The story of sunscreen photostability is really the story of how sunscreens work. The principal characters are photons and electrons. Their stages are tiny, and the acts in their dramas are extremely short-lived. Take, for example, a UVB photon with a wavelength of 300 nm (3000 Å). If one imagines a photon as a round object and its wavelength as its diameter, one can calculate the time it takes for it to pass a point in space by dividing its diameter, 3 x 103 Å, by its speed—the speed of light, c, or 3 x 1018 Å/sec. Thus the short journey of a 300-nm photon takes just 10-15 sec, a single femtosecond (i.e., one one-thousandth of a trillionth of a second), to complete.
Now take the interaction of a photon with a single electron, currently occupying a non-bonding (n) or π (pi) orbital in an organic molecule. The electron’s first role is to use all the energy in the photon to jump to a higher energy orbital. This initial transition to a higher energy orbital takes the molecule from the ground state to its first excited state, known as the singlet excited state. By doing so, it makes the photon completely disappear but it must act in a very short time since, as noted, it only takes the photon 10-15 sec to pass. If successful, the electron’s role quickly shifts to one that appears much less scripted and more improvisational; for example, the singlet excited state often transitions to the triplet excited state. Of course, each photon-absorbing electron is playing a small role in the larger drama performed by the sunscreen: that of protecting the wearer’s skin from UV radiation.
Lab Practical: Using Ethylhexyl Methoxycrylene
- Incorporate ethylhexyl methoxycrylene in the oil phase. If calculating the required HLB, use a value of 8.
- Use levels vary according to the difficulty of photostabilizing a given UV filter combination. 2% imparts substantial photostability to systems containing avobenzone alone or in combination with: oxybenzone, octocrylene, octisalate, homosalate, ensulizole; or with those currently not allowed in the United States including: enzacamene, bemotrizinol, bisoctrizole, octyl triazone, diethylamino hydroxybenzoyl hexyl benzoate, and bisdisulizole disodium.
- More difficult-to-photostabilize combinations such as avobenzone and octinoxate (OMC) require higher levels, i.e., 5%.
- Ethylhexyl methoxycrylene is an excellent solvent for crystalline UV filters such as avobenzone (25-30%), oxybenzone (> 30%), octyl triazone (> 25%), and bemotrizinol (> 30%).
- Use of ethylhexyl methoxycrylene will confer a light yellow or cream color to the emulsion.
- The efficacy of low levels of ethylhexyl methoxycrylene (1–3%) can be extended by adding one or more traditional photostabilizers. For example, adding 3% ethylhexyl methoxycrylene to 3% avobenzone will improve the retention of UVA absorbance-after 25 MED-from 35% to 90% and the retention of UVB absorbance from 70% to more than 97%.
- The addition of 3% octocrylene will improve the retention of UVA absorbance after 25 MED to 98%, and its retention of UVB absorbance to 100%.
- Every UV filter system and sunscreen formulation is different, and there is no substitute for experimenting with various combinations of UV filters with ethylhexyl methoxycrylene.