Water-Based Sunscreen Spray Formulation: DPDT Solubility, SPF/PA, and Practical Effectiveness

Question

Water-Based Sunscreen Spray Formulation Questions I have some questions regarding a water-based sunscreen spray formulation containing Ensulizole and Disodium Phenyl Dibenzimidazole Tetrasulfonate (DPDT): What percentage of pH adjusting agents (such as Triethanolamine, Sodium Hydroxide, and Citric Acid) is suitable or typically needed for this type of formula to achieve the desired pH? What SPF/PA value can be expected from this formula? (A low value is acceptable as the focus is on frequent reapplication). Can this type of water-based spray formula provide effective sun protection in practice, considering that product texture significantly impacts sun protection efficacy?

MySkinRecipes Expert · Accepted Answer

Dissolving Disodium Phenyl Dibenzimidazole Tetrasulfonate (DPDT)

Disodium Phenyl Dibenzimidazole Tetrasulfonate (DPDT) is an acidic UV filter that requires neutralization with a base (alkali) to become soluble in water. This is why bases like Triethanolamine and Sodium Hydroxide are used in formulations containing DPDT – they help dissolve the ingredient.

While bases at high concentrations can be irritating to the skin, formulators typically use a combination of bases and then adjust the final pH of the product down to a skin-friendly range, usually around pH 7. This final pH adjustment significantly reduces the potential for irritation from the bases used for dissolution.

Water-Based Sunscreen Spray Formulation

Regarding your proposed water-based sunscreen spray formula with Ensulizole and DPDT:

pH Adjuster Percentage: The exact percentage of pH adjusters (Triethanolamine, Sodium Hydroxide, Citric Acid) needed to achieve a final pH of around 7 cannot be provided without testing. You would typically dissolve the acidic UV filters using the bases and then add Citric Acid gradually while measuring the pH until it reaches the desired level (around 7).
SPF/PA Value: An exact SPF/PA value for this specific formula cannot be determined without laboratory testing. However, both Ensulizole and DPDT are effective UV filters. DPDT is known to provide good UVA protection. Theoretically, the combination at these percentages should yield a respectable SPF/PA value, although the precise number requires measurement.
Practical Effectiveness of Water-Based Spray: Yes, a water-based spray formula can provide sun protection. However, as you noted, the product vehicle significantly impacts efficacy. The major limitation of a simple water-based spray is its lack of water resistance. It can be easily removed by sweat or water, meaning the protection may not last long on the skin in practical use, especially if you sweat or are exposed to water. Frequent reapplication would be necessary to maintain protection.