In 1910, Beiersdorf launched the first commercial water-in-oil (W/O) emulsions based on lanolin. The product claimed improved skin smoothness and offered skin protective benefits. During the past 100 years, these W/O emulsions have come a long way. They still offer this protective benefit, but new skin care technology must meet additional challenges of modern W/O systems.  

Today’s formulators want stable emulsions that offer broad formulation flexibility, such as wide phase ratios, hot and cold processing, viscosity, pH range, electrolyte tolerance, as well as compatibility with active ingredients—all within a tight, economical balance. Water-in-oil formulations are ideal delivery systems for a wide application range due to their inherent moisturization and wearability properties.
 
 
Design of a New Emulsifier
 
Emulsifier performance is optimized by a combination of polyfunctionality and high molecular weight of the molecule. Optimal stabilization is provided by the polyfunctionality or comb structure on the backbone of the polymer. As molecular weight of the backbone increases, the stability of the emulsion stability at higher temperatures is enhanced since the energy needed to remove the emulsifier from the interface scales with molecular weight. Thus the combination of polyfunctionality and high molecular weight makes possible the creation of a robust emulsifier positioned for critical systems.
 
Evonik Industries developed such a new polyfunctional silicone-based W/O emulsifier with high molecular weight (81,000 g/mol). The new Abil EM 180 silicone emulsifier can provide superior performance with enhanced stabilization properties. It can be utilized as a single emulsifier at use levels down to 0.5%. It also can maintain stability at high storage temperatures to 70°C (158^oF).

The structure of this new emulsifier is depicted schematically in Figure 1.  Cetyl PEG/PPG-10/1 Dimethicone (high molecular weight (Emulsifier 180) is a water white, clear, viscous liquid with a HLB of approximately 5. It is efficient at low usage concentrations and easy to handle in cold or hot processing. Beside an outstanding cost-performance ratio, it provides superior thermal stability as well as high compatibility with electrolytes and active ingredients.
 
Expanding Formulation Opportunities
 
The following formulation examples demonstrate the advantages of this new emulsifier in various demanding systems.

Emulsifier 180 is excellent for stabilizing emulsions at a very low use level, which also can provide important cost advantages. Figure 2 shows a critical system utilizing Emulsifier 180 at 0.5% as the sole emulsifier. This emulsion also contains a high proportion of alcohol, a potential cause of instability in W/O systems.

Globally, formulators are looking for ways to decrease or eliminate preservatives, so the ability to successfully emulsify high alcohol loads is desirable. This system also contains a high loading of phenoxyethanol, a preservative blend. Phenoxyethanol is interfacially active, and in many W/O systems can displace the emulsifier at the interface and cause instability.

Another example of a critical system is quick-break emulsion technology. These systems can handle a high water phase (80 – 90%). When applied to the skin, the emulsion breaks and water is released, offering a visual trigger and a surprising sensation.  The key to this critical system is maintaining stability during pumping and filling as applied shear can easily lead to unwanted breaking of the emulsion.

Figure 3 contrasts the stability two types of Cetyl PEG/PPG-10/1 dimethicone: lower vs. higher molecular weight. Greater stability is achieved with the higher weight molecule.

A third example of a critical system is a sunscreen emulsion with a large oil phase (>50%). Figure 4 shows an example that combines all the US filters at maximum level. The stable sunscreen emulsion achieved a static result of 88 SPF (in-vivo, according to FDA) and 29 UVA-PF.

The final example of a critical system is a lotion across a range of electrolyte concentrations. This new robust emulsifier can handle low as well as high levels of electrolytes without impacting emulsion stability. Figure 5 shows that a range of 0-5% of a monovalent salt may be utilized. The lower molecular weight product (Emulsifier 90) requires a much narrower load range of 0.5-1.0%.

Temperature Stability

The higher molecular weight (Emulsifier 180) enhances the temperature stability of formulations, as the example shown in Figure 6. The emulsion containing Emulsifier 180 remains stable for more than two months under storage at 70°C whereas the system based on Emulsifier 90 shows degradation after one week.

Measurement of the interfacial shear modulus provides additional information for determining temperature stability. Besides observing macroscopic changes in the appearance of the emulsion, measurements permit insight into the emulsifier-interphase interaction. The device used was a “Double Wall Ring” as described by A. Franck, et. al.  The new device offers an advantageous geometry.  It can be mounted to a rheometer to permit measuring at very low deformations, which is crucial to probe interphases. The trough is designed in a way that shear rates on both sides of the ring are equal.  This set up is unique for measuring interfacial rheology. The complete setup and procedure is detailed elsewhere.¹

The results of comparison measurements with the formulation of Figure 6a based on Emulsifier 90 and Emulsifier 180 respectively, are displayed in the figure. In the graph, the interfacial shear modulus is plotted versus the applied strain. At room temperature (filled squares), the interfacial shear modulus provided by Emulsifier 180 is 4 times higher than the interfacial shear modulus of Emulsifier 90. The effect gets intensified when the measurement is performed at elevated temperatures (45°C, empty squares). For both emulsifiers, the shear modulus drops compared to room temperature, but less for Emulsifier 180 (one order of magnitude) than for Emulsifier 90 (two orders of magnitude). This means, that formulations based on Emulsifier 180 are more robust against higher temperatures than the comparable one based on Emulsifier 90 that is in line with the macroscopic results of the storage test discussed before.
 
Applications
 
Water-in-oil emulsions efficiently reduce evaporative water loss from the skin as they form an occlusive layer.   Skin protection and water repellency make W/O emulsions suitable for a range of skin care applications such as sun care products, dry-skin care and moisturizing systems, and color cosmetics (Formulations A, B and C).  In sunscreen products, W/O emulsions are preferred systems for sunscreen products due to their better water resistance properties compared to oil-in-water systems. In cosmetic make-up, water-in-oil systems provide long wear, a smooth application and a smooth and comfortable skin feel.
 
Summary
 
The choice of emulsifier is a determining factor in overall application performance and can yield important benefits in the final product. Formulators are always looking for ways to expand the boundaries of emulsion technology. They demand ingredients that maximize creativity and product performance while balancing costs. Emulsifier 180 is best-in-class for emulsification and stabilization properties in critical systems. This new product provides enhanced stabilization in difficult systems and at high temperatures. This new silicone emulsifier offers processing flexibility with demanding components such as electrolytes, UV filters or active ingredients. It can achieve performance at a use level as low as 0.5%, which can also offer cost savings.  As formulators look to the future, silicone emulsifier technology offers unmatched performance, and will continue to be a vital technology for W/O solutions.

                                               
References
1. A. Franck, S. Vandebril, J. Vermant, G. Fuller; ”A double wall-ring geometry for interfacial shear rheometry”, Rheol. Acta, 2009, 49,
     131 – 144.