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

In cosmetics, many formulations are based on combining two phases that do not naturally mix, such as water and oil. To stabilize 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 they can remain stable over time with no inherent tendency to separate.

The stability of emulsions and microemulsions relies on the specific organization of surfactants within the formulation. These molecules spontaneously assemble into structures called micelles, which are capable of trapping one phase within the other. In some cases, oil can be incorporated into the hydrophobic core of micelles dispersed in water: this is 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 formed. In these systems, surfactants create a kind of film at the interface between the two phases, which stabilizes these structures.

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

The reason microemulsions attract so much interest in cosmetics 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 especially difficult to overcome for hydrophilic actives, while some lipophilic compounds instead present challenges in terms of solubility or stability within formulations. In this context, 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 skin surface.

The very small droplet size of microemulsions is a major advantage. At the nanometer scale, the structures formed exhibit 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 organization into aqueous and oily compartments, stabilized by surfactants, gives them a high solubilization capacity for both hydrophilic and lipophilic molecules, often greater than that of conventional emulsions. In other words, they make it easier to incorporate certain active ingredients that would be difficult to disperse in a standard formulation. This effect is particularly relevant for poorly soluble ingredients, whose effectiveness may be limited if they are not fully dissolved. By improving their solubilization, the microemulsion can therefore help increase the availability of the active ingredient at the skin’s surface.

This interest is not only theoretical. In the scientific literature, several studies have shown that microemulsions can enhance the skin penetration of certain molecules compared to more conventional formulations. A frequently cited example concerns cold sore herpes (herpes labialis). When formulated as a microemulsion, penciclovir—a topical antiviral commonly used to treat cold sores—has shown, in an experimental model, a skin penetration capacity far superior to that of 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 thus potentially its local effectiveness.

If microemulsions are of such interest in cosmetics, it is also for more sensory and formulation-related reasons. Their small droplet size explains their often clear, translucent, or even transparent appearance, as well as their low viscosity. This allows them to give rise to fluid, lightweight, and easy-to-spread textures, which are particularly sought after in serums, lotions, or products intended 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 skin’s sensitivity, this can raise questions regarding skin tolerance, particularly for sensitive skin. In other words, microemulsions are not automatically superior to all other dosage forms: their value depends on the formulation, the active ingredients selected, the target skin type, and the balance achieved between efficacy, stability, and tolerance. This is precisely why they continue to be actively investigated in the field of cosmetics.

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 conditions. For example, various experimental and clinical studies have evaluated microemulsions containing plant extracts and have not observed erythema, edema, or inflammation after application to human or animal skin. In a trial conducted on 30 volunteers, a microemulsion serum containing 1% extract of Cordyceps militaris did not cause 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 specific retinoids, in a microemulsion can reduce their irritant potential compared to application of the ingredient alone. In other words, tolerance depends primarily on the overall composition of the formulation.

Tip : If you have sensitive skin, we recommend avoiding 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).