Sunscreen Formulation Stability and Separation Issue

Question

My sunscreen formulation containing various UV filters including Zinc Oxide, oils, silicones, and multiple emulsifiers, separates into an oil layer after sitting for 3-4 days despite looking good initially. Based on the provided ingredient list and mixing steps, what should I add or change in terms of emulsifiers or stabilizers to prevent this phase separation and improve stability?

MySkinRecipes Expert · Accepted Answer

Sunscreen Formulation Stability Issue

Your sunscreen formulation is separating into an oil layer after a few days, which indicates emulsion instability. This is a common challenge with complex formulas containing a high oil phase, solid UV filters like Zinc Oxide, and a mix of emulsifier types.

Your formula includes a combination of silicone-based water-in-silicone (W/Si) emulsifiers (Lauryl PEG-9 Polydimethylsiloxyethyl Dimethicone, PEG-10 Dimethicone, Warp Cream Maker XL) and an oil-in-water (O/W) emulsifier (Potassium Cetyl Phosphate), along with a high percentage of oil-soluble ingredients and Zinc Oxide. The separation suggests that the current emulsification and stabilization system is not robust enough to maintain the emulsion over time.

To significantly improve the stability and prevent oil separation, consider adding Magnesium Sulfate.

Magnesium Sulfate: This electrolyte is highly effective in stabilizing water-in-silicone and water-in-oil emulsions. It helps to strengthen the emulsion structure and prevent the oil and water phases from separating. Adding 1-2% Magnesium Sulfate to your water phase is recommended for this type of formulation.

Additionally, ensure your mixing process provides sufficient shear:

High-Shear Mixing: Proper high-shear mixing is crucial for creating stable emulsions, especially when incorporating solid particles like Zinc Oxide. Ensure you are using a high-speed blender during the emulsification step and continue blending until the mixture is completely uniform. Make sure the Zinc Oxide is well-dispersed in the oil/silicone phase before combining with the water phase.

By adding Magnesium Sulfate and optimizing your mixing technique, you should achieve a more stable sunscreen formulation.

Key ingredients in your formula include Zinc Oxide 200nm EasyDisperse™, DHHB, Octyl methoxycinnamate, Octocrylene, BEMT, Butyl Octyl Salicylate, LipidSoft SPF, LipidSoft C1215, Cyclopentasiloxane, Lauryl PEG-9 Polydimethylsiloxyethyl Dimethicone, SiliSolve, Warp Cream Maker XL, Potassium Cetyl Phosphate, Aristoflex AVC, Niacinamide, and Phenoxyethanol.