Surface Chemistry

Surface Chemistry

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Surface chemistry studies phenomena at the boundary between two phases — where molecules experience unbalanced cohesive forces and a residual interfacial energy (surface tension, γ, in J m⁻² or N m⁻¹). The two pillars of this NEET topic are adsorption and colloids.

Adsorption is the accumulation of a species (the adsorbate) at the surface of a solid or liquid (the adsorbent); absorption is bulk penetration. Physisorption uses weak van der Waals forces (low ΔH ≈ 20–40 kJ mol⁻¹, reversible, multi-layered); chemisorption forms chemical bonds (ΔH ≈ 80–240 kJ mol⁻¹, often mono-layered, may need activation energy). The Freundlich isotherm x/m = k·P^(1/n) fits heterogeneous surfaces; the Langmuir isotherm assumes a mono-layer on a uniform surface with finite sites.

Colloids are heterogeneous systems (1–1000 nm dispersed phase) classified as sols, gels, emulsions, foams, aerosols. The Tyndall effect (light scattering) distinguishes colloids from true solutions. Coagulation follows the Schulze–Hardy rule: ion valence dictates flocculating power (Al³⁺ > Mg²⁺ > Na⁺). High-yield terms: CMC, micelle, Gold number, zeta potential, Bredig’s arc method.

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Adsorption Isotherms and Their Meaning

The Freundlich isotherm is empirical, written as x/m = k·P^(1/n), where x is the mass of gas adsorbed, m is the mass of adsorbent, P is equilibrium pressure, and 1/n (between 0 and 1) measures surface heterogeneity. Plotting log(x/m) against log P gives a straight line of slope 1/n and intercept log k. It fails at high pressure because it predicts unlimited adsorption. The Langmuir isotherm assumes identical, independent sites forming a mono-layer: P/(V(P₀–P)) = 1/(V_m·C) + (C–1)/(V_m·C)·(P/P₀), where V is volume adsorbed, V_m is monolayer capacity, and C relates to adsorption heat. The BET equation extends this to multi-layer adsorption and is used to measure specific surface area of catalysts via nitrogen adsorption at 77 K.

Types of Colloids and Preparation

Class	Dispersed phase	Medium	Example
Sol	Solid	Liquid	Gold sol, sulphur sol
Gel	Liquid	Solid	Silica gel, jelly
Emulsion	Liquid	Liquid	Milk (O/W), butter (W/O)
Foam	Gas	Liquid	Whipped cream, froth
Aerosol	Solid/Gas	Gas	Smoke, dust, mist

Lyophilic sols (e.g., gum, starch, protein) are solvent-loving, stable, and reversible. Lyophobic sols (e.g., metals, metal sulphides) are solvent-hating, need a stabilising charge, and are easily coagulated. Preparation methods: Bredig’s arc method (metals in water using an electric arc), chemical reduction (e.g., HCHO reducing Au³⁺ to gold sol), and peptization of a freshly precipitated gel into a colloid using a small amount of electrolyte.

Coagulation and the Schulze–Hardy Rule

Colloidal stability arises from like charges on particles. Adding an electrolyte neutralises this charge; the minimum concentration (mmol L⁻¹) needed to coagulate a sol in 2 hours is the coagulation or flocculation value. Schulze–Hardy rule: the coagulating ion’s charge is decisive — for a negatively charged sol, Al³⁺ is ~1000× more effective than Na⁺ (effect ∝ z⁶). The Gold number (Zsigmondy) is the mg of protective colloid that prevents 10 mL of red gold sol from turning blue on adding 1 mL of 10% NaCl; a lower Gold number means stronger protective action.

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Thermodynamics of Adsorption

Adsorption is spontaneous, so ΔG < 0. Gas molecules lose translational freedom on the surface, so ΔS is negative. For ΔG = ΔH – TΔS to be negative, ΔH must be negative and large in magnitude — adsorption is always exothermic (Le Chatelier’s principle: raising temperature desorbs the gas). A plot of log K vs 1/T (van ‘t Hoff / Clausius–Clapeyron form) gives ΔH_ads from the slope: ln K = –ΔH/(RT) + const. Chemisorption often has an activation energy barrier (activated adsorption), so its rate first rises with T (kinetic regime) then falls (equilibrium regime), producing a maximum in the rate–T curve — a classic NEET assertion-reason trap.

Catalysis and Surface Reactivity

In heterogeneous catalysis, reactants adsorb on active sites, react at the 2D interface, and products desorb. A promoter (e.g., Mo in Fe–Haber–Bosch catalyst) enhances activity without itself being a catalyst; a catalytic poison (e.g., H₂S, CO for Pt) blocks sites by strong chemisorption. Shape-selective catalysis by zeolites (aluminosilicates with uniform 0.3–1 nm pores) restricts reactants/products by molecular size — used in ZSM-5 for methanol-to-gasoline. Enzyme catalysis follows Michaelis–Menten kinetics with extreme specificity. Autocatalysis (e.g., Mn²⁺ in permanganate–oxalate reaction) shows an S-shaped concentration–time curve because product catalyses its own formation.

Colloidal Properties and Emulsions

The Tyndall effect arises because colloidal particles (1–1000 nm) scatter light (Rayleigh scattering, intensity ∝ 1/λ⁴). Brownian movement (zig-zag motion from solvent bombardment) keeps particles suspended. Electrophoresis (movement under an electric field) and electro-osmosis measure the zeta potential; sols coagulate near the isoelectric point where ζ ≈ 0. Surfactants (amphiphiles with hydrophobic tail + hydrophilic head) form micelles above the critical micelle concentration (CMC); in O/W emulsions, the emulsifier (e.g., soap, casein in milk) stabilises droplets. Demulsification uses heat, centrifugation, or chemical breakers. Syneresis (gel contracting on standing, expelling liquid) and thixotropy (sol ↔ gel on shaking/standing) are gel-specific behaviours tested via short statements. Dialysis through a semipermeable membrane removes electrolytes; electrodialysis speeds this with an applied field — both rely on colloidal particles being too large to pass through the membrane.

NEET traps to avoid: (1) Confusing adsorption (surface) with absorption (bulk) — MnO₂ + H₂O₂ uses adsorption but MnO₂ + H₂ uses surface + bulk. (2) Saying “adsorption is endothermic” — never, it’s always exothermic. (3) Saying “Freundlich isotherm holds at all pressures” — it fails at high P. (4) Reversing Schulze–Hardy: for a positive sol, anions coagulate, not cations. (5) Calling milk a true solution — it’s a lyophilic O/W emulsion that passes through ordinary filter paper but not the Tyndall test’s resolution.

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