Cosmetics: what is a microemulsion and why is it useful?

In cosmetics, many formulations rely on the combination of two phases that do not naturally mix, such as water and oil. To stabilise these mixtures, formulators use different colloidal systems, including emulsions and microemulsions. In a conventional emulsion, one of the phases is dispersed as droplets within the other thanks to surfactants. However, these systems are thermodynamically unstable: over time, the droplets tend to aggregate and the formulation may eventually separate.

Unlike conventional emulsions, microemulsions are thermodynamically stable, meaning that they can remain stable over time with no natural tendency to separate.

The stability of emulsions and microemulsions relies on the specific organisation of surfactants within the formulation. These molecules spontaneously assemble into structures known as micelles, which are capable of trapping one phase within the other. In some cases, the oil can be incorporated into the hydrophobic core of micelles dispersed in water: these are referred to as oil-in-water emulsions (O/W), or indirect emulsions. Conversely, when small water droplets are dispersed in an oily phase and surrounded by a layer of surfactants, water-in-oil emulsions (W/O), also called direct emulsions, are obtained. In these systems, the surfactants form a kind of film at the interface between the two phases, which stabilises these structures.

Another important difference between emulsions and microemulsions concerns the size of the structures formed. In a conventional emulsion, the droplets generally range from a few hundred nanometres to several micrometres. In a microemulsion, they are much smaller, typically on the order of 10 to 100 nanometres. This extremely small size influences several properties of the formulation, particularly its appearance: microemulsions are often transparent or slightly translucent, because the structures are too small to scatter light effectively.

If microemulsions arouse so much interest in cosmetics, it is primarily because they can serve as highly efficient delivery systems for certain active ingredients. The skin, and in particular its outermost layer, the stratum corneum, forms an effective barrier against external aggressions, but also against the penetration of many molecules. This barrier is particularly difficult to cross for hydrophilic active substances, while certain lipophilic compounds raise problems more in terms of solubilisation or stability within formulations. In this context, the formulation does not depend solely on the nature of the active ingredient: the vehicle that carries it also plays an important role in determining its fate at the surface of the skin.

The extremely small droplet size of microemulsions is a major advantage. At the nanometric scale, the structures formed present a very large interfacial surface area, which promotes interaction between the formulation and the skin and thus facilitates the diffusion of active ingredients.

Microemulsions offer another important advantage: their nanometric organisation into aqueous and oily compartments stabilised by surfactants gives them a high solubilisation capacity for both hydrophilic and lipophilic molecules, often greater than that of conventional emulsions. In other words, they make it easier to incorporate certain actives that would be difficult to disperse in a standard formulation. This effect is particularly relevant for poorly soluble ingredients, whose efficacy may be limited if their dissolution is incomplete. By improving their solubilisation, the microemulsion can therefore help to increase the availability of the active at the skin surface.

This interest is not merely theoretical. In the scientific literature, several studies have shown that microemulsions can enhance the cutaneous penetration of certain molecules compared with more conventional formulations. A frequently cited example concerns penciclovir, a topical antiviral commonly used to treat cold sores (labial herpes). When formulated as a microemulsion, it demonstrated, in an experimental model, a markedly higher capacity for skin penetration than the standard cream. Of course, this is a pharmaceutical rather than a cosmetic example, but it clearly illustrates the principle. By modifying the vehicle, it is sometimes possible to improve the delivery of an active ingredient and therefore potentially its local effectiveness.

If microemulsions are of such interest in cosmetics, it is also for sensorial and formulation-related reasons. Their small droplet size explains theiroften clear, translucent or even transparent appearance, as well as their low viscosity. They can therefore give rise to fluid, lightweight textures that are easy to spread, which are particularly sought after in serums, lotions or skincare products designed for oily skin.

However, this picture needs to be qualified. Microemulsions often require relatively high amounts of surfactants, and sometimes co-surfactants, in order to form and remain stable.

However, depending on the nature of the molecules used and the sensitivity of the skin, this may raise questions of skin tolerance, particularly for sensitive skin. In other words, microemulsions are not automatically superior to all other dosage forms: their relevance depends on the formulation, the active ingredients chosen, the target skin type, and the balance achieved between efficacy, stability and tolerance. It is precisely for this reason that they continue to be actively researched in the cosmetic field.

This does not mean that microemulsions are systematically irritating because of their surfactant content.

Several studies show that certain well-designed formulations exhibit good skin tolerance, including under repeated application. For example, various experimental and clinical studies have evaluated microemulsions containing plant extracts and have not observed erythema, oedema or inflammation after application to human or animal skin. In one trial involving 30 volunteers, a microemulsified serum containing 1% Cordyceps militaris extract did not induce any signs of skin irritation after 72 hours of observation. Other studies have also shown that encapsulating certain potentially irritating substances, such as limonene or some retinoids, in a microemulsion can reduce their irritant potential compared with applying the ingredient alone. In other words, tolerance depends primarily on the overall composition of the formulation.

Advice : If you have sensitive skin, we recommend that you avoid certain harsh surfactants, such as sodium lauryl sulfate or sodium laureth sulfate.

• PALAZZO G. & al. Microemulsion microstructure(s): A tutorial review. Nanomaterials (2020).

• MACBEATH C. & al. Microemulsion composed of combination of skin beneficial oils as vehicle: Development of resveratrol-loaded microemulsion based formulations for skin care applications. Colloids and Surfaces B: Biointerfaces (2020).

• ZEMB T. & al. Using microemulsions: Formulation based on knowledge of their mesostructure. Chemical Reviews (2021).

• SILVA A. M. & al. Microemulsions and nanoemulsions in skin drug delivery. Bioengineering (2022).

• TEERANACHAIDEEKUL V. & al. Topical microemulsions: Skin irritation potential and anti-inflammatory effects of herbal substances. Pharmaceuticals (2023).