334 356

Wovelength (nm)


378 400

Figure 2 UV-Visible absorption spectra of 1x10' M aqueous solutions of several UV-B sunscreen active ingredients. Spectra were obtained in 1 cm quartz cuvettes versus solvent.

Benzophenone-8 did not undergo any measurable photolysis. Photolysis of each of the other sunscreen active ingredients followed first-order kinetics in all cases (Figure 3) with the exception of Octylmethoxycinnamate. In the case of Octylmethoxycinnamate, there was a rapid initial first-order reaction (presumably trans-cis isomerization) leading to a photostationary state in which no further reaction was observed. Photostability data are presented in Table 1 for all evaluated compounds except for Octylmethoxycinnamate.

In the Table, jsoisim is the experimentally determined first-order rate constant for photolysis of each of the sunscreen active ingredients in the solar simulator, jsuN is the normalized firstorder rate constant for photolysis in sunlight, and Ti/2,sun is the estimated half-life for sunscreen active ingredients when illuminated in sunlight. It should be noted that, since solar irradiance is dependent upon time of year, time of day, latitude, and factors such as cloud cover and haze, values for jsuN and Ti/2,sun are variable.

Table 1 Photolysis of sunscreen active ingredients in the solar simulator at 25 °C



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