The benefits of cosmetic acids in skin care formulas

Acids have a long history in skin care. When shiny, tight-fitting Italian leather shoes were in vogue, salicylic acid in colloidal suspensions was used to remove corns from the toes. Cotton pads soaked in salicylic acid in ethanol were then used to remove the outer layers of the stratum corneum; then, polishing the skin via exfoliation. The net result of this operation was to reduce the backscattering of the light hitting the skin, and therefore to decrease the visibility of small surface imperfections. A major breakthrough in skincare was Clinique’s development of leave-on creams containing salicylic acid that can perform 24-hour exfoliation.

It was a lucky hit because salicylic acid is both a keratolytic agent (hence a good exfoliant) and an anti-inflammatory agent (it is the precursor to aspirin). Faced with active cosmetic products, competing brands immediately searched for possible ingredients to formulate exfoliating creams and found that fruit acids are also good exfoliants.

Fruit acids, such as citric acid, glycolic acid, lactic acid, malic acid and the like, are palatable to consumers who insist that “natural is good”. The fact is that natural can be quite irritating to the skin.
The irritation was a major drawback as exfoliating action is mainly required around the eyes to reduce fine lines and wrinkles in the crow’s feet area and under the eyes. Skin care brands have struggled for years to formulate irritation-free, acid-containing products. Products containing acids have been marketed with different claims such as “it stings because it is active”, “removes scars”, “is an anti-acne product”, “smooths skin discoloration”, “smoothes lines and wrinkles” and “the best anti-aging ingredient ever.”

It should be noted that acid-containing topical products are not “peeling” products. Peeling is a medical practice that must be performed by a dermatologist. The peeling involves an aggressive treatment of the skin with high concentrations of phenolic or trichloroacetic acid or mixtures of lactic acid, salicylic acid and resorcinol. To be effective, peels may require several treatments, each taking several days or weeks for the skin to heal.

For reasons related to their chemical structure, the fruit acids mentioned above belong to a category called alpha-hydroxy acids (AHA) while salicylic acid belongs to a chemical category called beta-hydroxy acids (BHA).

A great addition to the panoply of exfoliants was ascorbic acid (AA). But AA, aka vitamin C, is neither an AHA nor a BHA. Rather, it is at the same time a powerful antioxidant, an irreplaceable catalyst for the maturation of collagen, a stimulator of ceramide synthesis and, in short, an excellent exfoliant. Being a powerful antioxidant, ascorbic acid is quite unstable (it gives an electron to those who need it) and changes color rapidly, thus affecting the organoleptic properties of the formulas that contain it.

Brands interested in developing vitamin C-containing products designed appropriate packaging so that dry vitamin C was stored in an oxygen-free cap and could be added to the serum in the jar below by hitting the cap; this was a perfect strategy developed by Helena Rubinstein to preserve vitamin C from accidental oxidation, thus allowing “fresh” vitamin C to be used. Unfortunately, under these conditions, “fresh” vitamin C is only available when the jar contains a single dose of the product, so a cap-jar combination had to be used for each application, which was not cost-effective. The winning strategy for delivering topical vitamin C has been to synthesize shelf-stable molecules that could be converted to ascorbic acid by stratum corneum resident enzymes, such as magnesium ascorbyl phosphate, ascorbyl glucoside, ethyl ascorbic acid and tetrahexyldecyl ascorbate ( THD ascorbate).

Both water soluble (sodium ascorbyl phosphate, magnesium ascorbyl phosphate, ascorbyl glucoside and ethyl ascorbic acid) and oil soluble (ascorbyl esters) are widely used in cosmetic and therapeutic preparations and one of the most popular forms is the tetrahexyldecyl ascorbate (THD vitamin C, THDC). Being an oil-soluble derivative, THDC not only improves chemical stability, but its lipophilic nature facilitates release into the epidermis and possibly the dermis where it undergoes intracellular enzymatic conversion to free ascorbic acid (AA). Importantly, the free AA must be released from THDC before its skin benefits can be realized. Therefore, THDC is a precursor of AA. When combined with other ingredients, THDC has been shown to reduce the signs of photodamage such as skin wrinkles and reduce hyperpigmentation.

For 25 years, THDC (CAS # 183476-82-6) and ascorbyl tetraisopalmitates (CAS # 183476-6) have been interpreted as equivalent structures. In fact, they represent two different chemicals with different CAS #. Although both are esters of ascorbic acid, THDC arises from the reaction of AA with 2-hexyldecanoic acid (CAS # 25354-97-6) while ascorbable tetraisopalmitate arises from the reaction of AA with isopalmitic acid (CAS # 4668-02- 7). Therefore, they originate from two different fatty acids which, esterified with ascorbic acid, give two different products. This misnomer has recently been corrected by Sytheon who works closely with the Personal Care Product Council (PCPC) and Chemical Abstract Services (CAS # provider) (personal communication). Recently, Sytheon has also shown that the stability of THDC in an oxidant-rich environment and its benefits for the skin can be enhanced by combining it with Acetyl Zingerone. This work was published in International J Molecular Sciences, 22: 8756, 2021.

This is to say that the precursors of ascorbic acid have not finished surprising us yet and it can be assumed that they will continue to be important ingredients for skin care products, valuable in “active cosmetics”.

Paolo Giacomoni, PhD
Consultancy for in-depth analysis

Paolo Giacomoni acts as an independent consultant for the skin care industry. He was executive director of research at Estée Lauder and was head of the biology department at L’Oréal. He has built a record of results through research into DNA damage and UV-induced metabolic deterioration, as well as the positive effects of vitamins and antioxidants. He is the author of more than 100 peer-reviewed publications and owns more than 20 patents.

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