The Effect of Adsorbed Sodium Dodecylsulfate at the Alumina–Water Interface on the Luminescence Quenching of Tris(2,2′-bipyridine) ruthenium(II) by Nitrophenols

The luminescence quenching of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+) by 2-, 3-, and 4-nitrophenol (NO2PhOH) was studied in the absence and presence of aggregates of sodium dodecyl sulfate (SDS) in the bulk solution and in the adsorbed state on α-alumina at pH 2. At this pH, the alumina has a superficial excess of positive charge. The uncorrected luminescence spectrum of Ru(bpy)32+ did not show any spectral shift in alumina suspension in the absence of SDS in relation to the spectrum in aqueous solution at pH 2 (λmax=609 nm). In the presence of 1 g/L alumina and concentrations of SDS between 0.6 and 0.8 mM and the critical micellar concentration (CMC), the surfactant forms surface aggregates—hemimicelles or admicelles. In these surface aggregates, the emission spectrum showed a red shift (λmax=637 nm). At SDS concentrations higher than the CMC, the surfactant forms micelles in the bulk solution and surface aggregates onto α-alumina. However, from the spectral shift observed with the increase in SDS concentration, it seems that, at high surfactant concentrations in the bulk, the metallic complex prefers to remain in the micellar instead of in the hemimicellar phases. The emission maximum in micelles was in 627 nm. The Stern-Volmer constants (KSV) of luminescence quenching were determined from steady-state emission intensity measurements at the maximum of emission in each case. The luminescence quenching of Ru(bpy)32+ showed that the NO2PhOHs are incorporated into both types of aggregates. On the basis of the KSV values as a function of the SDS concentration, the process of luminescence quenching of Ru(bpy)32+ by these compounds is more efficient in premicelles/micelles than in the surface aggregates onto α-alumina. This was observed up to a concentration of 5 mM SDS. These results can be interpreted as an effect of the increase in polarity of the micellar microenvironment on the electron-transfer mechanism for the quenching process. At SDS concentrations higher than 5 mM, the plot of KSV vs SDS concentration in the presence of 1 g/L alumina is shifted to higher SDS concentrations in relation to the observed plot in the absence of 1 g/L alumina. This fact can be explained in terms of the surfactant that forms surface aggregates onto alumina which do not contain the metallic complex.