Adsorption

Adsorption is a surface phenomenon in which substance molecules accumulate at the surface of a solid or a liquid rather than entering into the bulk phase. The substance that accumulates at the surface is called the adsorbate, and the solid or liquid material on whose surface the process takes place is termed the adsorbent.

Distinguishing Adsorption from Absorption

It is crucial to differentiate adsorption from absorption, as they represent distinct physical and chemical behaviors.

• Adsorption: A surface phenomenon where the concentration of the solute is higher at the surface than in the bulk. It is rapid at the beginning and its rate decreases gradually until equilibrium is attained.
• Absorption: A bulk phenomenon where the substance is distributed uniformly throughout the body of the solid or liquid. It occurs at a uniform rate throughout the process.
• Sorption: A term used when both adsorption and absorption take place simultaneously. For example, when a fabric dye is applied, the dye molecules are adsorbed onto the surface and absorbed into the deep fibers simultaneously.

Thermodynamic Parameters of Adsorption

Adsorption is universally an exothermic process, meaning it is accompanied by the release of heat energy.

• Enthalpy Change (Δ H): Always negative (Δ H < 0). When adsorbate molecules adhere to the surface, residual surface forces decrease, liberating energy.
• Entropy Change (Δ S): Always negative (Δ S < 0). The freedom of movement of the gaseous or liquid adsorbate molecules becomes highly restricted once they are anchored to the surface, reducing randomness.
• Gibbs Free Energy (Δ G): For adsorption to occur spontaneously at constant temperature and pressure, Δ G must be negative (Δ G < 0). This requirement is met because the highly negative Δ H overcomes the unfavorable negative TΔ S term in the fundamental thermodynamic equation:
  Δ G = Δ H – TΔ S

Classification of Adsorption

Depending on the nature of the forces operating between the adsorbate molecules and the adsorbent surface, adsorption is divided into two distinct classes.

Physisorption (Physical Adsorption)

Occurs when the adsorbate molecules are held to the adsorbent surface by weak intermolecular van der Waals forces without altering the chemical structure.

• Specificity: Completely non-specific; any gas can be physisorbed onto a solid surface under appropriate conditions.
• Reversibility: Highly reversible. Increasing pressure or decreasing temperature favors physisorption, while decreasing pressure or heating induces desorption.
• Enthalpy: Low enthalpy of adsorption, typically ranging between 20 kJ/mol and 40 kJ/mol.
• Layering: Forms multi-molecular layers on the adsorbent surface under high pressure.

Chemisorption (Chemical Adsorption)

Occurs when the adsorbate molecules are held to the surface by strong chemical bonds (covalent or ionic bonds), forming a surface compound.

• Specificity: Highly specific; it takes place only if there is a possibility of chemical bond formation between the adsorbent and the adsorbate.
• Reversibility: Strictly irreversible. The surface compound formed cannot be easily detached by simply lowering the pressure.
• Enthalpy: High enthalpy of adsorption, ranging between 80 kJ/mol and 240 kJ/mol due to chemical bond formation.
• Layering: Forms only a single uni-molecular (monolayer) layer, as the chemical affinity is satisfied once the surface sites are bound.

Comparison Summary between Physisorption and Chemisorption

Parameter	Physisorption	Chemisorption
Type of Forces	Weak van der Waals forces	Strong chemical bonds
Specificity	Non-specific in nature	Highly specific in nature
Reversibility	Completely reversible	Irreversible
Enthalpy of Adsorption	Low (20 to 40 kJ/mol)	High (80 to 240 kJ/mol)
Activation Energy	No appreciable activation energy required	Requires high activation energy in some cases
Layers Formed	Multi-molecular layers	Uni-molecular layer
Effect of Temperature	Decreases continuously with temperature rise	Increases initially, then decreases with temperature rise

Adsorption Isotherms

An adsorption isotherm is a mathematical and graphical representation showing the variation in the amount of gas adsorbed per unit mass of the solid adsorbent (x/m) with the equilibrium pressure (p) of the gas at a constant temperature.

Freundlich Adsorption Isotherm

Freundlich gave an empirical relationship for the isotherm over a limited range of pressure:

Where x is the mass of gas adsorbed, m is the mass of the adsorbent, p is the equilibrium pressure, and k and n are constants depending on the nature of the adsorbent and the gas at a particular temperature (n > 1).

• At Low Pressure: The graph is nearly straight and linear, meaning x/m ∝ p¹.
• At High Pressure: The adsorption becomes independent of pressure because all active surface sites are occupied. Thus, x/m ∝ p⁰.
• At Intermediate Pressure: The relationship is fractional, x/m ∝ p^1/n.

Key Applications of Adsorption

Adsorption plays a critical role in environmental management, consumer safety, and chemical manufacturing.

Activated Charcoal in Gas Masks

Activated charcoal is a highly porous material with a massive surface area per unit mass. It is utilized inside industrial and military gas masks to selectively adsorb toxic gases (such as carbon monoxide, methane, and chlorine) from polluted air, allowing pure oxygen to pass through safely for breathing.

Silica and Alumina Gels as Dehumidifiers

Silica gel and alumina gel possess strong affinities for water vapor. Packets containing these gels are universally placed inside leather goods, electronics, and pharmaceutical containers to adsorb ambient moisture, protecting sensitive items from fungal growth and water damage.

Heterogeneous Catalysis

Many industrial chemical manufacturing processes rely on solid catalysts to speed up reactions between gaseous reactants. The reactant gases are adsorbed onto the solid catalyst surface, increasing their localized concentration and weakening their internal bonds, which lowers the required activation energy.

• Example: The use of finely divided Nickel in the hydrogenation of vegetable oils, and the use of Iron in Haber’s Process for ammonia synthesis.

Clarification of Cane Sugar

Crude sugar juice extracted from sugarcane contains organic coloring matter that gives it a yellow or brown tint. Passing this crude syrup through beds of animal charcoal adsorbs the coloring pigments completely, producing clear, white granulated sugar crystals.

Chromatographic Analysis

Analytical separation methods such as Column Chromatography and Thin-Layer Chromatography (TLC) operate on the principle of selective adsorption. Components of a mixture travel at different rates over a stationary solid adsorbent phase based on their varying adsorption affinities, allowing complex mixtures to be separated and analyzed.

Last Modified: May 25, 2026