Coagulation of Colloids and Emulsions

A lyophobic sol is only kinetically stable, not thermodynamically stable, precisely because its particles carry a like electric charge that keeps them electrostatically repelled from one another; deliberately neutralizing that surface charge allows the particles to approach closely enough for weak attractive forces to take over, causing them to aggregate into progressively larger clumps that eventually grow too heavy to remain dispersed and settle out — a process called coagulation (or precipitation) of the sol.

The most common way to bring about coagulation is to add an electrolyte carrying ions of charge opposite to that on the sol particles; these oppositely-charged ions are adsorbed onto the particle surfaces and neutralize the charge that was keeping the particles apart. The effectiveness of a given ion at doing this is captured by the empirical Hardy–Schulze rule: the coagulating power of an ion of charge opposite to the sol increases sharply, and disproportionately, with the magnitude of that ion's charge — a trivalent ion is very much more effective, not merely three times as effective, as a coagulant than a univalent ion of the same sign. For the coagulation of a negatively charged sol such as $\text{As}_2\text{S}_3$, whose particles are neutralized by cations, the coagulating power therefore increases sharply in the order $\text{Na}^+ < \text{Ba}^{2+} < \text{Al}^{3+}$; the identical logic, with sign reversed, applies to positively charged sols coagulated by anions. Coagulation can also be brought about by mixing two oppositely-charged sols together (each neutralizes the other, a process called mutual coagulation), by prolonged heating (which increases the frequency and energy of particle collisions), or by prolonged dialysis (which removes the small stabilizing ions originally adsorbed on the particles).

Emulsions are a distinct class of colloid in which both the dispersed phase and the dispersion medium are liquids that do not mix with one another — most commonly, some combination of an oily/non-polar liquid and water. Two types of emulsion are possible depending on which liquid forms the continuous (dispersion) medium and which forms the dispersed droplets: an oil-in-water (o/w) emulsion has tiny oil droplets dispersed throughout a continuous water phase (milk, in which butterfat droplets are dispersed in an aqueous medium, is the standard example), while a water-in-oil (w/o) emulsion has tiny water droplets dispersed throughout a continuous oil phase (butter and cold cream, in which small amounts of water are dispersed through a continuous fatty phase, are standard examples). …