Clinical Overview
Glycolic acid, derived from sugar cane, has the smallest molecular weight of all the Alpha Hydroxy Acids (AHAs). This small size allows for rapid and deep penetration into the stratum corneum, making it the most potent—and potentially most irritating—chemical exfoliant.
Mechanism of Desquamation
Glycolic acid exfoliates the skin by interfering with ionic bonds between corneocytes. Specifically, it causes a reduction in calcium ion concentration in the epidermis, which leads to the breakdown of desmosomes (the cellular "glue"). Furthermore, at higher concentrations, it has been shown to stimulate dermal fibroblasts to increase collagen synthesis.
The Free Acid Value Equation
Like Salicylic Acid, Glycolic Acid's efficacy is dictated by its pKa (3.83). To act as an exfoliant, the acid must be in its un-ionized (free acid) state to cross the lipid-rich stratum corneum.
- At pH 3.83: 50% is Free Acid
- At pH 4.83: ~9% is Free Acid
- At pH 5.5 (Skin pH): Acts only as a humectant