A cosmetic emulsifier is a surface-active ingredient used to help combine and stabilize two normally immiscible phases, most commonly oil and water. Emulsifiers are fundamental to the development of creams, lotions, conditioners, cleansing products and many other cosmetic formulations.

What Is an Emulsion?

An emulsion contains one liquid phase dispersed as droplets within another continuous liquid phase. In an oil-in-water (O/W) emulsion, oil droplets are dispersed in water. In a water-in-oil (W/O) emulsion, water droplets are dispersed in oil.

Because oil and water naturally tend to separate, an emulsion requires an appropriate stabilization system. The emulsifier helps reduce interfacial tension and forms or supports an interfacial layer around the dispersed droplets.

How Emulsifiers Work

Many cosmetic emulsifiers are amphiphilic molecules, meaning that they contain both hydrophilic and lipophilic portions. This allows them to locate at the oil-water interface and help reduce the tendency of the two phases to separate.

The performance of an emulsifier depends on its chemical structure, concentration, the oil phase, water phase, processing conditions and the presence of co-emulsifiers or other stabilizing ingredients.

Choosing an Emulsifier

Emulsifier selection should be based on the desired emulsion type, oil phase composition, sensory profile, processing method and required stability. The Hydrophilic-Lipophilic Balance (HLB) concept can be useful as a starting point for selecting emulsifiers, particularly in conventional nonionic emulsifier systems.

However, HLB alone does not determine whether an emulsion will be stable. The complete formulation and manufacturing process must be evaluated experimentally.

Co-Emulsifiers and Stabilizers

Co-emulsifiers such as fatty alcohols can work together with a primary emulsifier to improve emulsion structure, viscosity and sensory properties. Rheology modifiers and polymers may also contribute to physical stability by increasing the viscosity of the continuous phase.

Processing Considerations

Processing conditions can have a major effect on the final emulsion. Phase temperatures, order of addition, mixing intensity, homogenization and cooling conditions can influence droplet size and the internal structure of the formulation.

An emulsifier that performs well in a laboratory formulation may behave differently during scale-up because the shear conditions and mixing efficiency can change substantially.

Emulsion Stability

A properly designed emulsifier system should help maintain acceptable physical stability during storage. Stability problems may appear as creaming, flocculation, coalescence, phase separation or phase inversion.

Finished formulations should therefore be evaluated using appropriate stability testing, including real-time storage and, where appropriate, accelerated temperature studies and other stress conditions.

Formulation Considerations

The emulsifier should be evaluated in the complete formulation rather than in isolation. Changes in electrolyte concentration, pH, oil composition, active ingredients or processing conditions can alter emulsifier performance.

The required concentration should also be established experimentally. More emulsifier does not necessarily result in a more stable or better-performing product and may negatively affect skin feel or other sensory properties.

References

Barel, A. O., Paye, M. and Maibach, H. I. Handbook of Cosmetic Science and Technology. CRC Press.

Griffin, W. C. "Classification of Surface-Active Agents by HLB." Journal of the Society of Cosmetic Chemists.

European Commission. CosIng – Cosmetic Ingredients Database.