Controlling Hydroxyl Radical Formulation in TiO2 Sunscreens

Feb 1, 2010 | Contact Author | By: Nai-Fang Chang and Chang-Chin Kwan, Providence University
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Title: Controlling Hydroxyl Radical Formulation in TiO2 Sunscreens
TiO2x catalasex hydroxyl radicalsx o/w emulsionsx photocatalysisx rutilex
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Keywords: TiO2 | catalase | hydroxyl radicals | o/w emulsions | photocatalysis | rutile

Abstract: Titanium dioxide (TiO2) is an effective sunscreen agent but its rutile form may produce hydroxyl radicals upon exposure to sunlight. Thus, the authors use salicylic acid as a capture agent to examine the development of free radicals in solutions or creams containing TiO2. They also explain how the choice of emulsion may reduce free radical generation.

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N-F Chang and C-C Kwan, Controlling hydroxyl radical formulation in TiO2 sunscreens, Cosm & Toil 125(2) 58-64 (Feb 2010)

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The use of titanium dioxide (TiO2) in cosmetics is extensive; in addition to makeup such as pressed powder cakes, liquid foundations, lipsticks and eye makeup, it is used in sunscreen creams. However, its rutile form for cosmetics may produce small amounts of hydroxyl radicals upon exposure to sunlight. Therefore, in the present article, the authors use salicylic acid (SA) as a hydroxyl radical capture agent to examine the development of free radicals in solutions or creams containing TiO2 to further examine its safety upon UV exposure.

Characterizing TiO2

TiO2 is a nontoxic, fine white powder consisting of uniform particles that has good dispersion and narrow particle size distribution. Its coloration and covering power are higher than zinc oxide or lead carbonate, and for particle sizes in the range of 0.2–0.5 μm, TiO2 has the largest reflective index (n = 2.7) among all white pigments. It also shows stable chemical properties such as being: insoluble in water and weak acids, slightly soluble in alkaline solutions, and soluble in hot sulfuric and hydrochloric acid. Crystal forms of TiO2 include anatase, rutile and brookite.

The anatase and rutile forms are the most often used for pigment and photocatalytic reactions. These forms are tetragonal structures and their molecule bonding type and curvatures are expressed as different densities and band gap energies. When the anatase form is heated to approximately 600°C, it transforms into the rutile form. Typical TiO2 on the market is a mixture of the anatase and rutile forms doped with other metal ions.

Photocatalytic Oxidation via TiO2 Catalyst

In 1972, the photocatalytic splitting of water on TiO2 electrodes marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts have been under way to understand the fundamental processes and to enhance the photocatalytic efficiency of TiO2. Such studies often relate to energy renewal and energy storage.

In a heterogeneous photocatalysis system, photo-induced molecular transformations or reactions take place at the surface of a catalyst. Depending where the initial excitation occurs, photocatalysis can generally be divided into two classes of processes. When initial photo-excitation occurs in an adsorbated molecule, which then interacts with the ground state catalyst substrate, the process is referred to as catalyzed photoreaction. If initial photo-excitation instead takes place in the catalyst substrate and the photo-excited catalyst then transfers an electron or energy into a ground state molecule, the process is called sensitized photoreaction.

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Table 1. Crystal forms and particle sizes of TiO2 samples tested

Table 1. Crystal forms and particle sizes of TiO<sub>2</sub> samples tested

Thirteen commercially available cosmetic grade TiO2 samples including both the anatase and rutile type were obtained from different companies and used to test for the generation of free radicals after UV irradiation. Shown here are their crystal forms and particle sizes of the individual TiO2 samples.

Figure 1. Structure of rutile and anatase TiO2

Figure 1. Structure of rutile and anatase TiO<sub>2</sub>

The anatase and rutile forms are the most often used for pigment and photocatalytic reactions. These forms are tetragonal structures and their molecule bonding type and curvatures are expressed as different densities and band gap energies.

Figure 2. Mechanism of excited catalyst particle

Figure 2. Mechanism of excited catalyst particle

The anatase type of TiO2 provides excellent photocatalysis due to a higher band gap energy.

Figure 3. Products formed from the attack of hydroxyl radicals upon salicylate

Figure 3. Products formed from the attack of hydroxyl radicals upon salicylate

By liquid-solid heterogeneous photocatalysis reaction principal, the authors used SA as a hydroxyl radical capture agent and produced catechol, 2,3-dihydroxy benzoic acid (2,3-DHBA) and 2,5-dihydroxy benzoic acid (2,5-DHBA).

Figure 4. Percentage of SA consumed under UV irradiation

Figure 4. Percentage of SA consumed under UV irradiation

Percentage of SA consumed in 0.3% TiO2 solution containing 1000 ppm SA under UV irradiation; the pH value of solutions varied from 3 to 9 under standard reaction conditions.

Figure 5. Percentage of SA consumed under standard reaction conditions

Figure 5. Percentage of SA consumed under standard reaction conditions

Percentage of SA consumed in varied TiO2 solutions under standard reaction conditions; specifications of the TiO2 samples are shown Table 1.

Figure 6. Percentage of SA consumed from varied oil phases of emulsions

Figure 6. Percentage of SA consumed from varied oil phases of emulsions

Percentage of SA consumed in varied TiO2 of varied oil phases of emulsions under standard reaction conditions; specifications of the TiO2 samples are shown Table 1.

Figure 7. Percentage of SA consumed from varied solutions or creams

Figure 7. Percentage of SA consumed from varied solutions or creams

Figure 7. Percentage of SA consumed in varied TiO2 solutions or creams under standard reaction conditions; specifications of the TiO2 samples are shown Table 1.

Formulas 1a-b. Emulsions used for described tests

Formulas 1a-b. Emulsions used for described tests

End products were measured hourly by HPLC with a UV detector. The same method was used to test the sample w/o and o/w creams shown here.

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