Water-Free Vitamin C Serum Formulation

Based on your interest in creating a water-free Vitamin C serum similar to the Kiehl's formula, here is a refined approach incorporating the feedback and addressing your questions.

Your proposed base of Propylene Glycol, Cyclopentasiloxane, and Glycerin is suitable for a water-free system.

Key Ingredients and Considerations:

• L-Ascorbic Acid: Using the Ultra-Fine grade of L-Ascorbic Acid is crucial in this water-free formula. While it has limited solubility in Propylene Glycol and Glycerin, the ultrafine powder disperses much better and can be suspended in the base, allowing it to deliver its benefits upon application to the skin (where it encounters skin moisture).
• Thickening and Suspension: The original Kiehl's formula uses Dimethicone Crosspolymer (Silicone Gel) for thickening and, importantly, to suspend the L-Ascorbic Acid powder evenly throughout the serum, preventing sedimentation. This ingredient is essential for the texture and stability of the formula. Products like Silicone Gel (Thick & Smooth, Cyclopentasiloxane Base) or Silicone Gel 9040 (e.q. DC 9040, Cyclopentasiloxane Base) are suitable options. While Xanthan Gum is a thickener, it is water-soluble and less effective at suspending powders in a silicone/glycol base compared to silicone gels designed for this purpose.
• Anti-Irritant: To help mitigate potential irritation from the low pH of the formula (due to L-Ascorbic Acid), adding an anti-irritant is recommended. Bisabolol is a good choice as it dissolves well in Propylene Glycol. Use around 0.5-1%. Allantoin, while a good anti-irritant, has very low solubility in Propylene Glycol, making it less suitable for this specific water-free base.
• Removed Ingredients: As discussed, Green Tea Extract and Licorice Extract are water-soluble and incompatible with a water-free formula. Salicylic Acid, if intended solely as a preservative, is not necessary in a truly water-free environment as bacteria require water to grow. Its acidity also contributes to the low pH.

Suggested Formula Structure (Conceptual):

• Propylene Glycol
• Cyclopentasiloxane
• Glycerin
• L-Ascorbic Acid (Ultra-Fine)
• Dimethicone Crosspolymer (Silicone Gel)

• Bisabolol

• (Adjust percentages based on desired texture and L-Ascorbic Acid concentration, ensuring L-Ascorbic Acid is within recommended usage rates and considering its solubility limits for dispersion).

General Mixing Instructions:

1. Combine Propylene Glycol, Cyclopentasiloxane, and Glycerin.
2. Slowly add the L-Ascorbic Acid Ultra-Fine powder to the liquid base while mixing. High-speed mixing may be needed to ensure even dispersion. Ensure the temperature is below 30°C when adding Vitamin C.
3. Add the Silicone Gel (Dimethicone Crosspolymer) and mix thoroughly until the desired gel texture is achieved and the powder is suspended.
4. Add Bisabolol and mix until incorporated.

Additional Recommendation:

If you experience significant irritation, consider applying a separate soothing gel containing ingredients like Allantoin or Safe-B3™ (Vitamin B3, Niacinamide) before applying the Vitamin C Silicone Serum. These ingredients are water-soluble and best used in a water-based product.

We hope this refined guidance helps you in formulating your Vitamin C serum!