Why Normal BHA Doesn't Dissolve Easily in Water

Thank you for your question and for sharing your formulation attempts. I understand your frustration with the normal Salicylic Acid (BHA) not dissolving properly in your water-based formula.

The core issue you're facing is the limited solubility of standard Salicylic Acid in water. While it dissolves well in certain organic solvents like Isopentyldiol and Propylene Glycol, adding a large amount of water significantly reduces the ability of the overall mixture to keep the BHA dissolved, causing it to precipitate out.

To successfully incorporate "normal" BHA into a water-based formula like a toner, you need to use sufficient amounts of co-solvents (solvents that help BHA dissolve alongside water) and/or solubilizers (ingredients that help disperse or dissolve BHA in the water phase).

Your initial formula (2% BHA, 7% Isopentyldiol, 91% water) did not contain enough co-solvent (Isopentyldiol) to keep 2% BHA dissolved once the large water phase was added. Adding 5% Propylene Glycol helped, but still wasn't enough.

A common approach for formulating "normal" BHA into a toner involves:

1. Dissolving the BHA first in a suitable co-solvent (like Propylene Glycol or Isopentyldiol) or a mixture of co-solvent and solubilizer (like Polysorbate 20). Gentle heating can sometimes assist this step.
2. Using a higher concentration of these co-solvents/solubilizers than in your initial attempt to ensure the BHA remains dissolved even after the water is added.
3. Slowly adding the water phase to the dissolved BHA mixture while mixing thoroughly.

The formula shared by user hydronium_ion3 in the thread (using 40-45% Propylene Glycol, 1.5-2% BHA, 4.5% Polysorbate 20, and water to 100%) is an example of how "normal" BHA can be formulated into a toner by using a high percentage of co-solvent (Propylene Glycol) and a solubilizer (Polysorbate 20) to keep the BHA dissolved in the water phase.

While it is technically possible to use "normal" BHA in water-based formulas, it often requires significant amounts of co-solvents or solubilizers, which can sometimes affect the texture or feel of the product (e.g., feeling sticky due to high glycol content). This is why alternative forms of BHA, such as water-soluble or encapsulated versions, are often recommended for easier formulation in water-based systems, as they bypass these solubility challenges.

If you are determined to use the "normal" BHA, you could try adjusting your formula to include a higher percentage of Isopentyldiol and/or Propylene Glycol, ensuring the BHA is fully dissolved in these solvents before carefully incorporating the water phase.