When formulating cosmetics, the optimal pH level of each ingredient is crucial for its stability, effectiveness, and compatibility with other ingredients. Below is an explanation of the optimal pH levels for each of the ingredients you mentioned, along with why their pH levels can affect their interaction when combined—highlighting cosmetic ingredients you shouldn’t mix.
1. Vitamin C (Ascorbic Acid) and Retinol (Vitamin A)
- Vitamin C (Ascorbic Acid):
- Optimal pH: <3.5 (acidic)
- Why it needs low pH: Vitamin C is most stable and effective in an acidic environment. At a pH lower than 3.5, ascorbic acid remains in its active form, which allows it to penetrate the skin efficiently and perform its antioxidant functions.
- Retinol (Vitamin A):
- Optimal pH: 5.0-6.0 (slightly acidic)
- Why they’re incompatible: When combined, the low pH of Vitamin C destabilizes the retinol, reducing its efficacy. Also, both are potent actives, and using them together may irritate sensitive skin, increasing the risk of dryness, peeling, or redness.
2. AHA (Alpha Hydroxy Acids) and Retinol
- AHA (Alpha Hydroxy Acids):
- Optimal pH: 3.0-4.0 (acidic)
- Why it needs low pH: AHAs like glycolic acid and lactic acid need an acidic environment to break down the bonds between dead skin cells, allowing for exfoliation. A lower pH enhances their exfoliating ability and makes them more effective at brightening the skin and reducing wrinkles.
- Retinol (Vitamin A):
- Optimal pH: 5.0-6.0 (slightly acidic)
- Why it needs higher pH: As mentioned above, retinol thrives in a slightly higher pH range, which helps maintain its stability and effectiveness.
- Why they’re incompatible: The acidic nature of AHAs can irritate the skin and may cause over-exfoliation, especially when used with retinol. Additionally, the low pH of AHAs can reduce the effectiveness of retinol by destabilizing it. The combination can also compromise the skin barrier, making it more sensitive and prone to irritation.
3. Vitamin C and Niacinamide
- Vitamin C (Ascorbic Acid):
- Optimal pH: <3.5 (acidic)
- Why it needs low pH: Vitamin C requires a low pH to remain stable and active, allowing it to act as an antioxidant and brighten the skin.
- Niacinamide (Vitamin B3):
- Optimal pH: ~6 (slightly acidic)
- Why it needs higher pH: Niacinamide works best in a slightly acidic environment. It functions at a pH that supports its ability to brighten the skin and reduce the appearance of pores, leading to a smoother, more even complexion.
- Why they might be incompatible: Vitamin C needs an acidic pH to work properly, while niacinamide works best in a slightly acidic environment. If used together, this combination can cause irritation for sensitive skin types. This irritation can result from hydrolysis when exposed to the low pH, where niacinamide breaks down into niacin, which can cause redness or flushing.
4. AHA and Peptides
- AHA (Alpha Hydroxy Acids):
- Optimal pH: 3.0-4.0 (acidic)
- Why it needs low pH: AHAs require an acidic pH to function effectively as chemical exfoliants. They need to penetrate the skin and break down the bonds between dead skin cells to promote cell turnover.
- Peptides:
- Optimal pH: 5.0-7.0 (slightly acidic to neutral)
- Why it needs neutral pH: Peptides, which are short chains of amino acids, are delicate and can be broken down in overly acidic environments. They are more stable and effective in a slightly acidic or neutral pH range.
- Why they’re incompatible: The acidic pH of AHAs can degrade the structure of peptides, reducing their effectiveness in stimulating collagen production and promoting skin repair. Additionally, peptides work best at a slightly acidic or neutral pH, and using them in a highly acidic formulation with AHA could lead to peptide degradation or hinder their absorption by the skin.
Chemist’s Recommendations
I don’t buy cosmetic products that contain ingredients that shouldn’t be mixed together. Instead, I purchase separate products for ingredients that need to be used alone. For example, I recommend using Vitamin C in the morning and retinol at night to avoid destabilizing either ingredient and to minimize irritation. Another option is to use them at different times of the day or alternate their use every other night. By respecting the optimal pH levels for each ingredient, you can help ensure that your skincare formulations are effective and gentle on the skin.
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