Salicylic acid is a weak organic acid that can be obtained in nature by isolating it from the bark of the white willow tree, Salix alba. Used since ancient times to treat aches and pains, salicylic acid is the chemical starting point for the modern pharmaceutical, aspirin. As a molecule, salicylic acid has a number of characteristics that contribute to its chemical and physical properties.
Physical Properties
According to the "CRC Manual of Chemistry and Physics," salicylic acid is a weak organic acid with a pKa, or measure of acid strength, of about 2.97. This makes it slightly more acidic than straight vinegar, and only slightly less acidic than phosphoric acid. It's a relatively light organic molecule with a mass of 138 g/mol, and is composed of seven atoms of carbon, six atoms of hydrogen, and three atoms of oxygen. The melting point of pure salicylic acid is 318 degrees Fahrenheit, and at room temperature, it's a white powder.
Chemical Reactivity--Internal
Taken internally, salicylic acid is an analgesic, antiinflammatory and antipyretic. This means it relieves pain and inflammation, as well as decreasing fever, notes Dr. Lauralee Sherwood in her book, "Human Physiology." While it's possible to use pure salicylic acid to produce these effects, a chemical group on the molecule called a phenol causes the molecule to be very irritating to the lining of the stomach. As such, modern pharmaceuticals that contain salicylic for internal use tend to contain a modified version of the chemical called acetylsalicylic acid.
Chemical Reactivity--External
Applied to the skin, salicylic acid acts as a keratolytic, meaning that it causes the outer, keratinized layers of the skin to slough and peel. This gives it utility in cosmetics, and it's a frequent ingredient in facial scrubs and peels meant to improve the quality of skin. The acidic nature of salicylic acid makes it toxic to bacteria, so it is used in acne creams and face washes. Explains Dr. Sherwood, the outer layers of skin lack nerve cells. As such, salicylic acid applied to the skin causes painless peeling of outer layers. This accelerates normal skin sloughing to the point that salicylic acid is sometimes used to remove warts and other skin lesions.
References
- "CRC Handbook of Chemistry and Physics"; David Lide; 1974
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
