Surface Chemistry — Notes

Adsorption: accumulation of a substance (the adsorbate) at the surface of another (the adsorbent), so its concentration is higher at the surface than in the bulk. Absorption: uniform distribution throughout the bulk (water vapour in anhydrous CaCl₂). Removal of adsorbate: desorption. Both together: sorption.

Surface particles have unbalanced attractive forces, which adsorption partly satisfies, so adsorption is exothermic ($\Delta H < 0$). The adsorbate loses freedom, so $\Delta S < 0$. Adsorption is spontaneous ($\Delta G < 0$) while $|\Delta H| > T|\Delta S|$; it reaches equilibrium when $\Delta H = T\Delta S$.

The same gas may be physisorbed at low temperature and chemisorbed at higher temperature (H₂ on Ni).

• Nature of gas: higher critical temperature → more physisorption (SO₂ > NH₃ > CO₂ > CH₄ > H₂ on charcoal).
• Surface area: finely divided, porous adsorbents (activated charcoal, silica gel) adsorb more.
• Pressure: at constant $T$, $x/m$ rises with $p$, steeply at first, then levels off (saturation).
• Temperature: physisorption falls with temperature (exothermic, Le Chatelier).

Freundlich isotherm (empirical, limited pressure range): $\dfrac{x}{m} = k\,p^{1/n}$, $n > 1$. Linear form: $\log \dfrac{x}{m} = \log k + \dfrac{1}{n}\log p$ — slope $1/n$, intercept $\log k$. For adsorption from solution, concentration $C$ replaces $p$.

Applications: gas masks (charcoal), humidity control (silica gel), decolourising sugar, heterogeneous catalysis (Fe in the Haber process, Ni in hydrogenation), high vacuum, chromatography, froth flotation, qualitative tests (magneson and Mg(OH)₂).

Particle size: true solution < 1 nm; colloid 1–1000 nm; suspension > 1000 nm. Colloids pass through filter paper but not parchment; do not settle; scatter light.

By phases (dispersed phase in dispersion medium): sol (solid in liquid: paint), aerosol (solid or liquid in gas: smoke, fog), solid sol (coloured glass), emulsion (liquid in liquid: milk), gel (liquid in solid: cheese, jelly), foam (gas in liquid: lather), solid foam (pumice).

By interaction: lyophilic (starch, gum, gelatin — made by mixing, reversible, stable) vs lyophobic (metals, sulfides, hydroxides — special methods, irreversible, coagulated by a little electrolyte).

By particle type: multimolecular (gold, sulfur sols), macromolecular (starch, proteins), associated colloids (soap micelles above the critical micelle concentration and Kraft temperature).

Preparation: hydrolysis ($\ce{FeCl3 + 3H2O -> Fe(OH)3 + 3HCl}$ in boiling water), double decomposition (As₂S₃ sol), Bredig’s arc (metal sols), peptisation. Purification: dialysis, electrodialysis.

• Tyndall effect: scattering of light by colloidal particles makes a beam visible from the side (seen when particles are much smaller than the wavelength yet large enough to scatter, and refractive indices differ).
• Brownian movement: zigzag motion from unequal bombardment by medium molecules; helps stability.
• Charge: from preferential adsorption of common ions; positive sols (Fe₂O₃·xH₂O, Al₂O₃·xH₂O, methylene blue, haemoglobin) and negative sols (Au, Ag, As₂S₃, starch, clay). Same charge on all particles → repulsion → stability. Electrophoresis shows the sign.
• Coagulation: neutralisation of charge (by electrolytes, electrophoresis, oppositely charged sols, boiling). Hardy–Schulze rule: the ion with charge opposite to the sol coagulates it; coagulating power rises with that ion’s charge (for negative sols Al³⁺ > Ba²⁺ > Na⁺; for positive sols [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻). Coagulation value: minimum millimoles per litre of electrolyte to coagulate a sol in two hours; smaller value = greater power.
• Protective colloids: lyophilic colloids shield lyophobic sols (gelatin in ice cream, gum in ink).
• Emulsions: O/W (milk, vanishing cream) and W/O (butter, cold cream); stabilised by emulsifiers (proteins, gums, soaps for O/W; long-chain alcohols, heavy-metal soaps for W/O).
• Uses: delta formation, alum to stop bleeding and purify water, Cottrell precipitator, blue sky, rain, soap cleaning.