Structure effects of amphiphilic and non-amphiphilic quaternary ammonium salts on photodegradation of alizarin red-s catalyzed by titanium dioxide

The role of surfactants such as single- and double-tailed tetraalkylammonium bromide and various nonamphiphilic tetraalkylammonium salts was investigated on the TiO2 photocatalyzed degradation of 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid sodium salt (Alizarin Red-S, ARS) in an air-equilibrated alkaline medium under UV light irradiation. Absorption spectral analysis showed that the photodegradation efficiency of the dye was significantly enhanced by the addition of cationic surfactants. Two interesting findings emerged from this study: ARS was almost completely degraded at surfactant concentrations close to 1 mM (values well above the cmc in the experimental conditions); moreover, on increasing the surfactant concentration, the photocatalytic reaction became less and less efficient and significantly dependent on the surfactant headgroup size. The presence of a maximum of efficiency depending on the surfactant concentration was due to the combination of catalytic and inhibiting processes. The first one likely depended on the ability of the surfactant to improve the ARS approach to the semiconductor through the formation of cationic bilayers on the TiO2 particles; this effect made the ARS more easily oxidized by TiO2. This catalytic action of the surfactant was opposed by the increase of the micellar aggregate number in the aqueous bulk, which competes with the TiO2 sites in associating to the dye. This was supported by the results obtained with the non-amphiphilic alkylammonium bromide. In this case a higher amount of salt must be added to reach the same maximum efficiency of ARS photooxidation. This is due to the lower capability of neutralization of these salts for both the ARS and the TiO2 surface; the inhibiting effect was not evidenced anymore.