Overview
This lecture explains why pH is important for the effectiveness of alpha hydroxy acids (AHAs) and vitamin C in skincare, focusing on chemical properties that influence skin penetration and product formulation.
Acid Basics in Skincare
- Acids in skincare (e.g., AHAs, BHAs, vitamin C) can donate hydrogen ions (H+).
- Common examples: glycolic, lactic, malic acids (AHAs); salicylic acid (BHA); L-ascorbic acid (vitamin C).
- Acids exist in skin care either as uncharged free acids or dissociated charged forms.
Skin Structure and Acid Penetration
- The stratum corneum (top skin layer) is made of cells in oily (non-polar) lipids.
- Non-polar (uncharged) molecules pass through skin more easily than charged (polar) ones.
- Only uncharged, free acid forms can efficiently penetrate through the skin barrier.
Factors Influencing Acid Form
- The bond strength holding H+ to the acid is measured by pKa; lower pKa means easier dissociation.
- Skincare acids are weak acids, so only a portion will dissociate in water.
- Product pH determines the concentration of hydrogen ions; lower pH increases uncharged acid fraction.
Practical Implications for Skincare Products
- Lower pH means more free acid available for skin penetration.
- Formulation example: 10% glycolic acid at pH 3 gives 8.7% free acid; at pH 4.5, 50% glycolic acid is needed for the same effect.
- Very low pH can irritate skin; recommended pH for leave-on AHAs is below 4, for ascorbic acid around 3.5.
Special Cases for Other Acids
- Hyaluronic acid is too large to penetrate skin and acts as a surface moisturizer.
- Retinoic acid is non-polar enough that its charge state is less important for penetration.
- Salicylic acid (BHA) is often recommended at pH below 3.5, but may not rely on pH for efficacy due to its non-polar nature.
Key Terms & Definitions
- Acid — Molecule capable of donating a hydrogen ion (H+).
- pKa — Measure of acid strength; lower pKa means stronger acid (more likely to dissociate).
- pH — Measure of the hydrogen ion concentration; lower pH equals more acidic.
- Free Acid — The uncharged, undissociated form of an acid that can penetrate skin.
- Stratum Corneum — The outermost layer of skin, acting as a barrier.
Action Items / Next Steps
- Review pH and pKa concepts.
- Check recommended pH and concentrations for products with acids.
- Consider skin sensitivity before selecting low pH acid products.