Stability and Solvent System for Anhydrous Sunscreen Formulation
Question
I would like to develop an anhydrous sunscreen formulation using the following sun filters and concentrations:
DHHB(Diethylamino Hydroxybenzoyl Hexyl Benzoate): 3.5%EHT(Ethylhexyl Triazone): 2.0%BEMT(Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine): 2.2%HMS(Homosalate): 7.0%OS(Octisalate): 4.5%Isododecane(IDD) as the main volatile base
Please evaluate if this solvent system is sufficient to keep the formulation stable, particularly regarding the solubility of the powder filters. Do I need to modify the formulation or add specific polar solvents to prevent recrystallization and ensure long-term stability?
Answer
Based on your anhydrous sunscreen formulation, here is an analysis of the stability and suggestions for the solvent system:
1. Solubility Analysis
Your formulation contains a high load of powder sunscreens (7.7% total) relative to the liquid sunscreens (11.5% total):
- Powders: DHHB (3.5%), EHT (2.0%), BEMT (2.2%)
- Liquid Solvents (Filters): Homosalate (7.0%), Octisalate (4.5%)
- Main Base: Isododecane (IDD)
The Challenge: Isododecane is a non-polar hydrocarbon. While it provides a light, non-greasy feel, it is a very poor solvent for polar organic powder filters like DHHB, BEMT, and EHT. If the ratio of polar solvents is too low, these powders will recrystallize over time, especially when "diluted" by a large amount of Isododecane.
2. Stability Evaluation
- DHHB: Soluble up to ~36% in Homosalate and ~30% in Octisalate.
- BEMT: Soluble up to ~16% in Homosalate and ~20% in Octisalate.
- EHT: Highly polar and requires significant polar esters for long-term stability.
With only 11.5% liquid filters, your system is currently at its limit. Adding a significant amount of Isododecane will likely cause the BEMT or EHT to precipitate (form crystals) because the overall polarity of the oil phase will drop.
3. Suggested Modifications
To ensure the formulation remains stable and clear (anhydrous), I recommend adding a polar ester to the solvent system:
- Add LipidSoft™ C1215 (C12-15 Alkyl Benzoate): This is the most effective ester for stabilizing chemical sunscreens. It helps prevent recrystallization and improves the skin feel.
- Recommended Ratio: Add 10-15% LipidSoft™ C1215 to the formula. This will act as a "bridge" between the polar filters and the non-polar Isododecane.
- Adjust Isododecane: Use Isododecane as the remaining balance to achieve the desired lightness and volatility.
4. Mixing Instructions
- Combine the liquid filters (Homosalate, Octisalate) and the polar ester (LipidSoft™ C1215).
- Add the powder filters (DHHB, EHT, BEMT).
- Heat the mixture to 80°C and stir until the powders are completely dissolved and the liquid is clear. (Do not exceed 30 minutes of heating).
- Once clear, allow it to cool slightly before adding the Isododecane and any other heat-sensitive components.
Recommended Ingredients:
- DHHB (Diethylamino Hydroxybenzoyl Hexyl Benzoate)
- Ethylhexyl Triazone (EHT)
- BEMT (Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine)
- Homosalate
- Octisalate (Ethylhexyl Salicylate)
- Isododecane
- LipidSoft™ C1215 (C12-15 Alkyl Benzoate) - Highly recommended for stability.
Related Products Mentioned
Isododecane (Germany)
