Between 40 and 50 million Americans battle acne vulgaris. As long ago as 2001, expenditure on this condition exceeded $1 billion. For one popular over-the-counter (OTC) acne treatment alone, revenues for 2010 were projected to exceed $800 million.¹

Face wash products are a considerable part of this market. One of the most widely used active ingredients in many OTC face washes is salicylic acid (SA), commonly at a 2% level. In nature, SA is found in the bark of the willow tree Salix alba. Salicylic acid is a β-hydroxy acid, making SA formulas milder than those based on benzoyl peroxide or α-hydroxy acids. It acts as an anti-comedogenic and a bactericide.

SA has a pK_a of 2.9 and causes desquamation by breaking the bonds between corneodesmosomes and corneocytes.¹ Typically, the pH of SA-based cleansers ranges from 3.7 to 4.8 so that SA will be ionized between 85% and 99%. As such, SA has both polar and non-polar structural moieties, which ultimately determine the solubility and formulation characteristics; i.e., the polar –COO^– and –OH groups, and the nonpolar phenyl group.

Optimizing the delivery of SA from face washes is important to the efficacy of these products, so noninvasive in vivo methods are desired to screen candidate formulas. And due to its phenyl group, SA fluoresces at an excitation and emission maxima near 302 nm and 350 nm, respectively (see Figure 1). The sensitivity of fluorescence spectroscopy can therefore be used to detect SA deposited onto skin from wash-off products; this is the subject of the present work.