SPF Serum Formulation: Stability and Eye Sting Prevention

Question

I am updating my SPF formula. Please let me know if everything works, as I want to make sure there is no eye sting this time. I am unsure whether Phenoxyethanol SA actually causes eye sting or not; I use it in serums without issues, but if there is a better preservative, I am open to using it. Also, could I get mixing instructions? I can't remember the stage for mixing the Silky Serum Base Plus with the water phase and the overall order of addition.

Formula (25g Batch):

Phase A
- Silky Serum Base Plus: 11.0% (2.75 g) - Reduced for stability
- Titanium Zinc NanoLiquid™ DEC: 12.5% (3.13 g) - Core SPF
- Quick Blur: 3.0% (0.75 g) - Blurring
- Cyclopentasiloxane: 2.0% (0.50 g) - Spread
Phase B
- Purified Water: 52.8% (13.20 g) - Increased to balance additions/removals
- Chamomile Water: 8.0% (2.00 g) - Soothing (reduced for lower load)
- Propanediol: 3.0% (0.75 g) - Humectant
- X-Glycerin (Polyglycerin-10): 1.0% (0.25 g) - Silky moisture
- Amino Matte: 1.5% (0.38 g) - Matte control
- Sepimax Zen: 0.6% (0.15 g) - Thickener
Phase C
- SiliSilk™ 5H: 0.5% (0.13 g) - Soft-focus
- MatteSilica 3™ (3 Micron): 0.5% (0.13 g) - Oil absorption + matte boost; pre-slurry
Phase D
- Genistein EZ™ (5% Water-Soluble): 1.5% (0.38 g) - Anti-aging, barrier, subtle pores
- Pep®-Geline (Trifluoroacetyl Tripeptide-2): 1.0% (0.25 g) - Firming, anti-sag
- Phenoxyethanol SA: 1.0% (0.25 g) - Liquid, gentle preservative
- Bisabolol: 0.3% (0.08 g) - Soothing
- Disodium EDTA: 0.1% (0.025 g) - Chelator/stabilizer (dissolve in water)

MySkinRecipes Expert · Accepted Answer

Formula Review & Recommendations

Your updated SPF formula is well-structured, but there are a few critical adjustments needed for stability and to ensure it is "sting-free" around the eyes.

1. Addressing Eye Sting (Preservative Choice)

Phenoxyethanol SA is a broad-spectrum preservative, but it can indeed cause a stinging sensation if used at high concentrations near the eyes. The manufacturer recommends using no more than 0.5% for eye-area products. At 1.0%, it is likely the cause of the irritation.

Recommendation: Switch to Mild Preserved Eco™. It is a "preservative-free" system (Ethylhexylglycerin and Propanediol) that is much gentler, non-irritating, and does not cause eye sting. Use it at 1.0% - 1.5%.

2. Stability & Texture

Magnesium Sulfate: For formulas using Silky Serum Base Plus, it is highly recommended to add 1% Magnesium Sulfate to the water phase. This acts as an electrolyte that significantly increases the stability of the water-in-silicone emulsion, preventing separation over time.
Water Ratio: The base performs best with at least 60-70% water. Your current water total (Purified + Chamomile) is ~60.8%, which is acceptable, but adding Magnesium Sulfate is vital at this ratio.

Revised Mixing Instructions (25g Batch)

Since Silky Serum Base Plus creates a "Water-in-Oil/Silicone" emulsion, the order of addition is crucial. You must add the water phase to the silicone phase very slowly.

Phase A: Silicone/Oil Phase

Silky Serum Base Plus (2.75 g)
Titanium Zinc NanoLiquid™ DEC (3.13 g)
QuikBlur™ (0.75 g)
Cyclopentasiloxane (0.50 g)
Bisabolol (0.08 g)
SiliSilk™ 5H (0.13 g) — Disperse this powder into the silicone phase first to prevent clumping.

Phase B: Water Phase

Purified Water (13.20 g)
Chamomile Water (2.00 g)
Propanediol (0.75 g)
X-Glycerin (0.25 g)
AminoMatte™ (0.38 g)
Disodium EDTA (0.025 g)
Genistein EZ™ (0.38 g)
Sepimax Zen (0.15 g) — Sprinkle this into the water and wait for it to fully hydrate into a clear gel before proceeding.
(Optional but Recommended) Magnesium Sulfate (0.25 g)

Phase C: Final Actives & Preservative

Pep®-Geline (0.25 g)
Mild Preserved Eco™ (0.25 g - 0.38 g)
MatteSilica 3™ (0.13 g)

Step-by-Step Mixing Process

Prepare Phase A: Mix all ingredients in Phase A together until uniform.
Prepare Phase B: In a separate beaker, dissolve the water-soluble ingredients. Add Sepimax Zen last and stir until a smooth gel forms.
Emulsification (The Most Important Step): While using a high-speed blender (or stirring vigorously), slowly add Phase B into Phase A in very small increments (a few drops at a time).
- Note: Do not pour the water in all at once. Wait for each addition to be fully incorporated before adding more. The mixture will gradually thicken into a creamy serum.
Final Phase: Once the emulsion is stable and smooth, add the ingredients from Phase C (Pep®-Geline, Preservative, and MatteSilica 3™). Blend until perfectly smooth.
Check pH: Ensure the final pH is between 5.0 - 6.0 for optimal stability and skin comfort.