Emollients are the biggest component, after water, in an emulsion system.  The functions of emollients are to soften, moisturize, lubricate, protect, film-form, condition, solubilize and disperse.  Even if emollients play such a pivotal role in emulsions, not all respective property functions might be obvious for formulators.

Let’s look at five basic physio-chemical properties of emollients that allow formulators to systematically review their options and choose emollients that meet their stability and sensory targets. These properties are viscosity, surface tension, spreadability, pour point/cloud point and polarity. These basic attributes allow a maximum differentiation of neat ingredients as well as blended cosmetic oils with minimal efforts. The emollient chart (Figure 1)¹ combines these properties.

1. Viscosity
Viscosity has the largest influence on the subjective fatty character of emulsions.

As the viscosity of the chosen emollient increases, the perception by the consumer of the emulsion seeming heavier and greasier is more pronounced. In water-in-oil emulsions with the same water-phase content and particle size, the viscosity directly correlates with the viscosity of the continuous phase; i.e., the oil phase. This phenomenon helps the formulator to determine the proper fatty character for the emulsion systems. One may choose a very high viscosity emollient (Tegosoft TIS, INCI: Triisostearin) for a water-in-oil night regimen to deliver very caring film formation on the skin, while a low viscosity emollient (Tegosoft AC, INCI: Isoamyl cocoate) is optimal for a daily wear facial emulsion to lend a light sensation on the skin.

2. Surface Tension
The surface tension of an emollient is the elastic-like force between the emollient and air; it has a strong impact on the wetting properties of an emollient on a surface. Surface tension, together with viscosity, correlates with spreadability. Low viscosity, low surface-tension emollients tend to favor emulsions that are fast spreading and leave a minimally perceptible residual film on the skin. The opposite is true for high viscosity, high surface-tension emollients.  These products tend to favor emulsions, which are slower to spread and leave behind a quite noticeable residual film on the skin, often described as exhibiting drag during rub-in and leaving a tacky after-feel. 

3. Spreadability
The spreadability parameter guides the formulator to match sensory targets with the individual performance of the emollients. For example, an application where low spreading emollients may be preferred would be lip care. The function of a lip care product is to place an occlusive barrier on the lips, and in many cases to beautify the lips with the addition of color. The choice of a low spreading emollient will aid in keeping the film in place while helping to prevent feathering or migration of the film into wrinkles.  A good fit for lip care products is an emollient that has low spreading characteristics which also aids in pigment wetting (Tegosoft OER, INCI: Oleyl erucate). 

When developing a facial or hand lotion, the choice of a high spreading emollient would be more advantageous. The high-spreading emollients quickly rub into the surface of the skin leaving a thin, low residue film that imparts a light sensation to the consumer (Tegosoft DEC; INCI: Diethylhexyl carbonate), an emollient that is high spreading and quickly rubs in.  The double branching of this molecule contributes to this attribute.

4. Pour Point/Cloud Point
The pour point is the temperature at which an emollient solidifies on cooling. Sometimes the emollient becomes cloudy before solidification; this temperature is the “cloud point.” It is understood that there is a good correlation between the freeze stability of water-in-oil emulsions and the pour point of the oils. The lower the pour point of the composite oils, the better the freeze stability of the emulsion; i.e., the emulsions can be stored at lower temperatures with a reduced risk of any separation or occurrence of inhomogeneity.

Most formulators run multiple lower temperature accelerated stabilities so they can be aware of the relationship between the emollients chosen and their individual pour points. If the pour point of the mixture is higher than the freeze stability temperatures being tested (-5°C, -15°C and/or -25°C), the oil phase can solidify and crush water droplets causing an irreversible instability or critical separation. Most formulators attribute accelerated stability problems to the emulsifier, but this is not always the case. 

5. Polarity
Of all the attributes, polarity is probably one of the most important to emulsion stability and the least understood. There are two key areas that the polarity of the oil phase affects in the emulsion:  stability and solubility of lipophilic crystalline materials. Polarity has a great impact on the stability of both oil-in-water and water-in-oil emulsions. It is well known that it is easier to obtain stable emulsions with non-polar emollients like mineral oil than with the polar synthetic or natural moieties (Tegosoft PBE, INCI: PPG-14 butyl ether or Tegosoft CT, INCI: Caprylic/capric triglyceride). Thus, as the polarity of the composite oil phase increases, the more difficult it is to emulsify. To address this stability concern, a polymeric, polyfunction oil-in-water or water-in-oil emulsifier should also be paired with the high polar oil phase to enhance stability. 

Very polar oils can also lead to Ostwald ripening, which is related to the molecular diffusion of oil molecules through the water phase from smaller oil droplets into the thermodynamically favored larger oil droplets (for O/W emulsions). This process can ultimately result in phase separation. Ostwald ripening can be reduced by adding oils of very low polarity to very polar oils. This balances the polarity, which makes the emulsion easier to stabilize. In the case of W/O emulsions, Ostwald ripening can be reduced by the addition of an electrolyte to the water phase. The role of the electrolyte is to slow down the diffusion process, which stabilizes the emulsion stability.

Another key feature of highly polar emollients is the ability to act as a solvent for lipophilic crystalline ingredients like sunscreen filters and skin care additives that are utilized in personal care products. Examples of some lipophilic sunscreen structures are butyl methoxydibenzoylmethane, benzophenone-3, ethylhexyl triazone, and bis-ethylhexyloxyphenol methyoxphenyl triazine. Highly polar cosmetic oils like Tegosoft XC (INCI: Phenoxyethyl caprylate), or C12-15 alkyl benzoate provide excellent solvency for these sunscreen active ingredients.²

Taking all of the aforementioned parameters into account can help formulators select the right emollient to meet desired performance targets.

References

1. T. Dietz, “Basic properties of cosmetic oils and their relevance to emulsion preparations,” SÖFW-Journal 125 (7), 1999.
2. A. Howe, D. Adkins, M. Soldato, B. Jha, O. Springer, “Phenoxyethyl Caprylate – A New Polar Emollient for Organic UV Filters,” SÖFW-Journal 137 (7), 2011.

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Anna Howe
Evonik Corporation

Anna Howe has over 25 years of experience in the cosmetic industry. As applied technology manager, North America, for the personal care business of Evonik Corporation (Hopewell, VA) her responsibilities include global product development and leading regional customer projects. Before joining Evonik, she worked with Inolex Chemical Company, Rhône-Poulenc (Rhodia) and Alcolac Chemical Corporation. A member of SCC, she holds several application patents and has authored a number of scientific papers.
More info: Anna Howe,  anna.howe@evonik.com; Evonik, Hopewell, VA, Tel: 804-541-8658