Molecule of the Day

A number of carboxylic acids are used as cosmetics. Familiar to many readers will be glycolic acid. It’s used in over-the-counter “cosmeceuticals” to improve skin tone. Also used in professional “chemical peels” is trichloroacetic acid. These acids are all closely related to the familiar acetic acid, but they have “electronegative” substituents.

In the aqueous systems we’re talking about, the strength of an acid is related to its tendency to liberate protons. An acid’s tendency to dissociate into protons is related to the thermodynamic stability of its “conjugate base” - the acid, minus a proton. These electronegative substituents withdraw some electron density from the negative charge on the conjugate base, giving a more potent acid.

This is one part of how these compounds seem to work; these mild acids help slough off dead skin cells, revealing the smoother skin underneath. Trichloroacetic acid is much stronger than acetic or even glycolic acid, and is, thus, only used by professionals.

Salicylic acid is another acid used in skin care; its hydroxyl group likely helps stabilize the conjugate base by virtue of its electronegativity alone, but it also can participate in a “hydrogen bond” (to a first approximation, the sharing of a proton between two electronegative atoms - usually oxygen, nitrogen, or fluorine). This gives a pretty good energetic “bonus,” stabilizing the conjugate base, salicylicate. A picture will hopefully help - the putative hydrogen bond is denoted by a dotted line:

Skin care is one of those fields that is full of crackpot ingredients (and incredibly overpriced ones), but these are among the few ingredients that are actually up to something.