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Can you combine ascorbic acid and retinol in an anhydrous glycol + oil emulsion?
Stabilities of ascorbic acid and retinol are pH-dependent in aqueous emulsions (O/W and W/O emulsions). Ascorbic acid is stable in acidic conditions (below its pKa), while retinol is stable in neutral pH conditions. Therefore, ascorbic acid and retinol are often not formulated together in an aqueous system.
When you dissolve ascorbic acid in glycols, ascorbic acid remains largely non-ionized and is more stable than in aqueous systems. When you incubate an ascorbic acid in glycol solution on a pH strip in a sealed chamber (to prevent moisture absorption from the atmosphere), however, it starts showing a pH of around 2.5 after about 30 minutes (initially the pH strip doesn’t change the color). This hints that there is still a little bit of ionization of ascorbic acid in the glycol solvent.
My question is, would this lower level of ionization and acidity of ascorbic acid in the glycol solvent be sufficient to decompose retinol when you combine ascorbic acid + retinol in an anhydrous glycol + oil emulsion? Any chemists here who can comment on this?
Many thanks in advance!
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3 Replies
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Good question, but I think you’re overthinking it.
In an anhydrous glycol and oil system, pH doesn’t really apply. pH requires water. If you’re seeing a pH reading after 30 minutes on a strip, it’s likely due to trace moisture or slow absorption from the air, not because the system is truly acidic in a way that affects stability.
Ascorbic acid and retinol can both be unstable, but in this type of system, pH isn’t relevant. If there’s degradation, it’s more likely due to oxidation, not acidity.
So yes, they might interact over time, but not for the reasons you’re assuming. If stability is a concern, use stabilized forms, encapsulation, or just don’t combine them.
Hope that clears it up.
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Thanks Perry! I know the pH calculations only apply to aqueous systems but organic solvents can still be acidified or alkalinized. For example we extract highly negatively charged lipids such as phosphoinositides from dry cellular materials by acidifying methanol:chloroform mixtures with a very small amount of concentrated HCl so that the phosphate groups on the lipids become protonated and more lipophilic. Without acidification, the recovery of highly acidic lipids is very low. Though this process wouldn’t be effective using a weak acid because the ionization activity is much lower in organic solvents.
You may be right that the pH strips absorb moisture over time. I used sealed chambers to prevent moisture absorption but there is still a little bit of air left in the chambers, so that might be the reason.
I purchased pure retinoic acid (tretinoin) powder and dissolved it in my glycol + oil anhydrous emulsion containing ascorbic acid along with a bunch of anti-oxidants (BHT, BHA, tocopherol, sodium metabisulfite). I wanted to use retinoic acid because it gives a bright yellow color to the emulsion when fresh, and it visibly turns orange when oxidized/decomposed. I made two preparations in dropper serum bottles, one was never opened after preparation and one was opened regularly. It’s been two months since then. The one that was never opened still has that fresh bright yellow color, while the regularly opened bottle looks very orange. Indeed, it was oxidation that played the major role in the stability of retinoic acid in my system, and not the very low ionization/acidity coming from ascorbic acid.
Thanks for your input!
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Interesting thought.
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Other than stability, irritation can also be a concern. If you’re in the anhydrous direction, I assume you’re stabilizing >10% of Ascorbic Acid and 0.1%-0.3% of Retinol.
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I haven’t seen a commercial product with both in a formulation yet - with high, functional %, not just marketing dose.
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My recent surprise of a pure C serum comes from Poems from the Lab - an experienced chemist’s brand. This Vitamin C stays almost clear even after 1 year, with water in it! Besides the excellent formulation, the packaging plays a big role. I encourage you to analyze this: https://poemsfromthelab.com/products/future-focus-vitamin-c-serum
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