Crystal Formation in Modified Vitamin C Serum

Question

I previously posted a question about a sticky Vitamin C serum formula (referencing post7729.html#p7729). To reduce the oiliness/stickiness (and potentially address crystallization, although I wasn't sure if it was related), I tried modifying the formula by reducing some ingredients.

After mixing the modified formula and refrigerating it for several weeks, I observed crystal-like particles in the bottle, which were not present immediately after mixing.

Here is the modified 100g formula:

Water (with Disodium EDTA 0.2%): 63g (Increased from 43.7g)
Triethanolamine: 2.5g
Ferulic Acid: 0.5g
Panthenol: Not included (Was 0.5g in original)
Ethoxydiglycol: 10g (Reduced from 20g)
Propylene glycol: 5g (Reduced from 10g)
Glycerin: Not included (Was 3g in original)
Laureth-23: 1.7g (Reduced from 3g)
d-Alpha Tocopherol: 1g
L-Ascorbic Acid: 15g
Hyaluronic Acid: 0.3g
Hyaluronic Acid Nano: 0.5g

I am unsure what caused this. Could it be a reaction between the alkaline Triethanolamine and the acidic L-Ascorbic Acid? I've heard that large amounts of acid and base can react to form salt crystals.

Or is it due to the ingredients I reduced? Could the significantly reduced Propylene glycol (cut by half), a solvent, be the reason the formula is unstable? Or is it the Laureth-23, reduced by almost half, causing poor oil-water emulsification? (Though I don't think Laureth-23 is related to crystallization).

Is it possible that the crystals are caused by poor water quality? I used water from a cement holding tank (which might contain sediment/impurities) that could react chemically with the serum ingredients. (However, the water was boiled and 0.2% Disodium EDTA was added).

What is the likely cause of the crystal formation in this modified Vitamin C serum formula?

MySkinRecipes Expert · Accepted Answer

Crystal Formation in Vitamin C Serum

Based on your description and the changes you made to the formula, the appearance of crystals in your Vitamin C serum after refrigeration is likely due to the reduced amount of solvents, specifically Propylene Glycol and Ethoxydiglycol.

Here's a breakdown of the potential causes:

Reduced Solvent Capacity: L-Ascorbic Acid (Vitamin C) has limited solubility in water. Solvents like Propylene Glycol and Ethoxydiglycol are crucial for keeping L-Ascorbic Acid dissolved in the formula, especially at higher concentrations (your formula uses 15%). By significantly reducing the amounts of Propylene Glycol (from 10g to 5g) and Ethoxydiglycol (from 20g to 10g), you decreased the formula's ability to keep the L-Ascorbic Acid dissolved. When stored in the refrigerator, the lower temperature further reduces the solubility of L-Ascorbic Acid, causing it to crystallize out of the solution.
Interaction between Triethanolamine and L-Ascorbic Acid: While it's true that mixing an acid (L-Ascorbic Acid) and a base (Triethanolamine) can form salts, and some salts can precipitate, the primary issue with high-concentration L-Ascorbic Acid serums is often the solubility of the L-Ascorbic Acid itself. However, if the pH of your serum is not within the optimal range for L-Ascorbic Acid stability (pH 2.0-4.0), it could contribute to degradation and potential precipitation issues.
Water Quality: Using water that contains mineral impurities, even after boiling and adding Disodium EDTA, could potentially contribute to instability. Disodium EDTA helps chelate metal ions that can degrade Vitamin C, but it might not be sufficient if the water has a very high mineral content. However, this is less likely to be the primary cause of the observed crystals compared to the significant reduction in solvents.

Conclusion: The most probable reason for the crystal formation is the insufficient amount of Propylene Glycol and Ethoxydiglycol to keep the 15% L-Ascorbic Acid dissolved, especially under refrigeration conditions.

Recommendations:

To prevent crystallization, consider increasing the concentration of Propylene Glycol and/or Ethoxydiglycol in your formula. You might need to revert to the original amounts or find an optimal balance that provides sufficient solubility without making the serum too sticky.
Ensure you are using distilled or deionized water to minimize mineral impurities.
Carefully measure and adjust the pH of your final serum to be in the range of 2.0-4.0 for optimal L-Ascorbic Acid stability.

Cutting down on the solvents like Propylene Glycol and Ethoxydiglycol to reduce stickiness had the unintended consequence of reducing the solubility of the L-Ascorbic Acid, leading to crystallization.