392 JOURNAL OF COSMETIC SCIENCE 100---------------------------- u C E ..... 0 C 0 80 60 C 40 !l cf. 20 c::::J 5% loading 10% loading Figure 4. Retention of vitamin C in Type II 0/W emulsion in the absence of inorganic UV absorbers and in conjunction with nanopartide Ti02 and Ti02:Mn over two hours of solar exposure. PHOTOPHYSICS OF Ti0 2 AND Ti0 2 :Mn The electronic band structure of rutile TiO 2 is interesting in that the direct and indirect transitions have very similar energy gaps, with a shallow barrier between f and M. Consequently, photogenerated electrons may thermalize down to either the direct or indirect transitions (26). The long lifetime of the indirect transitions renders the elec­ tron/hole pair likely to annihilate via surface states, resulting in the generation of ROS. It has been demonstrated that approximately 12% of electron/hole pairs annihilate via the particle surface to produce ROS (27). The introduction of manganese results in the substitution of Mn3 + ions on a Ti4+ site as a Mn3 + + hole p-type dopant. The manganese energy level site is essentially directly mid-gap, and therefore any hole associated with the site is strongly bound to the manganese site (28). In accordance with the uncertainty principle, the hole wavefunction is delocalized in k-space and allows electron/hole an­ nihilation from the indirect transition via the localized hole state. Consequently, ROS generation is reduced by 95% (26). Manganese oxygen anti-bonding states increase the density of states in the region of the bottom of the conduction band and increases the intrinsic UV A absorbance of the material (29). In addition, there is a split of manganese dopant states between Mn3 + and a lesser amount of Mn2+ . Mn2 + ions are associated with surface regions, as their size precludes incorporation into the TiO2 lattice. Such Mn2+ ions act to scavenge free radicals in the vicinity of the surface (13).

u C .E C a 100 80 60 !} 40 Q,) #- 20 TiO 2 :Mn IN SUNSCREENS 0 ------------- - 1% Loading C:=:J 2%Loading - 3%Loading 393 Figure 5. Retention of vitamin C in Type III W/0/W emulsion in the absence of inorganic UV absorbers and in conjunction with nanopartide Ti0 2 and Ti0 2 :Mn over two hours of solar exposure. CONCLUSIONS The addition of manganese to sunscreen grade TiO 2 results in an almost (97%) total elimination of photogenerated ROS due to electron/hole de-excitation via the manganese dopant ion and an ROS scavenging effect from manganese associated with the surface region of the particle. These properties result in photostabilization of organic sunscreen and cosmetics additives such as BMDM, OMC, vitamin E, and vitamin C. Significantly enhanced retention of active organics during solar exposure has been demonstrated. It has also been shown that photogenerated ROS from TiO2 acts to degrade anti-oxidant cosmetic components during topical use and that this degradation rate is directly related to TiO2 concentration. Only partial protection of UV-absorbing components is shown by TiO2 in comparison with TiO2:Mn. The beneficial effects on emulsion ROS load from incorporation of TiO2:Mn have been demonstrated during solar exposure, but it seems reasonable to postulate that any mechanism of ROS generation in emulsions, e.g., thermal generation, will also exhibit organic active stabilization effects in conjunction with TiO2:Mn. REFERENCES (1) B. L. Diffey, Sources and measurement of UV radiation, Methods, 28, 4--13 (2002). (2) W. A. Bruls, H. van Weelden, and J.C. van der Leun, Transmission of UV radiation through human