Molecular Weight and Photostability: The Two Numbers Behind Every UV Filter Decision

Three questions decide which ultraviolet (UV) filter goes into a sunscreen base. The first is spectral: which band does the molecule absorb? The second is regulatory: which markets allow it, and at what ceiling? Both were covered in earlier articles in this series. The third question is the one buyers ask least and should ask most: how big is the molecule, and how well does it survive the sunlight it absorbs?

Molecular weight decides where the filter stays — on the surface of the stratum corneum or across it. Photostability decides whether the filter is still a filter after two hours of beach sun. Neither number appears on a certificate of analysis, and neither can be inferred from the INCI name. This article puts both on the table for the eleven organic filters in our range.

The line-up, ordered by molecular weight

The 500 Da rule — and when it needs help

Bos and Meinardi proposed in 2000 that compounds above roughly 500 Da rarely cross intact skin, because the extracellular lipid matrix of the stratum corneum behaves as a size-exclusion barrier. The SCCS Notes of Guidance turns this into a screening set: low dermal absorption is indicated by any of molecular weight above 500 Da, a high degree of ionisation, log Pow ≤ −1 or ≥ 4, a topological polar surface area above 120 Ų, or a melting point above 200 °C. One criterion is enough to argue the point; the molecular weight is simply the easiest to check.

• The four triazine- and benzotriazole-type filters in the table above clear 500 Da by mass alone: Bemotrizinol 627.8, Iscotrizinol 766.0, Ethylhexyl Triazone 823.1, and Drometrizole Trisiloxane right at the line at 501.8.
• DHHB is only 397.5 g/mol, but its log Pow of 6.2 (SCCP opinion S83) means it prefers stratum corneum lipids to the aqueous compartment below — it stays where the UV is.
• Ensulizole is small at 274.3, yet its sulfonic acid group keeps it ionised and water-soluble; it sits in the water phase of a gel or mist and does not ride oil droplets into the skin.
• Ecamsule (562.7) is water-soluble and above 500 Da; in a radiolabelled human study, systemic absorption was below 0.1% of the applied dose.
• The counter-example is oxybenzone: 228.2, moderate polarity, and Franz-cell measurements of a conventional O/W emulsion give a 24-hour cumulative permeation of roughly 10–25 µg/cm² — an order of magnitude above the big filters.

Photostability: the active that disappears in an hour

Avobenzone absorbs UVA through an enol chromophore stabilised by an intramolecular hydrogen bond. Absorption triggers excited-state proton transfer to the diketo form, which is not photochemically competent — the energy exits as bond cleavage instead of as heat. Without a stabiliser, published work (Chatelain et al., 2001) puts the loss of UVA absorbance at 50–90% within one hour of simulated solar radiation. Co-formulated with a triplet-state quencher — classically octocrylene, and in modern bases bemotrizinol, which regenerates the enol by triplet–triplet energy transfer — the loss over the same window falls below roughly 20%.

Ethylhexyl methoxycinnamate fails more gently: E/Z photoisomerisation and dimerisation bleed off protection over hours rather than minutes, but the molecule also penetrates more readily. On Strat-M membranes (Olejnik et al.), cumulative permeation was about 2% for octinoxate versus 0.3% for avobenzone — the small, oily, cheap filter is the one that travels.

At the stable end of the scale, the triazines set the benchmark: iscotrizinol needs 25 hours of continuous irradiation to lose 10% of its SPF (3V Sigma data for Uvasorb HEB), and bemotrizinol and ethylhexyl triazone remain essentially intact after UV doses equivalent to 50 minimal erythemal doses.

The vehicle shifts all of these numbers. In vitro work on methoxycinnamate esters (Jimenez et al.) measured 24-hour permeation of 172.1 µg/cm² in a W/O emulsion versus 127.8 µg/cm² in O/W — the same filter, a 35% difference from the emulsion type alone. Water-resistance claims and penetration are the same conversation, not two.

What to actually write on the order

• Do not spec by molecular weight or λmax — both are fixed by the CAS number. Spec what can vary: HPLC assay (typically ≥98%), appearance, and for powders a melting point as an identity check.
• Spec solubility in your oil phase. The large triazines dissolve poorly in common solvents: bemotrizinol reaches only about 0.29% in C12-15 alkyl benzoate, and ethylhexyl triazone less. Budget the solvent or dispersant load at the design stage, or put DHHB in the UVA slot where a clear solution is possible.
• If avobenzone is in the formula, name the photostabiliser partner in the spec review — not just on the consumer label. An unstabilised avobenzone base is a refund waiting to happen.
• Check the ceiling per market. Under China’s Safety and Technical Standards for Cosmetics (2015), iscotrizinol is capped at 10% and drometrizole trisiloxane at 15%; the EU permits both at similar levels. Market compliance was covered in depth in our five-market compliance guide.

Selection quick map

• UVA-I protection without photo-loss: DHHB (oil phase) or ecamsule (water phase) first; avobenzone where regulations or cost mandate it, always with a named quencher such as bemotrizinol.
• High-load UVB with negligible penetration concern: ethylhexyl triazone or iscotrizinol.
• Water-phase UVB for clear gels and mists: ensulizole.
• One-molecule broad spectrum: bemotrizinol (oil) or drometrizole trisiloxane (silicone feel, water-resistant films).
• Cost-driven UVB: ethylhexyl methoxycinnamate or ethylhexyl dimethyl PABA — acceptable when the photolysis and permeation trade-offs are budgeted deliberately.
• Where oxybenzone is still permitted, treat its permeation profile as the reason modern bases migrate to DHHB — see our organic filter selection guide for the spectral pairing logic.

All eleven filters are stocked for B2B supply; sample quantities ship worldwide. Requests for specifications, photostability documentation, or formulation support are welcome via the enquiry form on any product page.