OIA1008 SURFACE & INTERFACIAL PHENOMENON II

Adsorption Definition

Surface phenomenon where molecules accumulate on the surface of a solid or liquid.

Absorption Definition

Bulk phenomenon: molecules are taken into the interior of the solid or liquid.

Main Interface Studied

Adsorption commonly occurs at the solid–gas interface.

Physical Adsorption (Physisorption)

Low heat (20–40 kJ/mol), reversible, van der Waals forces, forms multi-molecular layers.

Chemical Adsorption (Chemisorption)

High heat (40–400 kJ/mol), irreversible, forms monolayer, requires activation energy.

Adsorption Isotherm

Relationship between gas pressure and amount adsorbed at constant temperature.

Common Theories

Freundlich, Langmuir, BET (Brunauer-Emmett-Teller).

Freundlich Equation (1909)

xm=kP1/n or log⁡xm=log⁡k+1nlog⁡P

x/m Meaning

Mass of gas adsorbed per unit mass of adsorbent.

Graph Type (Freundlich Equation)

Linear plot of log(x/m) vs log P.

Limitations of Freundlich Equation

Empirical, no theoretical basis, valid only at moderate pressures.

Langmuir Isotherm Concept (1916)

Adsorption is limited to a monolayer.

Langmuir Equation

y= (ym X bp)(1+bp), where y = adsorbed amount, p = pressure.

Linear Form (Langmuir Equation)

py=1bym+pym

Assumptions of Langmuir Equation

Identical sites, no interaction between adsorbed molecules, equilibrium exists.

Limitations of Langmuir Equation

Only applicable to low pressure, monolayer, high temperature cases.

BET Equation (1938)

PV(P0−P)=1VmC+(C−1)(VmC)⋅PP0

Applications of BET Theory

Determines surface area of solids.

C Constant in BET Theory

Related to energy of adsorption.

Monolayer Capacity (Vmono)

Volume needed to form a single molecular layer.

BET Plot

Plot of PV(P0−P) gives straight line.

Type I Isotherm

Monolayer, fits Langmuir, e.g., N₂ on charcoal at -195°C.

Type II Isotherm

Multilayer after monolayer, e.g., N₂ on silica.

Type III Isotherm

No monolayer, weak adsorbent-adsorbate interaction, e.g., Br₂ on silica.

Type IV Isotherm

Multilayer + capillary condensation, e.g., benzene on silica.

Type V Isotherm

Capillary condensation, low affinity initially, e.g., H₂O on charcoal.

Adsorption Reaction

A(g)+B(s)⇌ABA(g) + B(s), AB

AA = gas, BB = surface, ABAB = adsorbed species.

Rate of Adsorption

r1=k1(1−θ)P

Rate of Desorption

r2=k2θ

Equilibrium Expression (Adsorption)

θ=bP1+bP

Surface Area

Higher surface → greater adsorption.

Temperature

Physical adsorption: ↓ with ↑ T,

Chemisorption: ↑ with ↑ T (initially).

Pressure

Increase in gas pressure ↑ adsorption (to a limit).

Nature of Adsorbent

Depends on porosity, polarity, chemical nature.

Activated Charcoal

Adsorbs poisons, toxins → used in emergency overdose treatment.

Kaolin

Used to adsorb bacteria and gases in GI tract.

Deodorants and Masks

Use adsorption to remove odors or gases (e.g., gas masks).

Chromatography

Separation technique based on differential adsorption.

Drying Agents

Alumina, silica gel, CaCl₂, P₂O₅ used to adsorb water vapor.

Wound Healing Preparations

Surface-active agents use adsorption to enhance contact and absorption.