Enhanced UV Protection of Aqueous PBSA Solutions by Incorporating Hydrophilic Polymers

Phenylbenzimidazole sulfonic acid (PBSA) is a representative water-soluble UV filter, yet its practical use is hampered by poor spreading and film formation in aqueous formulations, leading to limited UV-blocking efficiency. To address this, we examined the effect of hydrophilic polymers—poly(vinyl alcohol) (PVA), gelatin, polyethylene glycol (PEG), and dextran (DEX)—on PBSA performance. UV–Vis spectroscopy and fluorescence imaging showed that PVA and gelatin facilitated continuous film coverage and enhanced UV absorbance, whereas PEG and DEX produced discontinuous coatings with little improvement. Rheological analyses revealed that PVA–PBSA maintained network stability via hydrogen bonding, while gelatin–PBSA formed weaker gels and PEG/DEX–PBSA exhibited negligible effects. Interfacial adsorption measurements further confirmed synergistic film formation only for PVA and gelatin. Overall, our findings demonstrate that effective UV protection with water-soluble PBSA requires the combined presence of suitable bulk rheology and strong interfacial activity, establishing oil-free, water-based polymer–PBSA formulations as a sustainable route for next-generation sunscreens.