The surface of the skin carries a very fine film in which sebum, sweat and water mingle. This mixture of an oily phase and an aqueous phase behaves like a natural emulsion, constantly renewed, and the barrier it covers organises itself best when this environment remains acidic.
It is called the “hydrolipid film”: hydro for water, lipid for fats.
Two sources, two phases
The oily phase comes mainly from the sebaceous glands. Sebum is rich in triglycerides, wax esters and squalene. Added to this, in smaller quantities, are lipids produced by the cells of the epidermis: ceramides, cholesterol and free fatty acids. On gland-rich areas such as the face, sebum represents the bulk of the lipids collected at the surface (Pappas, 2009).
The aqueous phase comes from sweat and from water that diffuses from inside the skin. The sweat of the eccrine glands is composed almost entirely of water. It also contains salts, mainly sodium and chloride, as well as lactate and urea (Baker, 2019).
These two phases do not mix spontaneously, like oil and water in a glass. Yet they coexist at the surface of the skin, in a continuous film.
What an emulsion is
An emulsion is the dispersion of one liquid in another with which it does not mix. Milk, mayonnaise and most creams are emulsions. For it to hold, molecules are needed that can position themselves at the boundary between water and oil. These molecules have one part attracted to water and another attracted to fats: they are said to be amphiphilic.
The skin film contains such molecules. Free fatty acids, derived in part from the breakdown of sebum triglycerides, are one example (Pappas, 2009). Its stability remains far lower than that of a formulated cream. It is a dynamic mixture, which breaks down and reforms with secretions, perspiration and washing.
A film that changes throughout the day
The composition of the film is never fixed. A Chinese study followed 102 people during and after physical exertion. As soon as sweating begins, the hydration of the stratum corneum increases. The amount of sebum measured also varies, depending on whether sweat is light or abundant. The pH of the face changes during exertion, then returns to its usual acidic value in about one hour (Wang et al., 2013).
Other factors alter the balance of the film: temperature, air humidity, clothing, and above all washing. Water and cleansing products carry away part of the oily phase. The film then rebuilds itself as the glands secrete again.
An acidic emulsion
The hydrolipid film is slightly acidic. Measured on the forearm of 330 volunteers after 24 hours without showering or products, the pH of the skin surface is estimated on average at 4.7 (Lambers et al., 2006). Its origins are multiple and still being studied: gland secretions contribute to it, as do mechanisms specific to the stratum corneum.
This acidity matters above all for what happens just below. The lipids that form the barrier of the stratum corneum are assembled by enzymes that work best in an acidic environment; in mice, barrier repair is slowed at neutral or alkaline pH (Mauro et al., 1998).
In other words, the surface film is made of sebum, sweat and water, and the barrier it covers organises itself best when this film remains acidic.
In summary
- The hydrolipid film combines an oily phase, mainly sebum, and an aqueous phase, mainly sweat.
- It behaves like a natural emulsion, more mobile than a cream.
- Its composition changes with exertion, climate and washing, then rebuilds itself.
- It is slightly acidic, and the underlying barrier forms best in this environment.
This article is part of the PHYTO 5 Library.
Sources
- Baker LB. Physiology of sweat gland function: the roles of sweating and sweat composition in human health. Temperature. 2019;6(3):211-259. doi:10.1080/23328940.2019.1632145
- Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science. 2006;28(5):359-370. doi:10.1111/j.1467-2494.2006.00344.x
- Mauro T, Holleran WM, Grayson S, Gao WN, Man MQ, Kriehuber E, Behne M, Feingold KR, Elias PM. Barrier recovery is impeded at neutral pH, independent of ionic effects: implications for extracellular lipid processing. Archives of Dermatological Research. 1998;290(4):215-222. doi:10.1007/s004030050293
- Pappas A. Epidermal surface lipids. Dermato-Endocrinology. 2009;1(2):72-76. doi:10.4161/derm.1.2.7811
- Wang S, Zhang G, Meng H, Li L. Effect of exercise-induced sweating on facial sebum, stratum corneum hydration, and skin surface pH in normal population. Skin Research and Technology. 2013;19(1):e312-e317. doi:10.1111/j.1600-0846.2012.00645.x