PHOTODEGRADATION BY RUTILE-BASED PIGMENTS 521 In the case of the skin, the protection of which is the purpose of the pigment treatments, the wide variety of possible target molecules for photocatalytic degradation makes it very likely that some molecules transform like phenol upon reaction with Ti•v-øOH•,rf, and others like salicylic acid via electron-transfer processes. As a consequence, pigment treatments should assure protection to both classes of molecules. In real solar creams and gels, however, the presence of other components such as rheological modifiers may assure a certain degree of protection to skin molecules, but often at the expense of the modifier degradation. In fact, various rheological modifiers are able to inhibit the photocatalytic degradation of both phenol and salicylic acid, and in the case of salicylic acid they are more effective than pigment treatments (19). Interestingly, when adopting Lubrajel as rheological modifier (glyceryl polymethacrylate + propylene glycol), we have observed a marked inhibition of the photocatalytic degradation of both phenol and salicylic acid, without relevant degradation of the modifier itself, which could be an important result for solar cream formulations (19). PROPOSAL OF A NEW METHODOLOGY FOR THE ASSESSMENT OF PIGMENT TREATMENT EFFICIENCY The traditional way to assess the efficiency of pigment treatments is to choose a model molecule and then measure the degradation rate of this molecule upon irradiation in the presence of the treated pigments, the best pigment being that assuring the lowest degradation rate. The limitations of this single-molecule approach have already been described. The new approach we propose considers that the oxidative photoactivity of titanium dioxide is due to two distinct kinds of processes, namely degradation via reaction with Ti•v-øOH•urf and degradation via charge-transfer processes involving sur- face complexes. The first process can be evaluated by monitoring the phenol degradation rate, the second by monitoring the degradation of salicylic acid. In this way, the whole oxidative reactivity of titanium dioxide will be taken into account, and not just the fraction involving Ti•v-øOH•urf. The use of phenol instead of an alcohol as a molecule reacting with Ti•v-øOH•rf is due to our using the HPLC-UV to make the analyses (differently from most alcohols, phenol absorbs radiation above 200 nm) in another analytical setup the use of an alcohol (we suggest 2-propanol) might be more suitable. As to the molecule undergoing charge- transfer reactions, at the present state of knowledge the choice is restricted to benzoic acid and salicylic acid, but in the absence of other considerations, salicylic acid is preferable due to its higher reactivity and stronger complex formation with titanium dioxide (15,17). The setup for irradiation experiments, which can be built up with limited expense, requires a lamp emitting in the UVA range, a photochemical reactor, and a stirring device. It is not necessary to use the Solarbox equipped with the xenon lamp as an irradiation device: the use of a solarium lamp emitting in the UVA range (for instance, the 40 W Philips TL K05 lamps are excellent for this purpose) gives the same results at a much lower cost (20). In such a case, however, the distance between the light source and the irradiated samples should not exceed 5-10 cm if one wants to obtain reasonable degradation times. The cylindrical Pyrex glass cells shown in Figure 1 are among the best performing photocatalytic reactors available, and their manufacturing poses few problems in spite of their not being standard glassware. Their first advantage consists in their being thoroughly stirred systems, thus avoiding the stirring problems connected

522 JOURNAL OF COSMETIC SCIENCE with larger reactors. Irradiation of 5.0-ml suspensions has the double advantage of the limited optical path (0.4 cm), so that no dark zones will be present inside the suspension, and an optimal air-to-solution volume ratio, assuring that the aqueous suspension will not become anoxic due to oxygen consumption in photocatalytic degradation. The problems of incomplete stirring, the presence of dark zones, and the onset of anoxic conditions heavily limit the reproducibility of the results obtained with larger bench reactors (21,22). The Pyrex glass cells have to be stirred during irradiation and magnetic bar stirring is the most suitable technique. The screening of organic molecules for use as pigment treatments does not require the preparation of coated titanium dioxide. Inclusion of the compound to be studied, in the desired amount, into the suspension containing naked titanium dioxide and phenol or salicylic acid is the simplest procedure, at least in a first stage. When looking for a suitable compound to be used as organic pigment treatment, two strategies can be adopted. The first one is the single-treatment approach, consisting in looking for a single compound having the features required for an organic treatment. The second approach is to obtain the desired goal (inhibition of pigment photoactivity) with a mixture of more than one compound. In the single-treatment approach, the screening should consider both phenol and sali- cylic acid degradation rates, and the best performing organic molecule should inhibit to a sufficient extent degradation of both phenol and salicylic acid. Finding a compound having these features is highly desirable, but there is no assurance that a molecule behaving as such actually exists, since it should undergo efficient degradation both via reaction with Ti•v-øOHsu•f-and via electron-transfer processes. Moreover, it should also be usable in cosmetic formulations, and thus is should be nontoxic and compatible with the other formulation components, which narrows the list of candidate compounds. A possible way out of the problem might be in adopting two different compounds, one inhibiting phenol degradation and the other inhibiting degradation of salicylic acid. Many organic additives inhibit phenol degradation [for instance, 1,3-butanediol and dimethicone are very effective and are presently used in cosmetics (14)], and thus it should be sufficient to look for a compound effectively inhibiting degradation of salicylic acid, and to use the two compounds together as organic additives in the pigment treatment. In such a way, the ability of titanium dioxide to oxidize organic molecules of any kind should be prevented. At this point, it should be advisable to prepare a coated titanium dioxide using the two organic treatments, and to test this sunscreen for photocatalytic activity towards both phenol and salicylic acid. CONCLUSION The alumina surface coating and the organic treatments of titanium dioxide with stearic acid, dimethicone, and 1,3-butanediol protect phenol from photocatalytic degradation (Figure 2). When considering the degradation rate of phenol in the presence of Aldrich uncoated rutile as a reference, the pigments coated with stearic acid show a decrease in phenol degradation rate by 12% (pigment A, MT-100TV) and 56% (pigment C, M160). The pigments treated with 1,3-butanediol and dimethicone show a relevantly better performance, since the decrease in the degradation rate reaches 95% in the case of dimethicone (pigment B) and 94% in the case of 1,3-butanediol (pigment D). The