent. The substances that get adsorbed on the solid surface due to intermolecular attractions are called adsorbate.Charcoal, silica, gel, alumina gel are good adsorbents because they have highly porous structures and have large surface area. Colloids on account of their extremely small dimensions possess enoromous surface area per unit mass and are, therefore, also good adsorbents.

Examples of adsorption :

Adsorption of a gas by charcoal :  Finely divided activated charcoal has a tendency to adsorb a number of gases like ammonia, sulphur dioxide, chlorine, phosgene, etc. In this case, charcoal acts as an adsorbent while gas molecules act as adsorbate.

Adsorption of a dye by charcoal :  Animal charcoal is used for decolourising a number of organic substances in the form of their solutions. The discharge of the colour is due to the fact that the coloured component (generally an organic dye) gets adsorbed on the surface of the adsorbent (animal charcoal).

Sorption :  When both adsorption and absorption take place simultaneously.

Eg :  Dyes get adsorbed as well as absorbed in the cotton fibre i.e. sorption takes place.

Difference between adsorption and absorption :

The terms adsorption and absorption are different. Adsorption is a phenomenon in which there is higher concentration of another substance on the surface than in the bulk. On the other hand, absorption is a phenomenon in which the molecules of a substance are uniformly distributed throughout the body of other substance. For example, when silica gel is placed in the environment of water, it adsorbs the water vapour. The water vapour are present in high concentration at the surface of silica gel. On the other hand, when calcium chloride is placed in the environment of water, it absorbs water. The water vapour uniformly get distributed throughout the body of calcium chloride. Thus, silica gel adsorbs water vapour while anhydrous calcium chloride absorbs water.

Thermodynamics of adsorption : Adsorption is an exothermic process. Therefore DH of adsorption is always negative. When a gas is adsorbed the entropy of the gas decreases i.e. DS is negative. Adsorption is thus accompanied by decrease in enthalpy as well as entropy of the system, for a process to be spontaneous requirement is that DG must be negative. On the basis of equation, DG = DH - TDS, DG can be negative if DH has sufficiently high negative value as - TDS is positive. Thus, in an adsorption process, which is spontaneous, DS is negative, and DH is also sufficiently negative and as a combination of these two factors, DG is negative.

H becomes less and less negative as adsorption proceeds further and further. Ultimately DH becomes equal to TDS and DG becomes zero. This is the state at which equilibrium is attained.

Enthalpy of adsorption DHadsorption : It is the amount of the heat released when 1 mole of an adsorbate gets adsorbed on a particular adsorbent at adsorption equilibrium. It depends upon the nature of both the adsorbate as well as adsorbent.

Types of adsorption : The adsorption is classified into two types :

(i) Physical adsorption (i.e. physisorption) : When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van der Waal’s forces, the adsorption is called physical adsorption or vanderwaals adsorption.

(ii) Chemical adsorption (i.e. chemisorption) :

When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces, the adsorption is called chemical adsorption.

Difference between physical adsorption and chemical adsorption

Competitive adsorption : When an adsorbent is in contact with more than one species (adsorbate). There will be competition among them to get adsorbed on to the surface of the adsorbent. The one that is more strongly adsorbed gets deposited first in preference to the others. Further a strongly adsorbed substance may displace a weakly adsorbed substance.

Ex. NH3 can displace O2 or N2 from the surface of charcoal.

Adsorption of gases on solids :

The extent of adsorption of a gas on a solid surface is affected by the following factors:

The nature of the gas (i.e. nature of the adsorbate). The easily liquefiable gases such as HCl, NH3, Cl2 etc. are adsorbed more than the permanent gases such as H2, N2 and O2. The ease with which a gas can be liquefied is primarily determined by its critical temperature. Higher the critical temperature (Tc) of a gas, the more easily it will be liquefied and, therefore, more readily it will be adsorbed on the solid.

Gas    SO2      CH4     H2  

TC     330K   190 K   33 K

Nature of adsorbent. The extent of adsorption of a gas depends upon the nature of adsorbent. Activated charcoal (i.e. activated carbon), metal oxides (silica gel and aluminium oxide) and clay can adsorb gases which are easily liquified. Gases such as H2, N2 and O2 are generally adsorbed on finely divided transition metals Ni and Co.

Activation of adsorbent :

(a) Metallic adsorbents are activated by mechanical rubbing or by subjecting it to some chemical reactions.

(b) To increase the adsorbing power of adsorbents, they are sub-divided into smaller pieces. As a results, the surface area is increased and therefore, the adsorbing power increases.

Effect of temperature :Mostly the process of adsorption is exothermic and the reverse process or desorption is endothermic. If the above equilibrium is subjected to increase in temperature, then according to Le-Chaterlier’s principle, with increase in temperature, the desorption will be favoured. Physical adsorption decreases continuously with increase in temperature whereas chemisorption increases initially, shows a maximum in the curve and then it decreases continuously.

The initial increase in chemisorption with increase in temperature is because of activation energy required.

This is why the chemical adsorption is also known as “Activated adsorption”.

A graph between degree of adsorption (x/m) and temperature ‘t’ at a constant pressure of adsorbate gas is known as adsorption isobar.

 

Effect of pressure. The extent of adsorption of a 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 of the adsorbent) and the pressure is given as below. A graph between the amount of adsorption and gas pressure keeping the temperature constant is called an adsorption isotherm.  

It is clear from the figure-1 that extent of adsorption (x/m) increases with pressure and becomes maximum corresponding to pressure Ps called equilibrium pressure. Since adsorption is a reversible process, the desorption also takes place simultaneously. At this pressure (Ps) the amount of gas adsorbed becomes equal to the amount of gas desorbed.

Freundlich Adsorption isotherm :

The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich.

At low pressure the graph is almost straight line which indicates that x/m is directly proportional to the pressure. This may be expressed as :

(x/m) a p     or       (x/m) = kp   where k is constant.

At high pressure the graph becomes almost constant which means that x/m becomes independent of pressure. This may be expressed as :

(x/m) = constant    or     (x/m) a p0   (since p0 = 1)     or    (x/m) = kp0.

hus, in the intermediate range of pressure, x/m will depend upon the power of pressure which lies between 0 to 1, fractional power of pressure. This may be expressed as

 (x/m) a p1/n    or     (x/m) = kp1/n  

Where n can 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

Eq. (x/m) = kp1/n            we get                                                                      

 log (x/m) = logk + (1/n) log p.

One of the drawbacks of Freundlich isotherm is that it fails at high pressure of the gas. 

This equation applicable only when adsorbate substance form unimolecular layer on adsorbent surface. i.e. chemical adsorption.

Adsorption from solutions : The process of adsorption can take place from solutions also. It is observed that solid adsorbents adsorb certain solutes from solution in preference to other solutes and solvents. For example, animal charcoal decolouries impure sugar solution by adsorbing colouring dye in preference to sugar molecules.

The extent of adsorption from solution depends upon the concentration of solute in the solution as given by Freundlich isotherm :

(x/m) = k(c)1/n     (n 1)

where c is the equilibrium concentration of the solute in solution.

Temperature dependence here also is similar to that for adsorption of gases and in place of equilibrium pressure, we use equilibrium concentrations of the adsorbates in the solution.

Applications of adsorption :

In gas masks :  Activated charcoal is generally used in gas masks to adsorb poisonous and toxic gases from air. These masks are commonly used by the miners because there are poisonous gases like CO, CH4 etc. in the atmosphere in coal mines.

In dyeing of cloths : Mordants such as alums are used in dyeing of cloths. They adsorb the dye particles which, otherwise, do not stick to the cloths.

In dehumidizers : Silica gel is commonly used to adsorb humidity or moisture from air.

Removal of colouring matter : Many substances such as sugar, juice and vegetable oils are coloured due to the presence of impurities. They can be decolourised by placing them in contact with adsorbents like activated charcoal or fuller’s earth.

Heterogeneous catalysis : The phenomenon of adsorption is useful in the heterogeneous catalysis. The metals such as Fe, Ni, Pt, Pd, etc, are used in the manufacturing processes such as Contact process, Haber process and the hydrogenation of oils. Their use is based upon the phenomenon of adsorption.

Refining Petroleum : Silica gel is used as adsorbent in petroleum refining.

Chromatography : It is a method for separation of component and is based on preferential adsorption column is very common device used.

Creating vacuum : High vacuum can be created by removing gas by adsorption.

Adsorption Indicators : In volumetric analysis, adsorption indicator is used Surface of certain precipitates such as silver halide have the property of adsorbing some dye like eosin, fluorescein, etc In the case of precipitation titration (AgNO3 vs NaCI) of the indicator is adsorbed at the end point producing a characteristic colour on the precipitate.

In froth floatation process : (in metallurgy).

Softening of hard water : Ion exchange resins used for softening of hard water is based upon selective and competive adsorption of ions on resins.

Na2Z + Ca+2  CaZ + 2Na+

The organic polymers containing groups like –COOH, –SO3H and –NH2 etc. possess the property of selective adsorption of ions from solution.  These are quite useful in the softening of water.