Stabilizing Encapsulated Salicylic Acid (BHA) in Water-Based Serum to Prevent White Residue
คำถาม
I am developing a water-based serum containing 5% ActiveRelease Salicylic Acid (BHA encapsulated). Initially, when I dissolved it directly in water, it left a white residue/film on the skin. I have started using PEG-40 as a solubilizer, which seems to have resolved the white film issue, but I want to ensure the long-term stability of the formula.
Current Formulation (100g batch):
Part A:
- DI-Water: 76.20%
- Allantoin: 0.10%
- Mono Propylene Glycol: 1.00%
ActiveRelease Salicylic Acid(BHA encapsulated): 5.00%
Part A1:
- HEC (Hydroxyethyl Cellulose): 1.00%
- DI-Water: 10.00%
Part B:
- Natural PHA Witch Hazel Distillate: 0.50%
- Glycerin: 0.50%
- Witch Hazel Distillate: 0.50%
- Germall II: 0.50%
- Aloe Vera Gel Spray Dried-LC: 0.10%
Part C:
- Tween 20: 3.00%
- Tea Tree Oil: 0.10%
- Phenoxyethanol: 0.50%
Part D:
- Tomato Extract: 2.00%
Questions:
- Is this formulation structure appropriate for a stable water-based serum?
- Is there a risk of instability or the white residue returning over time?
- What adjustments should I make regarding pH, emulsifiers, or solvents to improve stability and skin feel?
คำตอบ
The formula structure is generally suitable for a water-based serum/gel, but the initial issue of the white film and potential long-term instability are critical points that require optimization, primarily related to solvent power and pH control.
1. Assessment of the White Film Issue and Stability Risk
The white film from ActiveRelease™ Salicylic Acid (Powder, Water-Soluble) (ID 10947) is likely due to insufficient wetting and dispersion of the encapsulated powder (Salicylic Acid 40% and Cyclodextrin 60%) before the Cyclodextrin fully solubilizes it.
- The PEG-40 Fix: Adding PEG-40 is an excellent solution. PEG (Polyethylene Glycol) compounds act as effective wetting agents and solvents, helping the powder disperse and dissolve completely, thus preventing the initial precipitation.
- Risk of Return: The white film (crystallization) could return if the final formula's pH is too low (below 3.5) or if the solvent system is not robust enough to maintain the stability of the Salicylic Acid component over time and temperature fluctuations.
2. Recommendations for Improvement
To ensure long-term stability and prevent the white film from returning, we recommend the following adjustments:
A. Solvent and Solubilizer Optimization
- Formalize the Solvent System: Integrate the solvent that solved the problem (PEG-40) formally into Part A. If you used PEG-400 or PEG-40 Hydrogenated Castor Oil, use 2.00% - 5.00% in Part A, replacing an equivalent amount of DI-Water.
- Increase Glycol: Increase Mono Propylene Glycol from 1.00% to 3.00% - 5.00%. Glycols are excellent solvents for BHA and will significantly boost the stability of the encapsulated complex in the water phase.
- Reduce Tween 20: The 3.00% Tween 20 is very high for only 0.10% Tea Tree Oil (a 30:1 ratio). This high concentration can lead to stickiness or foaming. Reduce Tween 20 to 1.00% - 1.50% (a 10:1 to 15:1 ratio) to maintain solubilization while improving the skin feel.
B. Critical pH Control
- Measure and Adjust: The most critical step for BHA stability and efficacy is the final pH. You must measure the pH of the final formula and adjust it to the optimal range for Salicylic Acid: pH 3.5 - 4.5.
- Adjustment: Use a 10% solution of Sodium Hydroxide (NaOH) or Triethanolamine (TEA) to raise the pH if it is too low.
C. Revised Mixing Procedure
- Part A (Solvent Phase): Combine DI-Water (adjusted), Mono Propylene Glycol (3-5%), and PEG-40 (2-5%). Heat to 60°C. Dissolve Allantoin (0.10%) and ActiveRelease™ Salicylic Acid (5.00%). Mix until completely clear. Cool to room temperature.
- Part A1 (Thickener): Disperse the HEC (1.00%) in its dedicated water portion (10.00%) and allow it to hydrate fully until a clear, lump-free gel forms.
- Part C (Solubilized Phase): Combine the reduced Tween 20 (1.00-1.50%), Tea Tree Oil (0.10%), and BPE-800 (0.50%). Mix until completely clear.
- Combine: Slowly add Part A (Solvent Phase) to Part A1 (Thickener Gel) while mixing. Add Part B and Part D ingredients (Witch Hazel, Aloe, Tomato Extract).
- Final Step: Add Part C (Solubilized Phase). Check and adjust the pH to 3.5 - 4.5. Q.S. with water if necessary.
