Surface Chemistry

Adsorption: Accumulation of molecules/atoms of one substance (adsorbate) on the surface of another (adsorbent). Surface phenomenon — different from absorption (bulk penetration).

Examples: Charcoal absorbing gases; silica gel absorbing water; dye on cloth.

Types of Adsorption:

Factors Affecting Adsorption:

1. Nature of adsorbent: Greater surface area → more adsorption (charcoal, silica gel, alumina, zeolites)
2. Nature of adsorbate: Easily liquefiable gases (higher critical T) adsorbed more
3. Surface area: Powdered/porous materials adsorb more
4. Temperature: Physisorption decreases with T; chemisorption increases first then decreases
5. Pressure (for gases): Adsorption increases with pressure (up to saturation)

Adsorption Isotherms: Plots of extent of adsorption vs pressure at constant T.

Freundlich Isotherm (Empirical): $$\frac{x}{m} = k \cdot p^{1/n} \quad (n \geq 1)$$

• $x/m$: mass adsorbed per gram adsorbent
• $p$: pressure
• $k, n$: constants ($n > 1$, so $1/n < 1$)

Logarithmic form: $\log(x/m) = \log k + (1/n)\log p$. Plot of $\log(x/m)$ vs $\log p$: straight line.

Limitations: Fails at high pressure (no saturation predicted).

Langmuir Isotherm: Assumes monolayer adsorption on equivalent sites. $$\frac{x}{m} = \frac{ap}{1+bp}$$

At low $p$: $x/m \approx ap$ (linear, like first order). At high $p$: $x/m \approx a/b$ (saturation, plateau).

Catalysis: Speeding of reaction by catalyst (not consumed). Categorized by phase of catalyst vs reactants.

Homogeneous Catalysis: Catalyst and reactants in same phase.

Examples:

1. Lead chamber process: $2SO_2 + O_2 \xrightarrow{NO\,(g)} 2SO_3$
2. Acid hydrolysis of esters: $CH_3COOCH_3 + H_2O \xrightarrow{H^+} CH_3COOH + CH_3OH$
3. Inversion of sucrose: $C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+} C_6H_{12}O_6 + C_6H_{12}O_6$

Heterogeneous Catalysis: Catalyst in different phase (usually solid catalyst, gas/liquid reactants).

Examples:

Adsorption Theory of Heterogeneous Catalysis:

1. Reactants diffuse to catalyst surface
2. Reactants adsorb on active sites
3. Adsorbed molecules' bonds weaken; new bonds form
4. Products formed on surface
5. Products desorb; diffuse away

Features of Solid Catalysts:

• Activity: How much it accelerates reaction
• Selectivity: Ability to direct reaction to specific products
• Promoters: Substances increasing activity (e.g., $K_2O, Al_2O_3$ in Haber)
• Poisons: Substances decreasing activity (e.g., As, S poisoning Pt)

Shape-Selective Catalysis (Zeolites):

• Microporous aluminosilicates
• Pores of specific size only allow certain molecules
• Used in petroleum cracking, ion exchange (water softening)
• ZSM-5 converts methanol to gasoline

Enzyme Catalysis:

• Enzymes: Globular proteins acting as catalysts in biological systems
• Highly specific for substrate and reaction
• Very efficient: rate increase $\sim 10^6$ to $10^{20}$
• Work at mild conditions: body T, neutral pH
• Examples:

Mechanism (Lock and Key): Substrate binds to active site of enzyme; conversion to product; product released.

Michaelis-Menten Equation: $v = V_{max}[S]/(K_M + [S])$

• $V_{max}$: maximum velocity
• $K_M$: Michaelis constant

Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration, inhibitors.

Colloidal Solution: Intermediate state between true solution and suspension. Particle size $1-1000$ nm (10⁻⁹ to 10⁻⁶ m).

Components:

• Dispersed phase: Like solute (particles)
• Dispersion medium: Like solvent (continuous phase)

Classification:

A. Based on Physical State:

B. Based on Interaction (Dispersion medium = water):

C. Based on Particle Type:

• Multimolecular: Many molecules together (Au sol)
• Macromolecular: Single large molecules (proteins, polymers)
• Associated colloids (micelles): Form only above critical concentration (CMC = critical micelle concentration); above Kraft T (KT)

Preparation of Colloids:

1. Dispersion methods (large → colloidal):

• Mechanical disintegration: Colloid mills
• Electrical disintegration (Bredig's arc): Metal electrodes in water; gives Au, Ag, Pt sols
• Peptization: Adding electrolyte (peptizing agent) to fresh ppt converts to sol; e.g., FeCl₃ to fresh Fe(OH)₃ ppt → red brown $Fe(OH)_3$ sol

2. Condensation methods (molecular → colloidal):

• Reduction: $2AuCl_3 + 3HCHO + 3H_2O \to 2Au + 3HCOOH + 6HCl$
• Oxidation: $H_2S + SO_2 \to S \downarrow + H_2O$ (sulfur sol)
• Hydrolysis: $FeCl_3 + 3H_2O \to Fe(OH)_3 + 3HCl$ (red-brown sol)
• Double decomposition: $H_2S + As_2O_3 \to As_2S_3 + H_2O$
• Excess reagent: $AgNO_3 + KI \to AgI + KNO_3$ (excess of either gives different charge sol)

Purification:

• Dialysis: Through semi-permeable membrane (parchment, cellophane)
• Electrodialysis: Faster; with applied electric field
• Ultrafiltration: Pressure through ultrafilter

Properties of Colloidal Solutions:

1. Colligative properties: Small (low number of particles).

2. Tyndall Effect: Scattering of light by colloidal particles. Path of light visible. Used to distinguish from true solution.

3. Brownian Motion: Random zig-zag movement of colloidal particles due to collisions with solvent molecules. Provides stability against settling.

4. Charge on colloidal particles: Colloid particles carry charge (positive or negative).

5. Electrophoresis: Movement of charged colloidal particles under electric field. Toward cathode if +, anode if -.

6. Coagulation (Flocculation): Process of settling/precipitating colloid by:

• Adding electrolyte (most common)
• Heating
• Electrophoresis
• Mixing oppositely charged sols

Hardy-Schulze Rule: Coagulation power of an ion increases with its charge. For a -ve sol, coagulating ions are cations: $Al^{3+} > Ba^{2+} > Na^+$ (most effective to least)

Helmholtz Electrical Double Layer: Around colloid particle, two layers of opposite charge — provides stability.

Zeta Potential: Potential difference between the two layers; related to stability.

Emulsions: Liquid dispersed in another liquid. Two main types:

Emulsifying Agent: Stabilizes emulsion; reduces interfacial tension.

• For O/W: soap, gelatin, proteins
• For W/O: long-chain fatty acids, gum, lanolin

Identification:

• Add water-soluble dye: dye spreads → O/W; dye doesn't spread → W/O
• Dilute with water: stable → O/W; separates → W/O
• Conductance: O/W conducts; W/O doesn't

Demulsification: Breaking emulsions by:

• Heating
• Freezing
• Centrifugation
• Adding electrolyte
• Adding more emulsifier of opposite type

Cleansing Action of Soap: Soaps are O/W emulsifiers.

• Soap molecule has hydrophobic tail (alkyl chain) + hydrophilic head ($-COO^-Na^+$)
• Head dissolves in water; tail in grease/oil
• Forms micelles: tails inward, heads outward; suspends grease in water → washes off

Applications of Colloids:

Industrial Applications:

Cottrell Smoke Precipitator: Electrostatic precipitation. Industrial smoke passed through chamber with positive electrodes. Charged smoke particles attracted, settle, removed.

Sewage Treatment: Sewage particles are charged colloids. Pass through tanks with electrodes; coagulate and settle.

Tear Gas Defense: Tear gases are aerosols; affect mucous membranes.

Surface Activity:

Surfactant (Surface-active agent): Substance that reduces surface tension at interface. Has hydrophilic head + hydrophobic tail.

Types:

• Anionic: Head is anion (soaps, sodium dodecyl sulfate)
• Cationic: Head is cation (quaternary ammonium salts)
• Nonionic: No charge (Triton X)
• Amphoteric: Has both + and - (lecithin)

Micelles: Aggregates of surfactant molecules above critical micelle concentration (CMC). Hydrophobic tails inside, hydrophilic heads outside.

Critical Micelle Concentration (CMC): Min concentration for micelle formation. For soap: ~$10^{-3}$ to $10^{-4}$ M.

Kraft Temperature: Below this T, surfactant exists as crystal; above, forms micelles.