Concept 05 / 06Surface Chemistry
Preparation, Purification and Properties of Colloids
Chemical and physical methods of preparing colloids (Bredig's arc, peptisation, double decomposition, redox, hydrolysis), purification by dialysis, electrodialysis and ultrafiltration, and properties of colloids: colligative properties, Tyndall effect, colour, Brownian movement, charge and its origin, electrical double layer and zeta potential, electrophoresis and electro-osmosis.
Preparation.
Chemical methods (condensation of small molecules into colloidal particles):
double decomposition: ;
oxidation: ;
reduction: ;
hydrolysis: (adding to boiling water).
Electrical disintegration (Bredig's arc method): an electric arc between metal electrodes under water (with a trace of KOH as stabiliser) vaporises the metal, which condenses into a colloid (Au, Ag, Pt sols).
Peptisation: converting a freshly prepared precipitate into a sol by shaking with the dispersion medium and a small amount of electrolyte (the peptising agent), which is adsorbed and gives the particles a charge that makes them repel: fresh precipitate with a little gives a red-brown sol.
Purification. Sols contain excess electrolytes, which destabilise them.
Dialysis: the sol is placed in a bag of parchment or cellophane (membrane) immersed in running water; ions and small molecules diffuse out, colloidal particles do not.
Electrodialysis: an electric field applied across the membranes speeds up removal of ions (only for electrolytic impurities).
Ultrafiltration: filter papers impregnated with collodion (ultrafilters) retain colloidal particles; ultracentrifugation separates them by settling.
Properties.
Colligative properties: very small, because colloidal particles are large aggregates and their number is small.
Tyndall effect (see figure): when a beam of light passes through a colloid, the particles scatter it and the beam path becomes visible when viewed at right angles. True solutions do not show it. Conditions: particle diameter not much smaller than the wavelength of light, and a large difference in refractive index between dispersed phase and medium. Used in the ultramicroscope (Zsigmondy) to count colloidal particles; explains the visible beam of a searchlight in fog and the blue sky.
Colour: depends on particle size and wavelength scattered: finest gold sol is red, larger particles purple, then blue, then golden.
Brownian movement: continuous zigzag motion of colloidal particles, caused by unbalanced bombardment by molecules of the medium; it opposes settling and helps stabilise sols.
Charge: colloidal particles carry a charge, all of the same sign in a given sol, and repulsion keeps them apart.
Positive sols: hydrated metal oxides (, , ), basic dyes (methylene blue), haemoglobin.
Negative sols: metals (Cu, Ag, Au), metal sulfides (, CdS), acid dyes (eosin, Congo red), sols of starch, gum, clay, charcoal.
Origin of charge: preferential adsorption of ions from the medium. With KI in excess, AgI adsorbs to give (negative); with in excess, (positive). The adsorbed layer attracts a diffuse layer of counter-ions: the Helmholtz electrical double layer; the potential difference between them is the zeta (electrokinetic) potential.
Electrophoresis: under an electric field, colloidal particles move towards the oppositely charged electrode and are discharged there (coagulated). Electro-osmosis: if the particles are held still, the dispersion medium moves instead.
Key Takeaways:
- Bredig's arc for metal sols; peptisation uses an electrolyte to redisperse a precipitate.
- Dialysis, electrodialysis, ultrafiltration purify sols.
- Tyndall effect distinguishes colloids from true solutions.
- sol positive; and gold sols negative.
- Charge from preferential adsorption of common ions; electrophoresis shows the sign.
- Pitfall: the sign of AgI sol depends on which reagent is in excess.
Brain-Teaser 1: What charge does a sol of AgI carry when (a) is added to excess KI, (b) KI is added to excess ?
Solution: (a) AgI adsorbs the common ion in excess, : negative sol . (b) It adsorbs : positive sol .
Brain-Teaser 2: Why does a beam of sunlight become visible in a dusty room but not through a solution of sugar in water?
Solution: Dust particles are of colloidal size and scatter light (Tyndall effect), so the path is visible from the side. Sugar molecules are far smaller than the wavelength of light and scatter negligibly, so the solution looks clear.
Brain-Teaser 3: Why is the osmotic pressure of a gold sol extremely small compared with a sugar solution of the same mass concentration?
Solution: Colligative properties depend on the number of particles. Each colloidal gold particle contains thousands of atoms, so the same mass of gold gives far fewer particles than the same mass of sugar gives molecules; the osmotic pressure is correspondingly tiny.
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