Counterion effect of cationic surfactants on the oxidative degradation of Alizarin Red-S photocatalysed by TiO2 in aqueous dispersion

Structure effects of amphiphilic and non-amphiphilic quaternary ammonium salts on photodegradation of Alizarin Red-S catalyzed by titanium dioxide

“…The graphics reported in Fig. 1 are very similar to that already reported for the same system, ARS/TiO 2 /pH 12, but in the presence of cetyltrimethylammonium salt with different counterions (CTAX), 19 at least as regards the trend of the degradation efficiency up to 1.0 mM surfactant concentration, which corresponds to the ARS degradation maximum. Also in this case the formation of positively charged close-packed bilayers on the TiO 2 surface is supposed, 11,30 which has the role of approaching the dye to the semiconductor, otherwise impossible being both negatively charged.…”

“…In a previous paper, we reported that the TiO 2 -photodegradation efficiency of ARS assisted by cetyltrimethylammonium salts improved by increasing the pH value up to 12. 19 The pH affected both the amount of hydroxyl radicals produced by oxidation of OH À ions available on the semiconductor surface 7 and also the stabilization, due to the electrostatic attraction between the photocatalyst surface (negatively charged at this pH) and the cation radical intermediates. On the basis of these results, all the experiments presented in this work were carried out at pH 12.…”

“…Phthalic acid was also detected by HPLC-HRMS analysis (see Experimental) in our experimental condition, where the degradation of Alizarin red was performed under UV light irradiation of the semiconductor in the presence of CTABr. In this case a photocatalytic degradation mechanism should be operative, 19 according to which conduction band electrons and valence band holes are formed by TiO 2 irradiation in the primary step. 8,9 Evidently, the same crucial intermediates, such as ARS radical cations and oxygen reactive species, are involved in the photodegradation process in both the irradiation conditions.…”

“…The concentration of 50 mM, in correspondence of which the photodegradation reached its maximum value, corresponds to the situation of maximum adsorbed amount of Alizarin on the semiconductor in the presence of different salts. It is plausible that the formation of positive charged close-packed bilayers of surfactant molecules adsorbed on the TiO 2 surface, which are known to assist the ARS degradation in micellar media, 19 was achieved at lower surfactant concentrations because of the importance of non-electrostatic hydrocarbon tail-tail interactions.…”

“…[12][13][14][15] Our long-standing interest in the mechanistic investigation of TiO 2 sensitized photooxidation of various organic compound has recently led us to focus our attention on the effect of the surfactant structure; [16][17][18] the photodegradation efficiency of ARS catalyzed by TiO 2 was studied in air-equilibrated basic aqueous dispersion under UV light irradiation using surfactants differing in their counterions. 19 A series of surfactants such as cetyltrimethylammonium salts (CTAX, with X ¼…”