Why disperse systems want to fall apart
Break an oil into millions of tiny droplets dispersed in water and you create an enormous amount of new oil-water interface. Every square metre of that interface stores energy, set by the interfacial tension. Nature wants to shed that energy, so droplets drift together and merge back into fewer, larger drops — eventually two clean layers. An emulsion is therefore thermodynamically unstable by nature; our job in formulation is to make it stable enough for long enough.
An emulsifying agent fights this on two fronts. First, by adsorbing at the droplet surface it lowers the interfacial tension, so less energy is stored per unit area and there is less driving force to merge. Second, and just as important, the adsorbed molecules form a tough, coherent interfacial film around each droplet — a mechanical and often electrical barrier that keeps droplets from touching and fusing.
Choosing the emulsifier: required HLB
Every oil has a required HLB — the HLB value of the emulsifier that stabilises it best for a given emulsion type. To hit it, formulators rarely use one surfactant; they blend a low-HLB and a high-HLB surfactant. The mixture's HLB is simply the weighted average of the two by mass fraction, so you can dial in almost any value. A blend also tends to form a denser, stronger interfacial film than a single surfactant alone.
Worked HLB blend: target an o/w cream, required HLB = 12 Available surfactants: Span 60 (sorbitan stearate) HLB = 4.7 Tween 60 (polysorbate 60) HLB = 14.9 Let x = mass fraction of Tween 60 (high-HLB): 4.7(1 - x) + 14.9(x) = 12 4.7 + 10.2 x = 12 10.2 x = 7.3 x = 0.716 So, per 5 g of total emulsifier: Tween 60 = 0.716 x 5 = 3.58 g Span 60 = 1.42 g -> blend HLB = 12.0 (matches the oil) Rule of thumb on type: blend HLB 8-18 -> oil-in-water (o/w) blend HLB 3-6 -> water-in-oil (w/o)
How emulsions fail — and the role of foam
- Creaming — droplets rise (or sediment) under gravity into a concentrated layer, but stay separate. Creaming is reversible: a gentle shake re-disperses them. It is unsightly but not fatal.
- Coalescence — droplets actually merge into bigger ones as their interfacial films rupture. Coalescence is irreversible and is true emulsion breakdown; left unchecked it ends in two separated layers (cracking).
- Phase inversion — an o/w emulsion flips to w/o (or vice versa), often when temperature or the phase ratio crosses a tipping point.
The same interfacial film thinking applies to a foam — gas bubbles dispersed in liquid, with surfactant films stabilising the thin liquid walls between bubbles. Foam can be wanted (a shaving foam, a pharmaceutical aerosol, a frothy syrup that signals a dose) or very much unwanted (foam during high-speed mixing or filling slows production and spoils dose accuracy). When foam is a nuisance, an antifoaming agent — a low-HLB, very oil-loving surfactant such as simethicone — spreads over the bubble films, destabilises them, and lets the foam collapse.