Acids have a long history in skincare. When shiny, tight Italian leather shoes were fashionable, salicylic acid in colloidal suspensions was used to remove calluses from the toes. Later, cotton pads soaked with salicylic acid in ethanol were used to remove the outer layers of the stratum corneum; thus, polishing the skin via exfoliation. The net result of this operation was to reduce the backscattering of light impinging on the skin, and therefore to decrease the visibility of minor surface imperfections. A major skin care breakthrough was Clinique’s development of leave-on creams containing salicylic acid that were able to perform exfoliation around the clock.

That was a lucky strike because salicylic acid is both a keratolytic agent (hence a good exfoliator) and an anti-inflammatory agent (it is the precursor of aspirin). Faced with active cosmetic products, competing brands immediately looked for possible ingredients to formulate exfoliating creams, and found that fruit acids are good exfoliators, too.


Acids in Skin Care

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

It should be noted that topical acid-containing products are not “peeling” products. Peeling is a medical practice that must be performed by a dermatologist. Peeling involves aggressively treating the skin with high concentrations of phenol or trichloroacetic acid or of mixtures of lactic acid, salicylic acid and resorcinol. To be effective, peelings might need several treatments, each of them needing several days or weeks for the skin to heal.

For reasons linked to their chemical structure, the fruit acids quoted above belong to a category called alpha-hydroxy-acids (AHAs) whereas salicylic acid belongs to a chemical category called beta-hydroxy-acids (BHAs).


A Multi-Purpose Acid

A great addition to the panoply of exfoliators was ascorbic acid (AA). But AA, aka vitamin C, is neither an AHA nor a BHA. Rather, it is simultaneously a powerful antioxidant, an irreplaceable catalyzer of the maturation of collagen, a stimulator of ceramide synthesis and, well, a great exfoliator. Being a powerful antioxidant makes it so that ascorbic acid is quite unstable (it gives an electron to whomever needs one) and rapidly changes color, thus affecting the organoleptic properties of the formulas that contains it.

Brands interested in developing vitamin C-containing products designed appropriate packaging such that dry vitamin C was kept in an oxygen-free cap, and could be added to the serum in the jar underneath by hitting the cap; this was a perfect strategy developed by Helena Rubinstein for keeping vitamin C from adventitious oxidation, thus allowing one to use “fresh” vitamin C. Unfortunately, in these conditions, “fresh” vitamin C is available only when the jar contains one single dose of product, so that one cap-jar combination had to be used for every application, and this was not economically viable. The winning strategy to deliver vitamin C upon topical application was to synthetize shelf-stable molecules that could be converted to ascorbic acid by enzymes resident in the stratum corneum, 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 tetrahexyldecyl ascorbate (THD vitamin C, THDC). As an oil-soluble derivative, THDC not only improves chemical stability, but its lipophilic nature facilitates delivery into the epidermis and perhaps the dermis where it undergoes intracellular enzymatic conversion to free ascorbic acid (AA). Importantly, free AA must be released by THDC before its skin benefits can be realized. Thus, THDC is a precursor of AA. When combined with other ingredients, THDC has been shown to decrease signs of photodamage such as skin wrinkling and to reduce hyperpigmentation.

For 25 years, THDC (CAS # 183476-82-6) and ascorbyl tetraisopalmitates (CAS # 183476-6) were construed as being equivalent structures. In reality, they do represent two different chemistries with different CAS#s. While both are esters of ascorbic acid, THDC comes from the reaction of AA with 2-hexyldecanoic acid (CAS# 25354-97-6) whereas ascorbyl tetraisopalmitates derives from reaction of AA with isopalmitic acid (CAS# 4668-02-7). Therefore, they originate from two different fatty acids that, when esterified with ascorbic acid, yield two different products. This misnomer was recently corrected by Sytheon working in close collaboration with the Personal Care Product Council (PCPC) and the Chemical Abstract Services (CAS # provider) (personal communication). Recently, Sytheon has also demonstrated that the stability of THDC under an oxidant rich environment as well as its skin benefits can be improved by combining it with Acetyl Zingerone. This work was published in the International J Molecular Sciences, 22: 8756, 2021.

This is to say that ascorbic acid precursors have not yet finished to surprise us, and it can be surmised that they will continue being relevant ingredients for skin care products, valuable in “active cosmetics.”

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Paolo Giacomoni, PhD
Insight Analysis Consulting
[email protected]
516-769-6904
 
Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as executive director of research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents.