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