Hair Serum Formulation: Adding Oil, Emulsifiers, and Texture

Regarding your remaining questions about the hair serum formula:

• Preservative: Yes, a formula with a high water content like this one definitely requires a preservative to prevent microbial growth. You should add a suitable preservative according to the manufacturer's recommended usage rate.
• Water Conditioner: Using RO water is a good choice for cosmetic formulations. While specific water conditioners can be used depending on the desired properties or to chelate trace minerals, I cannot provide specific recommendations for this formula without more context on the specific RO water quality and desired outcome.
• Penetration Enhancer: Penetration enhancers help ingredients absorb into the hair shaft. For a serum primarily designed to coat the hair (like one containing silicone), deep penetration might not be the main goal. Adding penetration enhancers could potentially affect the texture and might lead to stickiness, as you are concerned about. Whether to add one depends on the specific performance goals for the serum and careful selection of the enhancer type and concentration to avoid unwanted texture issues.