Preparation of Colloids

What this figure shows. A colloid mill consists of two metal plates set very close together, rotating in opposite directions at a very high speed of nearly 7000 revolutions per minute; a coarse dispersion of the solid to be reduced is fed in between the plates. The intense mechanical shearing force generated between the two rapidly counter-rotating plates grinds the solid particles down to colloidal dimensions, and the exact particle size obtained can be tuned by adjusting the gap between the two plates. This mechanical dispersion method is …

What this figure shows. Two metal electrodes are dipped into water, itself surrounded by a bath of ice to keep the system cool, and connected to a source supplying about 1 amp at 100 V. Striking an electric arc between the electrodes vaporises a small quantity of the electrode metal; the metal vapour, immediately surrounded by the cold water, condenses instantly into colloidal-sized particles, and a little alkali hydroxide is added as a stabilising agent to keep the resulting sol from coagulating. George Bredig first prepared a brown colloidal solution of platinum this way in 1898, and the same electro-dispersion method is used for colloidal solutions of copper, silver and gold. Svedberg later modified the technique, using a high-frequency alternating current instead of a simple arc, so that non-aqueous, inflammable liquid …

What this figure shows. An electrical oscillator drives a quartz generator immersed in a vessel of water; the generator's vibration is transmitted first through a layer of oil and then into a smaller container holding mercury, all connected through the intervening liquids. The quartz generator produces sound waves of frequency well above 20 kHz — beyond the audible limit — and these high-frequency ultrasonic vibrations are energetic enough to shatter a coarse suspension into colloidal dimension. Claus used exactly this ultrasonic dispersion technique to obtain a mercury sol by subject …

Worked out. Peptisation is the process by which a freshly formed precipitate is converted back into a colloidal solution by the addition of a suitable electrolyte, called a peptising or dispersing agent; the ions of the added electrolyte are selectively adsorbed onto the surface of the precipitate particles, and the resulting like-charge on every particle causes them to repel each other and break apart into colloidal-sized fragments instead of remaining clumped as a bulk precipitate. A classic example is freshly precipitated silver chloride, which is peptised back into a colloidal AgCl sol by the addition of dilute hydrochloric …

Worked out. Condensation methods build colloidal particles up from species that start out smaller than colloidal size — atoms, molecules or ions — and must be controlled carefully, since going too far causes ordinary bulk precipitation instead of a stable colloid. Oxidation gives, for example, a sol of iodine from hydroiodic acid and iodic acid ($\text{HIO}_3 + 5\text{HI} \rightarrow 3\text{H}_2\text{O} + \text{I}_2\text{(sol)}$) and a sol of selenium when $\text{O}_2$ is passed through $\text{H}_2\text{Se}$. Reduction gives, for example, a gold sol by reducing auric chloride with formaldehyde ($2\text{AuCl}_3 + 3\text{HCHO} + 3\text{H}_2\text{O} \rightarrow 2\text{Au(sol)} + 6\text{HCl} + 3\text{HCOOH}$). Hydrolysis gives sols of metal hydroxides such as $\text{Fe(OH)}_3$ from ferric chloride ($\text{FeCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3 + 3\text{HCl}$). Double decomposition gives water-insoluble sols, such as a yellow $\text{As}_2\text{S}_3$ sol when $\text{H}_2\text{S}$ gas is passed through a solution of arsenic oxide. Decomposition gives a coloured sulphur sol when a few drops of acid are added to dilute sodium thiosulphate, the freed sulphur clustering into small colloidal-sized particles. Finally, exchange of solvent gives sols of substances such as phosphorus or sulphur that are soluble in alcohol but not in water: dissolving t …