Surface Chemistry

Surface chemistry deals with phenomena occurring at surfaces and interfaces. Adsorption is a surface phenomenon where molecules (adsorbate) accumulate on a surface (adsorbent), while absorption is a bulk phenomenon where molecules penetrate into the bulk. When both occur simultaneously, the process is called sorption.

Physisorption vs Chemisorption — the two types differ fundamentally: Physisorption involves weak van der Waals forces (enthalpy 20–40 kJ/mol), is reversible, non-specific, forms multilayers, requires no activation energy, and decreases with increasing temperature. Chemisorption involves chemical bond formation (enthalpy 80–240 kJ/mol), is irreversible, highly specific, forms monolayer only, requires appreciable activation energy, and first increases with temperature (overcoming Ea) then decreases at high temperature (bond breaking).

The Freundlich adsorption isotherm relates the amount adsorbed to pressure: x/m = kP^(1/n), where 1/n lies between 0 and 1. In logarithmic form: log(x/m) = log k + (1/n) log P (a straight line with slope 1/n and intercept log k). The Langmuir isotherm describes monolayer adsorption with saturation at high pressure.

Factors affecting adsorption: nature of adsorbent (high surface area favours adsorption — finely divided or activated charcoal is effective), nature of adsorbate (easily liquefiable gases with higher critical temperature are adsorbed more strongly), surface area, temperature, and pressure.

Solved Example 1: Freundlich isotherm x/m = 0.5 × P^($\frac{1}{3}$). At P = 27 atm: x/m = 0.5 × (27)^($\frac{1}{3}$) = 0.5 × 3 = 1.5 units adsorbed per gram

Catalysis: Homogeneous catalysis (catalyst and reactants in same phase — e.g., acid catalysis in solution), heterogeneous catalysis (different phases — e.g., Haber process: Fe catalyst for N₂ + 3H₂ → 2NH₃, Contact process: V₂O₅ for 2SO₂ + O₂ → 2SO₃). Activity measures how much a catalyst increases the rate; selectivity directs the reaction to a specific product (e.g., CO + H₂ → CH₃OH with ZnO-Cr₂O₃, but → HCHO with Cu, and → CH₄ with Ni).

Enzyme catalysis follows the lock-and-key model (substrate binds at active site). Characteristics: high specificity, optimal pH and temperature, Michaelis-Menten kinetics. Inhibition can be competitive (similar substrate blocks active site) or non-competitive (binds elsewhere, changes shape).

Colloids have particle sizes of 1–1000 nm (between true solutions <1 nm and suspensions >1000 nm). Lyophilic colloids (solvent-loving: starch, gelatin, proteins) are reversible, stable, and self-stabilizing. Lyophobic colloids (solvent-hating: gold sol, As₂S₃ sol, Fe(OH)₃ sol) are irreversible, unstable, and require stabilizers.

Types: multimolecular (S₈ sol), macromolecular (starch, polymers), and associated colloids/micelles (soap, detergents) formed above the critical micelle concentration (CMC) at the Kraft temperature.

Preparation: chemical methods (reduction for Au sol using HCHO, hydrolysis for Fe(OH)₃ sol using FeCl₃ in boiling water), Bredig's arc method, peptization (dispersal of precipitate using electrolyte). Purification: dialysis, electrodialysis, ultrafiltration.

Colloidal properties: Tyndall effect (light scattering — path visible), Brownian motion (zig-zag due to molecular bombardment), electrophoresis (migration under electric field), and coagulation (neutralization of charge causes settling).

Solved Example 2: Coagulating power for Fe(OH)₃ sol (positive charge): [Fe(CN)₆]⁴⁻, SO₄²⁻, Cl⁻. By Hardy-Schulze rule: higher charge on the flocculating ion (opposite to colloidal charge) = greater coagulating power. For positive Fe(OH)₃ sol, anions coagulate: [Fe(CN)₆]⁴⁻ > SO₄²⁻ > Cl⁻ (charge 4 > 2 > 1)

Protective colloids prevent coagulation. The gold number = mg of protective colloid that just prevents coagulation of 10 mL standard gold sol by 1 mL of 10% NaCl. Lower gold number = better protective action: Gelatin (0.005) < Albumin (0.1) < Starch (25) — gelatin is the best protective colloid.

Emulsions: Oil-in-water (O/W: milk, vanishing cream — dilutable with water) and water-in-oil (W/O: butter, cold cream — dilutable with oil). Emulsifiers (soap, proteins) stabilize emulsions.

The key testable concept is distinguishing physisorption from chemisorption, applying the Hardy-Schulze rule for coagulation ordering, and interpreting gold number values.