Emulsions are mixtures of two immiscible liquids, such as oil and water, stabilized by the introduction of an emulsifier. Common examples of emulsions include vinaigrettes, milk, or even paint. The key challenge with emulsions is their inherent instability. Oil and water will tend to inevitably separate due to their different densities. To overcome this, we use emulsifiers that sit at the droplet interface to reduce interfacial tension and stabilize the oil droplets through the formation of either steric barriers or through electrostatic repulsion.
Emulsifiers also play a role in the aesthetics of cosmetic formulation, influencing the consistency, texture and skin feel. The initial rub out of a cosmetic formulation is important and is driven mostly by emulsifiers. Emulsifiers can also help to reduce stickiness or tackiness but too much emulsifier can also create soaping or heavy waxy feeling. Finally, they can be functional as well, bringing benefits to skin.
In the realm of emulsification stabilization mechanisms, we encounter several distinct categories: First, there are systems that stabilize emulsions through electrostatic repulsion, utilizing anionic, cationic or amphoteric emulsifiers. Next, we have systems dominated by steric stabilization, which involve non-ionic emulsifiers guided by their Hydrophilic-Lipophilic Balance (HLB). A more specialized category includes liquid crystal or lamellar gel systems, exemplified by lecithin-based or olive-based emulsifiers. These systems form structured layers at the interface, enhancing stability through their unique molecular arrangements.
Additionally, polymeric microgel systems, typically composed of liquid acrylic acid polymer suspensions, provide another method of stabilization. These polymers form a network around droplets, offering stability through their gel-like properties.
Lastly, pickering emulsions are stabilized by insoluble particles, which may or may not be coated. These particles, often functioning through steric repulsion, create a robust barrier that prevents droplets from merging.
Let’s dive deep on their differences:
- Surfactant-based emulsions
Most emulsifiers are surfactant-based. Surfactants are small amphiphilic molecules, which offer reduced surface coverage of the oil droplets. They can be ionic or nonionic types of ingredients: Both have a hydrophilic head and a lipophilic tail, and they need low energy for adsorption/desorption at the Oil-Water interface.
Surfactant-based emulsifiers also have a high affinity to the oil phase, and they can easily be dispersed at the oil-water interface.
But they also create reversible dynamic structures. The low energy interaction at the oil-water interface means that the system can easily be destabilized, which will result in phase separation of the emulsion.
Surfactants can also have a huge impact on the consistency of cosmetics formulations. Thickening mechanisms in surfactant-based systems rely on the interplay of several factors, including the size of the surfactant micelles, surfactant concentration, the type and ratio of surfactants used, temperature, and micelle charge density. Surfactant solutions achieve viscosity primarily through the formation of rod-like micelles, facilitated by reducing charge density or crosslinking micelles. The optimal thickener selection relies on the specific formulation requirements, such as pH, suspended ingredients, and the types and concentrations of surfactants.
Surfactants also present significant environmental concerns. Many surfactants are derived from palm oil, a resource linked to substantial environmental challenges and supply chain disruptions. Additionally, surfactants sourced from PEG (polyethylene glycol) compounds raise health issues due to the presence of ethoxylates. Furthermore, surfactants impact the skin’s microbiome by stripping away its natural flora.
- Pickering emulsions
Pickering emulsions were reported in 1907 by Percival Spencer Umfreville Pickering, a British chemist.
Instead of relying on traditional emulsifiers, pickering emulsions are based on small solid particles coating the droplets of the dispersed phase in an emulsion. These particles adsorb at the oil-water interface, creating a protective barrier that prevents coalescence. While commonly used in the food industry, they are still under-utilized in the personal care industry.
The type of emulsion (oil in water) or (water in oil) can be determined by the wettability of the particles. If the particles used are hydrophilic (< 90-degree contact angle), such as silica or clay, an oil-in-water emulsion is formed. If the particles are hydrophobic (> 90-degree contact angle) such as carbon black, a water-in-oil emulsion will be formed.
Pickering emulsions offer several advantages over traditional emulsions. They provide enhanced stability over time because they create an irreversible emulsification mechanism thanks to the high energy required for adsorption/desorption of the particles at the oil interface. This enhanced stability ensures a longer shelf life for cosmetic products.
Pickering emulsion also set formulations apart in term of aesthetic. They provide unique texture which are smoother and silkier. They can create lightweight, non-greasy formulations that spread easily and feel pleasant on the skin. This is particularly beneficial for products like moisturizers, sunscreens, and makeup, where a smooth, non-oily finish is desirable.
While pickering emulsions are great, they also come with some challenges. Beyond the choice of solid particles and understanding their interactions in various conditions, they also need high-shear equipment to anchor the particles at the oil-water interface.
SeaBalance® 2000 Technology, a seaweed-based upcycled ingredient, creating pickering emulsions
SeaBalance is a blend of Sargassum seaweed extract with xanthan gum and pentylene glycol. It is made through a proprietary cold extraction process that uses green chemistry to facilitate and enhance the value of natural seaweed polymers. It is an anionic multifunctional ingredient that can be used in both hot and cold processes – an important consideration for formulators looking to reduce overall energy consumption.
SeaBalance provides excellent stabilization capabilities and creates stable pickering emulsions via the following mechanisms.
The Sargassum seaweed extract contains an anionic cellulose fiber complex with amphiphilic properties that assist in creating cellulose-based pickering emulsions.
This amphiphilic natural complex offers high surface coverage of oil droplets. It has hydrophilic regions and lipophilic domains, and it needs high energy for adsorption / desorption at the oil-water interface.
In parallel, a 3D stabilization mechanism is created through a polymeric network, which increases steric stabilization. Finally, SeaBalance uses the electrostatic repulsion between the cellulose adsorbed and non-adsorbed at the interface to prevent oil droplets coalescence.
Overall, the combination of electrostatic repulsion and steric stabilization help to create stable emulsions. Unlike surfactant systems in which amphiphilic molecules are reversibly adsorbed at the oil-water interface, pickering emulsions are irreversible once they are established. However, it is important to highlight that shear force is an important factor in setting up the initial emulsion stability for pickering emulsions. To ensure a uniform distribution across oil droplets, a high-dispersibility of the active ingredient is required. This dispersibility can be improved by high-shear mixing, due to the shear-thinning properties of SeaBalance.
The emulsions made with SeaBalance are luxurious with a great skin-feel. It is an incredibly versatile ingredient that can hold a wide range of emollients, can be used in a wide range of pH (3.5 to 9.0) and has a great resistance to electrolytes.
In conclusion, cellulose-based pickering emulsions hold great promise and represent an innovative breakthrough in the cosmetics industry. The unique stabilization mechanism of pickering emulsions not only enhances the stability and sensory properties of cosmetics products but also aligns with the industry’s move towards natural, safe and effective ingredients. This makes them a promising and innovative solution in the development of next-generation cosmetics formulations.
