Surfactant effect on titanium dioxide photosensitized oxidation of 4-dodecyloxybenzyl alcohol

The TiO2 photosensitized oxidation of 4-dodecyloxybenzyl alcohol (1), water insoluble, was investigated in aqueous surfactant solutions. A variety of surfactants of different charge type was used and in some cases the pH effect was investigated. The considered surfactants are: cationic (cetyltrimethylammonium with various counterions, CTAX with X = Cl, Br and OMs), zwitterionic (dimethyldodecylamine oxide, DDAO, tetradecyltrimethylammonium propane sulfonate, SB3-14, and tetradecyltripropylammonium propane sulfonate, SBPr3-14) and anionic (sodium dodecylsulfate, SDS, sodium dodecylbenzenesulfonate, SDBS). The reaction, that is practically absent in water, is greatly enhanced by several surfactants at concentrations higher than cmc and the effect is strongly dependent on the nature of the surfactant. A rationale for that is suggested, with micellar aggregates rather than monomeric surfactants playing a main role. The effects of surfactant concentration on the conversion to the corresponding benzaldehyde were studied. We suggest that the increase of the surfactant concentration leads to an increase of the number of substrate molecules, solubilized (through co-micellization) in micelles, which can be transported close to the TiO2 particle surface where they react. After a certain concentration which varies with the nature of the surfactant, the presence of the competitive partition between the co-micellized substrate 1 adsorbed on TiO2 (where reaction occurs) and the co-micellized substrate in the bulk tends to limit more and more the beneficial kinetic effect of the surfactant. The surfactant that gives in water the best performance in terms of 4-dodecyloxybenzaldehyde yield (42%) is SB3-14 0.015 M. Moreover, a better yield (55%) is obtained in the presence of surfactant CTABr 0.05 M with added HCl 0.1 M.