Cosmetic emulsions case study
The proposed methodology was tested in the design of cosmetic products, specifically skin moisturizers. These products are used to keep in good condition the skin by maintaining its balance of oil and water.43 Skin moisturizers are generally oil in water (O/W) emulsions, and commercially they are usually divided into two main types: creams and lotions. Lotions are low-viscosity emulsions, while creams are much higher viscosity materials, generally presented as semi-solid emulsions.44 The O/W emulsions are characterized by a low internal phase ratio, typically containing 10 to 35% dispersed phase.45 For this reason, the addition of a suitable emulsifier agent along with the application of mechanical agitation is necessary to create a stable emulsion.46In this work, apart from some mandatory ingredients like water or humectants (i.e. Glycerol) that are kept constant, three main types of ingredients are considered for the modeling of O/W emulsions: Emollients in the dispersed phase, thickeners in the continuous phase, and emulsifiers in the interface. The selection of these groups of ingredients is based on several studies that evaluated their impact on the sensorial, rheological and textural properties of cosmetic emulsions.19,46–50
When formulating cosmetic emulsions, the dispersed phase should be first selected,3 and emollients constitute the main component of this phase, particularly in O/W emulsions. Emollients are required in the dispersed phase because they help to prevent soaping, they improve spreadability, and they are responsible for the consumer-perceived benefits after evaporation of volatile materials.46,48 Performance of emollients is generally related to greasiness, and the emulsifying properties are dependent on other physical properties such as density, viscosity, melting point, and the required HLB (RHLB).51,52
Emulsifiers are essential ingredients to stabilize the emulsions.45 The type of surfactant and its physicochemical properties will influence the droplet size and stability of the emulsion.52 HLB is a used here to predict the emulsifying properties of surfactants,37 and to correlate with some sensorial properties of the emulsions, such as color, odor, and consistency.46,53
Thickeners are used to increase the viscosity of the continuous phase and to mitigate the phase separation.46,51 It has also been shown that thickeners could have a relevant impact on skin feeling, namely when removing cream from the container, and when spreading the product.54