Concept 01 / 06Surface Chemistry
Colloids: Classification, Lyophilic and Lyophobic Sols, Associated Colloids and Micelles
True solutions, colloids and suspensions by particle size, the eight types of colloidal systems by physical state of dispersed phase and dispersion medium, lyophilic and lyophobic sols, multimolecular, macromolecular and associated colloids, micelle formation with critical micelle concentration and Kraft temperature, and the cleansing action of soaps.
A colloid is a heterogeneous system in which one substance (the dispersed phase) is dispersed as very fine particles in another (the dispersion medium). Particle size decides the type of mixture:
true solution: below (homogeneous, particles pass through filter paper and parchment);
colloid: to (appears homogeneous, passes through filter paper but not parchment or ultrafilters);
suspension: above (visible particles, settle, do not pass filter paper).
Classification by physical state (dispersed phase in dispersion medium; eight types, since gas in gas is always a true solution):
solid in gas: aerosol (smoke, dust); liquid in gas: aerosol (fog, mist, cloud, insecticide sprays);
solid in liquid: sol (paints, cell fluids, gold sol); liquid in liquid: emulsion (milk, hair cream); gas in liquid: foam (froth, whipped cream, soap lather);
solid in solid: solid sol (coloured glass, gemstones); liquid in solid: gel (cheese, butter, jellies); gas in solid: solid foam (pumice, foam rubber).
A sol with water as the medium is a hydrosol (aquasol); with alcohol, an alcosol.
Lyophilic and lyophobic sols (by affinity for the medium).
Lyophilic ("solvent-loving"): starch, gum, gelatin, rubber. Formed directly by mixing with the medium; reversible (can be re-formed after evaporation); quite stable; highly solvated.
Lyophobic ("solvent-hating"): metals (Au, Ag), metal sulfides, metal hydroxides. Prepared only by special methods; irreversible; unstable, easily coagulated by small amounts of electrolyte; need stabilising agents.
Classification by type of particles.
Multimolecular colloids: aggregates of many atoms or small molecules (diameter below ) held by van der Waals forces: gold sol, sulfur sol ( units).
Macromolecular colloids: single large molecules of colloidal size: starch, cellulose, proteins, enzymes (natural); polythene, nylon, polystyrene (synthetic). They are usually lyophilic and stable.
Associated colloids (micelles): substances that behave as strong electrolytes at low concentration but aggregate into colloidal particles (micelles) at higher concentration. Micelles form only above the critical micelle concentration (CMC, typically to for soaps) and above the Kraft temperature (). Soaps (sodium stearate, ) and detergents (sodium lauryl sulfate) are examples. A micelle may contain or more ions.
Micelle structure and cleansing action (see figure). A soap anion has a long hydrophobic hydrocarbon tail and a hydrophilic carboxylate head. In water above the CMC, the ions cluster with tails pointing inward (away from water) and heads outward. Oily dirt dissolves in the hydrophobic core; the micelle, with a negatively charged surface, is carried away by water, and repulsion between micelles keeps the dirt from re-depositing.
Key Takeaways:
- Colloid particles: to .
- Eight colloidal systems; no gas-in-gas colloid.
- Lyophilic: reversible, stable; lyophobic: irreversible, needs stabiliser.
- Multimolecular (gold, sulfur), macromolecular (starch, proteins), associated (soaps above CMC and ).
- Pitfall: an associated colloid behaves as an ordinary electrolyte below the CMC.
Brain-Teaser 1: Identify the dispersed phase and dispersion medium: (a) milk, (b) fog, (c) pumice, (d) butter, (e) smoke.
Solution: (a) liquid fat in liquid water (emulsion); (b) liquid water in gas (aerosol); (c) gas in solid (solid foam); (d) liquid water in solid fat (gel, a water-in-oil emulsion); (e) solid carbon particles in gas (aerosol).
Brain-Teaser 2: Why does soap not form micelles in very dilute solution or in cold water?
Solution: Micelle formation needs enough soap ions to aggregate (above the CMC) and enough thermal energy for the ions to be soluble and mobile (above the Kraft temperature). Below either, the soap exists as separate ions and does not form a hydrophobic core to trap grease, so it cleans poorly.
Brain-Teaser 3: A starch sol, once dried, re-forms when warmed with water; a gold sol, once coagulated, does not. Explain.
Solution: Starch is lyophilic: its large molecules interact strongly with water (solvation), so they disperse spontaneously again (reversible sol). Gold is lyophobic: its sol is held only by surface charge; once the particles aggregate and lose that charge, nothing drives them to redisperse (irreversible).
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