| Surfactant-free emulsions might be formulated as so-called Pickering emulsions. In this case, a stable interfacial film with good protection against coalescence can be achieved by densely packed solid particles in the o/w interface. The key factor for the use of particles as a stabilizing agent is their wetting by the two phases, the oil and the aqueous phase. However, the affinity to each of the two phases should be different. This is expressed by the contact angle (see figure 1). | | | Figure 1. Influence of the contact angle on the stabilizing action of solid particles. | Characterising Pickering Emulsions and How to Guard Against Coalescence Pickering emulsions require sufficiently small particles which arrange in the o/w interface. This means that the solid particles usually are at least 10-fold smaller in size than the dispersed droplets of the emulsion. Capillary forces can support the formation a particulate network in the interface (see figure 2). This serves as a mechanical barrier to prevent the coalescence of the droplets. The protection against coalescence is based on the energy to expel the particles from the interface into the dispersed droplets. This energy depends on the contact angle, which ideally should be close to 90°. | 
| | Figure 2. Solid particles stabilizing an emulsion. | Some Examples of Particulate Emulsifiers Table 1. Examples of particulate emulsifiers. | | | Alumina | W/O | | Betonite | O/W | | Magnesium Aluminum Silicate | O/W | | Fat Crystals | W/O | | Magnesium Oxide | W/O | | Magnesium Trisilicate | W/O | | Titanium Dioxide (coated) | O/W, W/O | | Silica | O/W | | Tin Oxide | O/W | Features That Can Affect the Stability of Pickering Emulsions The following factors mainly influence the stability of Pickering emulsions: · Contact Angle, · Particle Size, · Solid Concentration, · Interparticulate Interaction. Further critical parameters are the nature of the oil, the phase volume fraction and, last but not least, the order of addition during processing. Processing Techniques for the Preparation of Particle-Stabilized Emulsions So far, hints for the preparation of particle stabilized emulsions have been scant and only few formulations are available in literature and patents. Table 2 (below) lists a suitable base for the formulation of an o/w emulsion. For the preparation, both phases are mixed and homogenized for 2 minutes. After the pH has been adjusted to approximately 6, the emulsion is again homogenized for 10 minutes. Table 2. Emulsion stabilized by flocculated solid particles. | | | Water Phase | Hydroxypropylcellulose | 0,1 g | | (50 ml) | Silica | 1,13 g | | | Aqua | 49,5 g | | Lipid Phase | Paraffinum Liquidum | 150 ml | Pickering Emulsions - Potential Applications in Sun Care Products Although very interesting not only from theory but also from an application point of view, Pickering emulsions are not en vogue as a research topic and cosmetic products only make rare use of this principle. However, particle-stabilized emulsions seem to be of great value in the formulation of sun care products. If such products use physical sun screen substances, e.g. TiO2 or ZnO, these physical filters can be distributed uniformly and they can act concurrently as emulsion stabilizers. Note: A complete list of references can be found by referring to the original text. |