Colloidal and adsorption

Colloids & Adsorption

1. Colloids — basics

Dispersion with particle size 1–1000 nm (between true solution < 1 nm and suspension > 1000 nm): shows Tyndall effect (scatters light), Brownian motion, doesn't settle, passes filter paper but not semipermeable membrane.

Type	Dispersed / medium	Examples
Sol	solid in liquid	paint, ink, blood, $Au$ sol
Gel	liquid in solid	jelly, curd, silica gel
Emulsion	liquid in liquid	milk, cream, oil–water
Aerosol	solid/liquid in gas	fog, smoke, mist
Foam	gas in liquid	soap lather, whipped cream

• Lyophilic (solvent-loving: starch, gum) — reversible, stable. Lyophobic (solvent-hating: metal sols) — irreversible, need stabilizer.
• Preparation: condensation (aggregation upward) vs dispersion (grinding/peptization downward). Purification by dialysis.
• Stability from surface charge (zeta potential); electrolytes coagulate lyophobic sols — Hardy–Schulze rule: higher counter-ion valency = stronger coagulation ($Al^{3+} > Ca^{2+} > Na^+$). Gold number measures protective-colloid power.

2. Adsorption — basics

Accumulation of molecules on a surface (adsorbate on adsorbent), unlike absorption (bulk uptake).

• Physisorption: van der Waals forces, low heat (~20–40 kJ/mol), reversible, multilayer, fast, falls with rising T.
• Chemisorption: chemical bond, high heat (~80–400 kJ/mol), irreversible, monolayer, needs activation energy.

Point	Physisorption	Chemisorption
Forces	van der Waals	chemical bond
Heat	low	high
Layers	multilayer	monolayer only
Reversibility	reversible	irreversible

3. Adsorption isotherms

• Freundlich: $x/m = kp^{1/n}$ → $\log(x/m) = \log k + \frac{1}{n}\log p$; straight-line plot gives $k, n$ (works at moderate pressure only).
• Langmuir: monolayer, uniform sites, no lateral interaction:

Plot of $p/(x/m)$ vs $p$ is linear — slope gives monolayer capacity $a$, intercept gives $K$.

4. Engineering applications

• Activated charcoal: gas masks, decolourizing sugar, water purification, solvent recovery.
• Silica gel/alumina: desiccants, chromatography adsorbents.
• Catalysts (Fe in Haber, Pt in converters) work by adsorbing reactants; zeolites as molecular sieves + ion exchangers.
• Emulsions stabilized by surfactants (soaps lower oil–water interfacial tension); froth flotation separates ores using selective adsorption.

Exam Points

• Colloid types table + Tyndall/Brownian/Hardy–Schulze (3 marks).
• Physisorption vs chemisorption table (3 marks, very frequent).
• Freundlich and Langmuir equations + linear plots + which gives monolayer capacity (5 marks favourite).