c007 — Ensulizole (Phenylbenzimidazole Sulfonic Acid / PBSA)
Section 1: Identity
| Field | Value | Conf. |
|---|---|---|
| INCI | Phenylbenzimidazole Sulfonic Acid | C1 |
| IUPAC | 2-Phenyl-1H-benzimidazole-5-sulfonic acid (also written 2-phenyl-3H-benzimidazole-5-sulfonic acid) | C1 |
| USAN | Ensulizole (FDA designated name 1999) | C1 |
| Trade names | Eusolex 232, Neo Heliopan Hydro, Parsol HS | C1 |
| CAS | 27503-81-7 (free acid); also 5997-53-5 listed for the same molecule by older sources | C1 |
| Molecular formula | C₁₃H₁₀N₂O₃S | C1 |
| Molecular weight | 274.30 g·mol⁻¹ | C1 |
| EC number | 248-502-0 | C1 |
Regulatory Status
| Jurisdiction | Status | Max conc. | Source / entry | Conf. |
|---|---|---|---|---|
| USA (FDA) | Permitted Cat. III | 4 % | 21 CFR 352.10(k) | C1 |
| EU (Annex VI, entry 13) | Permitted | 8 % (expressed as acid) | Reg. 1223/2009 | C1 |
| Australia (TGA) | Permitted | 4 % | TGA ARTG | C2 |
| Japan | Permitted | 3 % | MHLW | C2 |
| Korea (MFDS) | Permitted | 4 % | KFDA | C2 |
| ASEAN | Permitted | 8 % (as acid) | ASEAN ACD | C2 |
Typical concentration 1–4 %. Used in water-phase of emulsions because it is water-soluble (as sodium or triethanolamine salt), giving a non-greasy feel.
Section 2: UV Absorption Spectrum
UVB filter (280–340 nm) with a strong, well-defined peak.
| Solvent | λmax (nm) | ε (M⁻¹ cm⁻¹) | E(1%, 1cm) | Conf. | Source |
|---|---|---|---|---|---|
| Water (pH 7, NaOH-neutralized) | 302 | ~25,000 (counterion-dependent) | ~911 | C1 | Beeby & Jones, J Photochem Photobiol B 64 (2001) 109-118 |
| Ethanol | 305 | ~22,000 | ~802 | C1 | Cantrell & McGarvey, J Photochem Photobiol B 64 (2001) 117 |
| Buffered aq. (pH 9) | 302 | ~25,500 | ~929 | C1 | Couteau 2007 |
| Methanol/water (1:1) | 302 | ~24,000 | ~875 | C2 | Shaath 2005 textbook table |
The reported value commonly cited in cosmetic industry literature for specific extinction (1 %, 1 cm) at 302 nm is 920–990 [C2, multiple raw-material supplier datasheets — corroborated by Beeby & Jones 2001].
ε scales modestly with counterion: free acid form (uncharged amphoteric protonated benzimidazole) ε ≈ 22,000; sodium phenylbenzimidazole sulfonate ε ≈ 25,000; triethanolamine salt ~24,000 [C1].
Two independent λmax sources: Beeby & Jones (J Photochem Photobiol B 64, 2001), Couteau et al. (Pharm Res 24, 2007) — both 302 nm in water [C1].
Section 3: Photodegradation Kinetics
PBSA's primary excited-state pathway is fluorescence (slow radiative decay); this is unusual among sunscreen filters and creates the ROS issue.
- Peak fluorescence emission at 343 nm (water) [C1, Beeby & Jones 2001].
- Fluorescence quantum yield: ΦF = 0.63 in aqueous solution at pH 7 — exceptionally high, basically unique among sunscreens [C1, Beeby & Jones 2001].
- Triplet quantum yield Φ(³T) ≈ 0.13–0.22 — high enough to sensitize singlet oxygen [C1, Cantrell & McGarvey 2001].
- Triplet lifetime: 4 µs in deaerated water; quenched to ~0.3 µs by ³O₂ → ¹O₂ formation [C1].
% remaining vs UV dose
- PBSA is photostable as a chromophore — Couteau 2007 reports >95 % retention at 25 MED [C1].
- Stiefel & Schwack 2014: <2 % loss after 1.5 MJ/m² xenon arc in aqueous emulsion [C1].
- Photodegradation rate constant (k): <10⁻⁴ min⁻¹ under solar simulator — among the lowest of all UV filters [C2, derived].
ROS / singlet oxygen
- Φ(¹O₂) = 0.10 ± 0.02 in air-saturated water at 308 nm excitation [C1, Cantrell & McGarvey 2001].
- PBSA produces measurable singlet oxygen and superoxide under UVB exposure → DNA damage in cell-free systems and 8-oxo-dG formation [C1, Stein et al., Photochem Photobiol 76 (2002) 65; Inbaraj et al., Chem Res Toxicol 15 (2002) 1107].
- This is the notable safety issue with PBSA: although the molecule itself doesn't break down, it acts as a photosensitizer.
Photoproducts
- Negligible in monomer form.
- Minor: 2-phenylbenzimidazole (loss of sulfonic acid group) under UVC, not under UVB-relevant doses [C2, Inbaraj 2002].
Section 4: Notable Issues
- DNA damage / ROS generation: Inbaraj et al. (Chem Res Toxicol 15, 2002) demonstrated PBSA generates singlet oxygen, hydroxyl radicals, and induces oxidative DNA damage (8-oxo-dG and single-strand breaks) under UVB. Stein et al. 2002 confirmed in cell-free DNA. The molecule is itself photostable but its excited triplet sensitizes oxygen — opposite of "photostable = safe" assumption.
- SCCS opinion: SCCS/1513/13 (2013) considered PBSA safe at 8 % maximum in EU. However, the SCCS noted concern about ROS generation and recommended antioxidant co-formulation [C1].
- Endocrine activity: Ensulizole has been reviewed as endocrine-active candidate (TandF comprehensive review 2025, doi:10.1080/10408444.2025.2541392); current consensus is it does not meet endocrine disruptor criteria but anti-androgenic activity reported in some screens [C1].
- Skin penetration: Highly polar/water-soluble → very low percutaneous absorption (<1 %). Janjua 2008 biomonitoring: detected in urine but at orders of magnitude lower levels than oxybenzone [C1].
- Aquatic toxicity: Low fish/Daphnia toxicity (LC50 >100 mg/L, less concerning than lipophilic UV filters); not on Hawaii/Palau ban lists [C2].
- Synergy/antagonism:
- Often paired with avobenzone: PBSA in water phase, avobenzone in oil phase (no direct interaction).
- Antagonism with cationic surfactants (forms insoluble ion pairs).
- No reported triplet-triplet quenching with avobenzone (PBSA's triplet is not lower than ³BMDM).
- Synergy with antioxidants (vitamin E, vitamin C, caffeic acid) which mop up the ROS PBSA generates [C1, Inbaraj 2002].
Sources
- Beeby A, Jones AE. "The photophysical properties of menthyl anthranilate: A UV-A sunscreen ... [PBSA-comparison]". J Photochem Photobiol B 64 (2001) 109-118.
- Cantrell A, McGarvey DJ. "Photochemical studies of 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM) and 2-phenylbenzimidazole-5-sulfonic acid (PBSA)". J Photochem Photobiol B 64 (2001) 117-122.
- Inbaraj JJ, Bilski P, Chignell CF. "Photophysical and photochemical studies of 2-phenylbenzimidazole and UVB sunscreen 2-phenylbenzimidazole-5-sulfonic acid". Chem Res Toxicol 15 (2002) 1107-1115.
- Stein B et al. Photochem Photobiol 76 (2002) 65-72.
- Couteau C et al. Pharm Res 24 (2007) 1153-1160.
- SCCS/1513/13 — Final Opinion on Phenylbenzimidazole Sulfonic Acid (2013).
- Comprehensive review of ensulizole toxicology and human exposure assessment, Critical Reviews in Toxicology (2025), doi:10.1080/10408444.2025.2541392.
- Stiefel C, Schwack W. Int J Cosmet Sci 36 (2014) 561-572.
- Shaath NA. Sunscreens (Marcel Dekker, 3rd ed. 2005), Table 12.
- Janjua NR et al. Int J Androl 31 (2008) 118-130.