Hydroxy Acids (AHAs & BHAs)

Chemical exfoliants that weaken corneodesmosomes to facilitate desquamation.

Alpha Hydroxy Acids (AHAs)

Water-soluble acids that act on the surface of the stratum corneum. They increase epidermal thickness and upregulate hyaluronic acid production.

Glycolic Acid: Smallest molecular weight. Deepest penetration, highest efficacy, but highest potential for irritation.
Lactic Acid: Larger molecule. Slower penetration. Part of the skin's Natural Moisturizing Factor (NMF), making it inherently hydrating.
Mandelic Acid: Largest molecule. Slowest penetration. Excellent for sensitive skin or deeper skin tones prone to PIH.

Beta Hydroxy Acids (BHAs)

Salicylic Acid is lipid-soluble, allowing it to penetrate through sebum into the pilosebaceous unit. It is keratolytic, comedolytic, and inherently anti-inflammatory (structurally related to aspirin).

The Importance of pKa and pH

The efficacy of an acid depends entirely on the formulation's pH relative to the acid's pKa (the pH at which 50% of the acid is free/active and 50% is ionized/salt).

For an acid to penetrate the lipid barrier, it must be in its free acid state (protonated). If the formulation pH is significantly higher than the acid's pKa, the acid exists mostly as a salt, rendering it a moisturizer rather than an exfoliant.

Example: Glycolic acid has a pKa of 3.83. A formulation at pH 3.83 contains 50% free acid. At pH 4.83, only ~10% is free acid.