Adsorption

We have dealt with a large number of chemical changes up till now which actually occur at the surfaces of interacting species. Matter that is at or near an interface is not in the same state as matter in the deep cleft, which includes the active site at which substrate is bound. The exterior strata of a solid substance have really distinguished facets and features than the substances constituting the deep inner bulk. Understanding of behaviour of surface of a solid and its specific affectination towards a chemical species makes the matter for the heterogeneous reactions involving solid and liquid lucid.

Definition

When a fluid comes in contact with a solid or liquid and the concentration of the fluid in the interfacial layers between two phases is greater than in the bulk of either phase, then the substance is said to be adsorbed at the interface.

Many solid substances have the power of adsorbing moisture. Particularly do glass and porcelain possess this property. Some porous substances have the power of adsorbing gases.

In this case adsorbed molecules remain only at the surface and do not go deeper into
the bulk.

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

It is found that those substances which lower the surface tension of a solvent in which they are dissolved become concentrated in the surface layer while the concentration of substances which raise the surface tension is less in the surface layer than in the bulk of the solution.

Distinction Between Absorption and Adsorption

Adsorption is different from absorption. The essential difference between absorption and adsorption lies in the fact that the latter is a surface phenomenon, and the former concerns the whole mass of the absorbent. In practice, it will be expected that in the case of adsorption, equilibrium will be attained rapidly, whereas in the case of absorption it will be reached more or less slowly. The term adsorption is used to cover the whole phenomenon of the taking up of a gas by a solid. The surface effect is adsorption, the solution in the interior of the substance is absorption.

Mechanism of Adsorption

Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the particles inside the bulk. Inside the adsorbent all the forces acting between the particles are mutually balance but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides and hence they have 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 adsorbent at a given temperature and pressure.

Another important factor regarding adsorption is the heat of adsorption. ΔH of adsorption is always negative. When a gas is adsorbed the freedom of movement of its molecules becomes restricted. This amounts to decrease in the entropy of the gas after adsorption i.e., ΔS is negative. Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system. For a process to be instantaneous, the thermodynamic requirement is that Δ G must be negative i.e., there is decrease in free energy. On the basis of equation ΔG = Δ H − TΔS.ΔG can be negative if ΔH has sufficiently high negative value as −TΔS is positive

Thus, in an adsorption process, which is spontaneous

ΔS is negative,

ΔH is also sufficient negative

and as of combination of these two factors,

ΔG is negative.

ΔG becomes less and less negative as adsorption proceeds further and further. Ultimately ΔH becomes equal to TΔS and ΔG becomes zero. This is the state at which equlibrium is attained.

Types of Adsorption

There are two types of adsorption of gases on solids.

(i) Physical Adsorption (ii) Chemical Adsorption

If accumulation of gas on the surface of a solid occurs on account of weak van der Waal’s forces, the adsorption termed as physical adsorption or physisorption. When the gas molecules or atoms are held to solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. The chemical bonds may be covalent or ionic in nature.

Some of the important characteristics of both types of adsorption are following

Characteristics of physical adsorption

(i) Lack of specificity: A given surface of an adsorbent does not show very strong attraction for a particular gas as the van der Waals forces are universal.

(ii) Nature of gas : The amount of gas adsorbed by a solid depends on the nature of gas. In general more easily liquefiable gases is adsorbed to a greater extent.

(iii) Reversible nature : Physical adsorption of a gas by a solid is generally reversible. The gas adsorbed can be removed by reversing the conditions temperature and pressure. Thus.

Gas Gas/Solid + Heat

More of gas is absorbed when pressure is increased as the volume of the gas decreases and it 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). As the activation energy in the physical adsorption is more or less zero, the rate of adsorption is not affected even at low temperature.

(iv) Surface area of adsorbent: The extent of adsorption increases with increase of surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas as good adsorbents.

(v) Heat of adsorption: No doubt, physical adsorption is an exothermic process but it heat of adsorption is quite low (20−40) kJ  mole^-1 since the attraction between gas molecules and solid surface is due to weak van der Waal’s forces.

Characteristics of chemisorption

(i) High specificity: Chemi−sorption is highly specific and it will only occur if there is some possibility of chemical bonding. For example, oxygen is adsorbed on metals by virture of oxide formation and hydrogen is adsorbed by transition metals with unpaired d−orbitals leading to hydride formation.

(ii) Nature of gas: Chemisorption will occur if there is some possibility of chemical action between the gas and the solid adsorbent.

(iii) Irreversibility: As chemisorption involves compound formation, it is commonly 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, it often increases with rise of temperature. A gas adsorbed at low temperature by physical adsorption may change into chemisorption at high temperature.

High pressure is favourable for chemisorption.

(iv) Surface area: Like physical adsorption, chemisorption also increases with increase of surface area of the adsorbent.

(v) Heat of adsorption: Heat of adsorption is high enough (80−240 kJ mol–1 or
20−60 kcal mol–1) as chemisorption involves chemical bond formation.

Adsorption Isotherms

The relation between the quantity of gas adsorbed by a solid and the pressure of the gas over the solid when equilibrium has been reached is referred to as adsorption isotherm because the measurements are made at a fixed temperature. The gas adsorption isotherm is always of the form.

The quantity adsorbed (expressed as a member of molecules, or mole or a weight) at first rises rapidly with the pressure, but ultimately, when the available surface is fully covered, the quantity adsorbed can not rise further and is then independent of the equilibrium pressure.

A relation of a similar form can be observed in the adsorption of a solute from a solution.

Freundlich adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas absorbed by unit mass of solid adsorbent and pressure at a particular temperature called Freundlich adsorption isotherm which is expressed mathematically by the relation.

,

Where x is the mass of gas adsorbed, and m the mass of the adsorbent, and p is the pressure of the gas, where a and n are constants. Since this relationship was obtained empirically by Freundlich, it has no theoretical significance, and holds only for medium gas pressures.

In the case of adsorption from a solution, the equation becomes

,

where c is the equilibrium concentration of the adsorbed substance in the solution.

Taking logarithms, we have

log x − log m = 1/n  log c + log a,

so if log x is plotted against log c, straight line is obtained.

The adsorption isotherm bears a certain formal similarity to the Distribution Law which says that if the concentration of a substance distributed between two immiscible liquids is c1 in the first liquid and c2 in the second, and if n is the degree of association of the solute in the second solvent,

= k

where k is a constant.

Hence, c₁ = ,

It will be seen that this is very similar to the adsorption isotherm. It does not mean however, that the value of n indicates the extent of association in the adsorbed layer.

Chromatography

Substances can be selectively adsorbed from solution, and an application of this fact is now widely used in analysis for carrying out separations which would involve very lengthy and difficult procedures by ordinary chemical method under the name chromatography.