Undergraduate Degree Surface Chemistry:Freundlich's adsorption isotherm, BET theory , application of adsorption

B.Sc. Second year Undergraduate degree course (CBCS Pattern)

Semester Third

Physical Chemistry (CHE-312)

Chapter – Surface Chemistry

Freundlich's adsorption isotherm

Freundlich proposed an empirical relation in the form of a mathematical equation.

Where w is the mass of the gas adsorbed on a mass m of adsorbent at a pressure P; k and n are constants depending on the nature of the gas and the adsorbent and on temperature. This relation is generally represented in the form of a curve obtained by plotting the mass of the gas adsorbed per unit mass of adsorbent (w/m) against equilibrium pressure.

Freundlich isotherm is not applicable at high pressures. Taking logarithms on both sides of Freundlich equation, we have

This is equation for a straight line. Thus a plot of log (w/m) against log P should be a straight line with slope 1/n and intercept log k. However, it is actually found that the plots were straight lines at low pressures, while at higher pressure they showed a slight curvature, especially at low temperature. This indicated that Freundlich equation is approximate and does not apply to adsorption of gases by solids at higher pressures.

Limitations of Freundlich Adsorption Isotherm

1. Freundlich equation is purely empirical and has no theoretical basis.

2. The equation is valid only up to a certain pressure and invalid at higher pressure.

3. The constants k and n are not temperature independent, they vary with temperature.

4. Frendilich’s adsorption isotherm fails when the concentration of the adsorbate is very high.

BET theory

The BET equation, short for Brunauer-Emmett-Teller equation, is another important equation in the field of surface chemistry and adsorption. It is used to model the adsorption of gases on solid surfaces, particularly in the context of multilayer adsorption. The BET equation is an extension of the Langmuir adsorption isotherm, which assumes monolayer adsorption, to situations where multiple layers of molecules can be adsorbed on a surface.

The BET equation is given by:

The BET equation assumes that gas molecules adsorb onto a surface in layers and that there is a monolayer coverage limit. It provides a way to calculate the specific surface area of a solid material from experimental data, particularly the adsorption isotherm (the relationship between the amount of gas adsorbed and the equilibrium pressure) of a gas on the material's surface.

Keep in mind that while the BET equation is a widely used model, it makes certain assumptions about the behavior of gas adsorption, and its applicability might depend on the nature of the material and the gas being adsorbed. Additionally, variations and improvements to the BET equation have been developed over time to better account for specific cases and complexities.

Application of adsorption

Extensive application of adsorption is been listed below

1. Production of high vacuum

In Dewar flasks activated charcoal is placed between the walls of the flask so that gas entering into the annular space gets adsorbed.

2. In Gas masks

Activated carbon is used in gas masks to adsorb poisonous gases (e.g. oxide of sulphur, NO₂ etc.) and purifies air for breathing.

Figure 1.6: Adsorption of poisonous gases using activated charcoal

3. In desiccation or dehumidification

Certain substances can be used to remove water vapour or moisture present in the air. Silica gel and alumina are used for dehumidification in electronic equipment.

4. In clarification of sugar

Sugar is decolorized by treating sugar solution with animal charcoal powder which removes the colour producing substances.

5. In paint industry

The paint should not contain dissolved gases as it inhibits the adherence capacity of paint to the surface to be coated. The dissolved gases are therefore, removed by suitable adsorbents. This is done by adding suitable liquids which adsorbs these films. Such liquids are called wetting agents.

Eg. Use of spirit as wetting agent in furniture painting.

6. Adsorption chromatography

Analytical method, in which molecules are separated according to their adsorptive properties, where a mobile fluid phase is passed over an immobile solid adsorptive stationary phase.

7. In adsorption indicators

Various dyes which possess adsorption property have been introduced as indicators mainly in precipitation titrations. For example KBr is titrated with AgNO₃ using eosin as an indicator.

8. Heterogeneous Catalysis

In heterogeneous catalytic reactions adsorption of gaseous reactants on solid catalyst occurs. The adsorption mechanism is responsible for the greater efficiency of the catalyst in the finely divided state and helps us to understand the action of catalyst promoters and poisons. Eg. Finely powdered nickel is used for the hydrogenation of oils.

9. In adsorption indicators

In many precipitation titrations many dyes are used as indicators which work on the principle ofadsorption.

10. In curing diseases

Some pharmaceutical drugs have the capacity to adsorb the germs on them and kill them and protect us from diseases.

11. Separation of inert gases

Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low temperatures.

12. In softening of hard water

The use of ion exchangers for softening of hard water is based upon the principle of adsorption chromatography. The ion exchange resins helps to remove hardness causing ions from water and make it useful for industrial and domestic applications.

13. Arsenic Poisoning

Colloidal ferric hydroxide is administered which adsorbs arsenic and removes it from body by vomiting.

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