Techniques · Intermediate ·
Emulsions explained: how oil and water stay together
Why lotions do not separate, what emulsifiers actually do, and what to check when yours does.
Key takeaways
- An emulsifier has a water-loving end and an oil-loving end, so it sits between droplets and keeps them apart.
- O/W emulsions are lighter; W/O emulsions are richer and more protective.
- Heat, mixing energy and the right emulsifier level decide whether an emulsion holds.
- Fatty alcohols and gums make an emulsion more stable and add body.
Why oil and water do not mix
Oil molecules cling to each other and push water away. Shake them together and they will form tiny droplets for a moment, but those droplets rejoin and the layers separate again. An emulsion is what you get when something stops them rejoining.
What an emulsifier does
An emulsifier molecule has one end that loves water and another that loves oil. It lines up around each oil droplet with the oil-loving tails pointing in and the water-loving heads pointing out, forming a shield that stops droplets merging.
Two kinds of emulsion
| O/W (oil-in-water) | W/O (water-in-oil) | |
|---|---|---|
| What it is | Oil droplets in water | Water droplets in oil |
| Feel | Light, fresh, absorbs quickly | Rich, protective, can feel greasy |
| Typical products | Lotions, face creams, serums | Cold creams, salves, some sunscreens |
| Usually uses | Higher HLB emulsifiers (about 8-18) | Lower HLB emulsifiers (about 3-6) |
| Washes off | Easily | Needs a cleanser |
What HLB means
HLB (hydrophilic-lipophilic balance) is a 0-20 scale describing how water-loving an emulsifier is. Many stable creams combine two emulsifiers, one low and one high. In the ingredient library, for example, Steareth-2 has an HLB of 4.9 and Steareth-20 an HLB of 15.3. Used together they form very tight, stable structures. The HLB calculator works out the ratio for your oils.
The ingredients of a stable emulsion
- Emulsifier, typically 2-5% of a lotion. Higher oil loads need more.
- Co-emulsifier / fatty alcohol (cetyl or cetearyl alcohol, 1-5%) for body and stability.
- Gum or polymer (xanthan gum, 0.1-0.5%) to slow droplet movement.
- Chelator so metal ions do not interfere.
Technique matters as much as the recipe
- Heat both phases to about 70-75°C. They should be within a few degrees of each other when combined.
- Add oil to water (for O/W) while mixing.
- Blend with energy using a stick blender in short bursts. Without enough shear, droplets stay large and will separate.
- Keep stirring gently while it cools, and add heat-sensitive ingredients below 40°C.
Why emulsions fail
- Too little emulsifier for the amount of oil.
- Phases at very different temperatures, or too little mixing.
- Electrolytes, such as aloe powder, sodium PCA or salts, which can thin or break some emulsions and gels.
- A pH outside the emulsifier's working range.