Formulation Stacking Rules
A complete guide to chemical compatibility, pH dependence, and pKa values for layering actives.
The pH Ladder Protocol
Wait times are a myth. The sequence of application matters strictly due to pH dependency. Applying a pH 6.0 niacinamide serum directly over a pH 3.0 L-Ascorbic Acid serum will shift the LAA out of its functional pKa range, rendering it biologically inert before it can penetrate the stratum corneum.
Rule 1: Ascending pH
Always apply products from lowest pH to highest pH, not thinnest to thickest.
- Step 1: L-Ascorbic Acid (pH 2.5 - 3.5)
- Step 2: AHAs / BHAs (pH 3.0 - 4.0)
- Step 3: Retinoids (pH 5.5 - 6.0)
- Step 4: Niacinamide / Peptides (pH 5.0 - 7.0)
"The pKa of an acid dictates its bioavailability. For Glycolic Acid (pKa 3.83), a formulation at pH 3.83 yields 50% free acid. At pH 4.83, only 10% remains active."
Chemical Incompatibilities
| Ingredient A | Ingredient B | Result |
|---|---|---|
| L-Ascorbic Acid | Copper Peptides | Oxidation (Cancel out) |
| Retinol | Benzoyl Peroxide | Degradation of Retinol |
| Niacinamide | LAA (Low pH) | Niacin Flush (Flushing) |