36 TEXTURES
The revolution of natural w/o emulsions in skin care
n Flora Bollon – Gattefossé, France
Despite the growing interest of consumers in natural formulations, brands struggle to develop innovative and sensory products. The majority of these products are based on an O/W system, which limits possibilities in terms of textures. A solution could be to use a W/O system but it suffers from a poor reputation among formulators. Why are W/O emulsions not used more in skin care? What are their limits? What solutions could be provided to help formulators to move “beyond the box” of traditional cosmetic emulsions?
W/O emulsions are not popular in skin care Different reasons explain why W/O emulsions are not used more in skin care. A W/O emulsion is composed of an
internal aqueous phase and an external oil phase, that is why it is often perceived as greasy, tacky or heavy on the skin. This structure is also responsible for thick textures, difficult to spread on the skin. Moreover, W/O emulsions are more
difficult to formulate than O/W emulsions. Indeed, the upstream construction of the formulation must be rigorous, in order to avoid problems of compatibility between the ingredients (in particular in the oil phase). In addition, the emulsification step is meticulous since it is necessary to add the aqueous phase very slowly to the oil phase. Finally, the W/O formulation has specific features such as, for example, the addition of salts to prevent problems of instability over time and the need to stir very strongly during emulsification. This type of emulsion therefore requires a longer development
time and a special attention to the stability of the formula over time. Finally, the actual offer of W/O
emulsifiers is not satisfying. Indeed, over the last decade, skin care has gone natural, progressively but with great success. It is undeniable that natural skin care today is performant, sensorial and pleasant. It is particularly true for O/W emulsifiers, thanks to research carried out by scientists across the board, from raw materials to formulation. W/O emulsions are a different story altogether. The natural offer is not as efficient and sensorial as the synthetic one, that is why formulators do not take the risk of using this type of formulation.
How to formulate effective and stable W/O emulsions First of all, it is necessary to understand why W/O emulsions are complicated to stabilise and how to solve it. Here are key concepts to enhance stability when formulating W/O emulsions in general.
High energy is required High energy brings several benefits to W/O systems, altogether contributing to a stable emulsion. l Create high energy dissipation rates 1,2 Figure 1 is a simplified energy flow chart when you are creating a W/O emulsion. Turbulence, initiated by the high-
shearing rotor stator, transfers its energy into kinetic energy within the W/O system. That energy of motion is converted into the large velocity gradients of the dispersed droplets of various sizes. And finally, the energy of the rapidly moving particles is
converted into heat through dissipation.2 This is a very simplified flowchart, as
there are many other variables involved, but the take-home message here is that the higher the conversion of energy into heat, the higher the dissipation energy, the more kinetically stable emulsion. Essentially, we do not want any energy left in the emulsion, especially in those water droplets because that can lead to instability. l Control particle size of the dispersed phase of the emulsion 1,2
As mentioned before, high energy processes will use rotor stators/homogenisers, while a low energy process may use simpler blades (turbine stirrer, propeller, or blade stirrer). High energy leads to a finer, more evenly dispersed emulsion which is more stable. But if homogenisation is too mild, due to
the equipment and/or shearing speed, the W/O emulsion will be highly poly- dispersed, resulting in wide size distribution in the dispersed droplets and leading to instability. l Reduce interfacial tension 1,2 High energy reduces the interfacial tension (energy present at the water-oil interface) in W/O systems. This benefit is a result of the other two:
The more heat that is given off, the less energy and mobility in the dispersed water phase. And the smaller the average droplet size of the dispersed phase, the better the stability. Altogether, there is less energy remaining at the water-oil interface, so water droplets are less likely to coalesce and will remain stable within the continuous oil phase.
Turbulence
Kinetic energy
Molecular velocity
Heat
Figure 1: Energy flow chart. PERSONAL CARE ASIA PACIFIC November 2020
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