Surface Chemistry

Surface Chemistry – Introduction

Surface chemistry deals with the nature of surfaces and the changes occurring at the surfaces of solids.

Adsorption & Related Terms

• Adsorption: Higher concentration of molecules (gas or liquid) on the surface of a solid than in the bulk.
• Desorption: Removal of adsorbed substance from the surface.
• Sorption: When adsorption and absorption occur simultaneously.
• Positive adsorption: Concentration of adsorbate is higher on surface than in bulk.
• Negative adsorption: Concentration of adsorbate is lower on surface than in bulk.

Factors Affecting Adsorption of Gases on Solids

• Nature of gas: Easily liquefiable gases (CO₂, NH₃, Cl₂) are adsorbed more.
• Nature of adsorbent
• Surface area: Greater surface area → greater adsorption.
• Pressure: Adsorption increases with pressure.
• Temperature: Adsorption decreases with increase in temperature.

Types of Adsorption

Physical Adsorption (Physisorption)

• Weak Van der Waals forces
• Reversible
• Low heat of adsorption
• Occurs at low temperature

Chemical Adsorption (Chemisorption)

• Chemical bond formation
• Irreversible
• High heat of adsorption
• Specific in nature

Adsorption Isotherms

Adsorption isotherm shows variation of x/m with pressure at constant temperature.

Freundlich Adsorption Isotherm

x/m = kP^1/n

• k and n are constants
• Valid at low pressure
• log(x/m) = log k + (1/n) log P
• Slope = 1/n, Intercept = log k

Langmuir Adsorption Isotherm

x/m = aP / (1 + bP)

• Assumes monolayer adsorption
• Valid mainly for chemisorption
• At high pressure: x/m = a/b
• At low pressure: x/m ∝ P

Adsorption Isobars

Graph between x/m and temperature at constant pressure.

• Physisorption: adsorption decreases with temperature
• Chemisorption: adsorption first increases, then decreases

Catalysis

Catalysis is the phenomenon of altering the rate of reaction using a substance called a catalyst, without change in its mass or composition.

Types of Catalysis

• Homogeneous catalysis: Catalyst and reactants in same phase
• Heterogeneous catalysis: Catalyst and reactants in different phases

Positive & Negative Catalysis

• Positive catalysis: Increases reaction rate
• Negative catalysis: Decreases reaction rate (e.g. alcohol preventing oxidation of chloroform)

Enzyme Catalysis

• Enzymes are biochemical catalysts
• Highly specific in nature
• Examples:
  • Glucose → Ethanol (zymase)
  • Alcohol → Acetic acid (mycoderma aceti)
  • Urea hydrolysis (urease)

Colloidal State

Classification Based on Particle Size

• True solution: Homogeneous
• Suspension: Heterogeneous, large particles
• Colloidal solution: Intermediate particle size

Based on Interaction with Medium

• Lyophilic colloids: Strong affinity for dispersion medium
• Lyophobic colloids: Little or no affinity

Preparation of Colloidal Solutions

Dispersion Methods

• Grinding
• Peptization
• Bredig’s arc method (metal sols)

Condensation Methods

• Exchange of solvent
• Excessive cooling
• Double decomposition
• Hydrolysis
• Oxidation
• Reduction

Purification of Colloids

• Dialysis
• Electrodialysis
• Ultrafiltration
• Ultracentrifugation

Properties of Colloids

Colligative Properties

• Osmotic pressure
• Elevation in boiling point
• Depression in freezing point

Optical Property (Tyndall Effect)

• Scattering of light by colloidal particles

Electrical Properties

• Colloidal particles carry charge
• Metal sols are generally negatively charged

Electrophoresis

• Movement of colloidal particles under electric field

Coagulation

• Precipitation of colloids by excess electrolyte
• Hardy–Schulze rule: Higher valency of ion → greater coagulating power
• Flocculation value: minimum electrolyte concentration causing coagulation

Protection of Colloids

• Stabilisation of lyophobic sols using lyophilic colloids
• Gold number: Minimum amount of protective colloid preventing coagulation

Emulsions

• Emulsion: Dispersion of one liquid in another immiscible liquid
• Oil in water: Milk
• Water in oil: Butter, cold cream

Emulsification

• Formation of emulsion using emulsifiers (soap, detergents)

Micelle Formation

• Aggregation of surfactant molecules above CMC
• Example: Sodium stearate
• Hydrophobic tail + hydrophilic head
• CMC ≈ 10⁻³ mol L⁻¹