Salicylic Acid Solubility and Precipitation in Toner
Question
I am trying to formulate a toner with 2% Salicylic Acid. I dissolved the Salicylic Acid in Propanediol first, then added water. However, the solution becomes cloudy or precipitates after cooling. I thought Salicylic Acid was highly soluble in Propanediol (up to 15%). Why is it precipitating in my toner formula? How can I make a clear, stable 2% Salicylic Acid toner?
Answer
Based on the information from the staff, the high solubility of Salicylic Acid in Propanediol (up to 15%) is observed when Propanediol is the primary solvent, not in a water-based formula like your toner.
When you add water to the mixture of Salicylic Acid and Propanediol, the solubility of Salicylic Acid decreases significantly, causing it to precipitate or become cloudy, as you observed. Heating temporarily helps dissolve it, but as the solution cools, the Salicylic Acid recrystallizes and settles out because it exceeds its solubility limit in the water/Propanediol mixture.
Therefore, even with 15% Propanediol, ordinary Salicylic Acid at 2% concentration will not remain fully dissolved and stable in your water-based toner formula over time.
As the staff recommended, to achieve a stable and clear 2% Salicylic Acid solution in a water-based formula, you would need to use the water-soluble "Complex Salicylic Acid" at a concentration of 4-5% (which provides 2% active Salicylic Acid). With the ordinary Salicylic Acid you currently have, it is not possible to create a stable 2% solution in your proposed toner formula.
Related Products Mentioned
Salicylic Acid (BHA, Pharma Grade)
Natural Betaine (Crystal)
