Surface Chemistry

Surface Chemistry is a branch of chemistry that tells us the characteristics of surfaces or phase boundaries and the chemical changes occurring at a surface or interface.

Surface chemistry tells about the phenomena that occur at the surfaces or interfaces. The surface is characterised by separating the bulk phases by a slash. For example, the surface between a solid and a gas is represented by solid-gas or solid/gas.

ADSORPTION

• Adsorption is the phenomenon of higher concentration at the surface brought about by the action of surface forces Or we can say It is change in the concentration in the interfacial layers between two faces of the system
• Adsorption was introduced by Scheele during the discovery of uptake of gases by charcoal and adsorption was developed by Kayser and Raymonds.

Examples for adsorption

1. If a gas like H₂, O₂, Cl₂ etc is taken in a vessel containing powdered charcoal, pressure of the gas slowly decreases as the gas is adsorbed on the surface of charcoal.
2. In the presence of silica gel, the air becomes dry due to water adsorption on the gel’s surface.
3. When a litmus solution or a dye solution like methylene blue is shaken with animal charcoal, the colouring material adsorbs and the solution becomes colourless.

• Absorption: The distribution of molecules uniformly throughout the bulk of the adsorbent. Example, anhydrous Calcium chloride absorbs water.It occurs inside the surface and a slower process. It is endothermic (deltaH is positive) by nature.
• The adsorption is called physisorption when gas molecules aggregate on the surface of materials due to weak van der Waals’ forces of attraction.
• Chemisorption occurs when molecules of gases are retained on the surface of a material by chemical bonding.

Positive and Negative Adsorption

• In positive adsorption, the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk while in negative adsorption the concentration of the adsorbate is less relative to its concentration in the bulk.

Types of Adsorption

• Chemical Adsorption

• Molecules are held on the surface by strong chemical bonds which may be rapid or slow depending upon the nature of bonding.
• It is very specific in nature.
• It is irreversible in nature.
• Gas forming compounds with adsorbent shows chemical adsorption.
• The adsorption energy is large here, ranging from 40 to 400 kJ/mole.
• It is endothermic
• Firstly it increases with temperature then decreases.

• Physical Adsorption

• Adsorption energy is low and the value is 20-40 kJ/mol
• Multi-layers are formed.
• There is no need for activation energy.
• Example: Adsorption of nitrogen on mica and gases on charcoal.

Characteristics of Physisorption

1. Non-specific nature: Because the van der Waals forces are universal, an adsorbent has no preference for a gas.
2. CO2, SO2, and other easily liquefiable gases are adsorbed fast.
3. Reversible nature: A gas’s physisorption by a solid can be reversed.
4. Increases in response to increased pressure
5. Adsorbent surface area: As the adsorbent surface area rises, more gas is adsorbed, i.e. the amount of adsorption increases.
6. Adsorption enthalpy: The enthalpy of physisorption adsorption is quite low (20-40 KJ mol-1). Due to the exothermic nature of adsorption, physisorption occurs easily at low temperatures and desorption occurs at higher temperatures.

Characteristics of Chemisorption

1. High specificity: It is highly specific and occurs only if chemical bond takes place between adsorbate and adsorbent.
2. Irreversibility: Chemisorption is irreversible as the chemical bond formed is tough to break.
3. Chemisorption increases with temperature.
4. Increases with increase in pressure. 
5. Surface area of adsorbent: As the surface area of the adsorbent increases, more gas is adsorbed, due to which there is increased adsorption.
6. Enthalpy of adsorption: Enthalpy of adsorption of chemisorption is high (80-240 KJ mol-¹)

Factors Influencing Adsorption

1. Surface Area of the Adsorbent

We can observe that Same gas is adsorbed to different areas by different solids at the same conditions. More the surface area of the adsorbent, the higher the volume of gas adsorbed.

2. Temperature

Adsorption of a gas generally lowers with rise in temperature. This is because adsorption is exothermic and increases of temperature favours the backward desorption.

3. Pressure

Adsorption of a gas by an adsorbent at constant temperature increases with rise of pressure.

Surface Chemistry Adsorption Isotherms

Freundlich Adsorption Isotherm

x/m = K [P^(1/n)]

‘x’ denotes  mass of adsorbate on the mass of adsorbent

⇒ log K/m = 1/n (log P + log K)

It is not applicable for the adsorption of gases on solids at a higher pressure

Langmuir Adsorption Isotherm

• The amount of gas which is adsorbed on the solid adsorbent is more thick.

• The adsorbed layer is symmetrical  all over the adsorbent.

• Adjacent adsorbed molecules do not interact.

COLLOIDAL STATE

• Thomas Graham (1862) studied the process of diffusion of dissolved substances through a parchment paper or an animal membrane and divided substances into two classes. Substances like sugar, urea, common salt, etc., which readily passed through the membrane when in the dissolved state were called crystalloids. Substances like starch, glue, gelatin, etc, which in the dissolved state either did not pass at all or passed very slowly through the membrane were called colloids.

• According to the modern view, Colloidal state is an intermediate state between solution and suspension.

APPLICATIONS OF ADSORPTION

1. Preparation and operation of sorbents

2. Solid reactants

3. Catalysts for clean fuels and chemicals production 

4. Pollution cleanup

5. Photocatalysts

6. Fuel cells

7. In batteries

8. Production of High Vacuum

9. Softening of Hard Water

10. Refining of Petroleum

11. Chromatographic Separation

Conclusion

We know that the surface of a liquid is in a state of unsaturation due to the unbalanced or we can say residual forces which act along the surface of a liquid. Similarly, the surface of a solid may also have residual forces due to which  the surface of a solid has a tendency to attract and to retain molecules of gas or liquids with the surfaces to which they come in contact. This phenomenon of surfaces is termed as adsorption. Adsorption is a surface phenomenon and absorption is a bulk phenomenon in which the substance is uniformly distributed throughout the body of a solid or liquid to form a solution or a compound.