March 2022 • Vol. 28 • Issue 3
History of Salicylic Acid— Chemistry, Properties, and Cosmetic Applications
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by Paul Thau
alicylic acid is a mild acid derived from the bark of the Willow Tree. The bark of this tree has been used as a traditional remedy for pain, inflammation, and fever since ancient times. Today, salicylic acid has become a popular treatment for acne and psoriasis, and is used for chemical peels to ameliorate the appearance of facial lines, wrinkles, blemishes, and sun damage. The medicinal use of salicylic acid was first documented by the ancient Greeks in about 500 B.C. John Buchner, at the University of Munich, was the first to distill Willow Bark to produce the substance he called “Salicin” in 1829. A year later, the French chemist, Henri Le Roux, perfected the extraction technique. In the 20th century, salicylic acid began to be used as a treatment for skin conditions such as acne and psoriasis. Salicylic acid has the capacity to soften skin and can be applied at various concentrations to remove dead skin cells and debris that can build up to cause acne.
Chemistry of Salicylic Acid
Salicylic acid is a carboxylic acid, which like most acids, donates hydrogen ions if a base is present to accept them. Salicylic acid reacts this way with all bases, both organic (for example, the amines) and inorganic. Their reactions with bases, called neutralizations, are accompanied by the evolution of substantial amounts of heat.
Solubilization of Salicylic Acid
Salicylic acid does not dissolve well in water and many other common solvents. Here is a list of its solubility: • Water: 0.2% at 20 °C; 1.7% at 75 °C (adding 5% sodium phosphate or another neutral salt, the solubility of salicylic acid in water increases to 1.8%). • Alcohol (ethanol): 14% in pure ethanol; 0.5% in 20% ethanol. • Glycerin: About 1.6%. • Octyldodecanol: About 15-20%.
Ensure that you measure the pH after adding salicylic acid to your formula as salicylic acid lowers the pH. An exceptionally low pH may destabilize an emulsion and cause separation. To increase the pH again you can add an alkali, such as triethanolamine. Typically, the pH should not be below 3.5 for most cosmetic products.1
Applications for Salicylic Acid
The rapid onset and limited absorption of salicylic acid has demonstrated it to be an effective agent against comedones and keratolysis.2 Varying concentrations of salicylic acid provide benefits for local and general topical use as well as for targeted and repetitive applications. Products containing salicylic acid optimize its effect to support penetration of other agents and devices. In (continued on Page 6)
CLEAN BEAUTY – MINIMALISM IN FORMULATION • MARCH 30th ...see pages 4-5 for more information.