Which liquid is inside?
An emulsion is one liquid dispersed as droplets through a second liquid that won’t mix with it — almost always oil and water. The naming follows what’s inside versus outside. In an oil-in-water (o/w) emulsion oil droplets float in a water continuous phase; it feels light, washes off, and is the usual choice for oral medicines and most lotions. In a water-in-oil (w/o) emulsion water droplets sit in an oily continuous phase; it feels greasy and occlusive, good for barrier creams and night ointments.
Why an emulsion needs an emulsifier
Shake oil and water and you create huge new oil–water interface, which carries interfacial tension — energy the system wants to shed by merging droplets straight back into two layers. Without help, an emulsion breaks in seconds. An emulsifying agent (usually a surfactant) does two jobs: it lowers interfacial tension so droplets form more easily, and it builds a tough interfacial film around each droplet that physically blocks them from fusing.
There’s also a neat rule of thumb for *which* type you’ll get: the phase in which the emulsifier is more soluble tends to become the continuous phase (Bancroft’s rule). A water-loving emulsifier favours o/w; an oil-loving one favours w/o. That single idea links directly to the number we use to pick emulsifiers — HLB.
Matching emulsifier to oil with HLB
The hydrophilic–lipophilic balance (HLB) is a 0–20 scale for how water-loving a surfactant is. Low HLB (3–6) surfactants are oil-loving and make w/o emulsions; high HLB (8–18) surfactants are water-loving and make o/w. Each oil has a *required HLB* — the value its emulsifier system must hit. You rarely use one surfactant; you blend a high- and a low-HLB pair and calculate the mix to match the oil.
Blending two surfactants to hit a required HLB Goal: an o/w emulsion of an oil whose required HLB = 12. Available surfactants: Tween 80 HLB = 15.0 (high, hydrophilic) Span 80 HLB = 4.3 (low, lipophilic) Let x = fraction of Tween 80 in the emulsifier blend. Blend HLB = x(15.0) + (1 - x)(4.3) = 12 15.0x + 4.3 - 4.3x = 12 10.7x = 7.7 x = 0.720 So the emulsifier blend = 72.0% Tween 80 + 28.0% Span 80. If total emulsifier in the formula = 5.0 g: Tween 80 = 0.720 x 5.0 = 3.60 g Span 80 = 0.280 x 5.0 = 1.40 g Check: 3.60/5.0 x 15.0 + 1.40/5.0 x 4.3 = 10.80 + 1.20 = 12.0 OK
Get the HLB right and droplets form readily and the film holds; get it wrong and the emulsion is coarse, separates fast, or forms the type you didn’t want. A semisolid o/w emulsion thickened for the skin is simply a cream — same physics, stiffer continuous phase. The ways an emulsion can still fail are the subject of the final guide.