White Cast Issue in Oil Control Gel with Silicone and Silica

Based on your description, the white cast you are experiencing is likely due to the Silica powder not being fully or stably dispersed in the Cyclomethicone / Cyclopentasiloxane base.

Here's a breakdown of why this happens and how to potentially fix it:

Why the white cast occurs:

• Silica is a powder: Silica is a solid particle. When mixed into a liquid base, especially one as light and volatile as Cyclomethicone / Cyclopentasiloxane, it needs to be properly suspended.
• Dispersion issues: If the Silica is not thoroughly dispersed, or if it settles over time, the particles can aggregate on the skin surface as the volatile silicone base evaporates, leaving a visible white residue.
• Base viscosity: Lighter, less viscous bases like Cyclomethicone / Cyclopentasiloxane are not as effective at keeping solid particles suspended compared to thicker bases. This explains why you don't see the issue with a thicker silicone base – the higher viscosity helps keep the Silica particles from settling or clumping.
• Concentration: Using too high a concentration of Silica can also make it difficult to disperse and increase the likelihood of a white cast.

How to avoid the white cast:

• Improve Dispersion: Ensure you are mixing the Silica into the silicone base very thoroughly. You might need to use a high-shear mixer if available, or mix vigorously for an extended period. The goal is to break up any aggregates of Silica particles.
• Check Silica Particle Size: Finer, micronized Silica powders generally disperse better and are less likely to cause a white cast than coarser grades. If possible, check the specifications of the Silica you are using.
• Reduce Silica Concentration: Try reducing the percentage of Silica in your formulation. You might find that a lower amount still provides sufficient oil absorption without the white cast issue.
• Consider a Dispersing Agent: Some ingredients can help improve the dispersion and stability of powders in silicone bases. However, finding a compatible dispersing agent might require some research and testing.
• Acknowledge Base Choice: As you've observed, a thicker silicone base inherently helps suspend the Silica better. If possible and suitable for your desired product feel, using a slightly more viscous base could be a simpler solution.

In summary, the problem is likely related to the dispersion of Silica in the very light Cyclomethicone / Cyclopentasiloxane base. Focusing on improving dispersion or adjusting the concentration/particle size of the Silica are the key steps to solving this.