Emollients: Chemical structures, physicochemical and sensorial properties

Emolliency has become a must in cosmetics, and is used to describe a product or cream that provides a certain level of comfort on application, and "softens" and "smoothes" the skin or hair. Emollience covers a set of sensory characteristics associated with properties such as softness, elasticity, spreadability and shine of the skin or hair. Originally, the term "emollient" was used in the medical field to designate a substance with the property of softening and relaxing the body's tissues. Emollient creams are thus recommended in dermatology to nourish and care for dry or atopy-prone skin. Vegetable, animal, mineral and synthetic oils and fats, insoluble organic compounds, are emollients par excellence. The more emollient an oil, the softer it feels when applied to the skin. This phenomenon is closely linked to the ease with which the product spreads over the skin, and the formation of a protective film on the cutaneous surface. This so-called "occlusive" film helps combat skin dehydration. As a result, oilier emollients spread over the skin with greater difficulty and have a greater occlusive effect than less oily substances. As a result, night creams are formulated mainly with lipid-rich emollients, which promote occlusion. Medium-fat emollients are used in day creams, while more volatile emollients are used to formulate body milks intended for application to large surfaces such as arms and legs. The sensoriality offered by an emollient is directly linked to its physicochemical properties and chemical structure. For example, ease of spreading or a light, non-sticky feel depend mainly on the emollient's viscosity and surface tension, which in turn derive from its molecular structure. Establishing and mastering such relationships is proving to be a major asset for emollient designers and formulators, who are constantly on the lookout for new, innovative emollient structures to meet not only consumer expectations but also societal challenges such as the replacement of certain petrosourced ingredients, decried for their ecological footprint.

Following an overview of the emollient market and a description of the structural organization of the skin, this article presents in detail the main families of emollients, namely silicones, esters and hydrocarbons. The main physicochemical and sensory properties used to characterize emollients are detailed, and the relationships between their chemical structure, physicochemical properties and sensory profile are developed. Finally, regulatory and toxicological aspects are addressed, using the example of silicone oils, increasingly criticized since the early 2000s for their persistence in the environment.