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)(3 )(2+)) by 2-, 3-, and 4-nitrophenol (NO2PhOH) was studied in the absence an d presence of aggregates of sodium dodecyl sulfate (SDS) in the bulk soluti on and in the adsorbed state on a-alumina at pH 2. At this pH, the alumina has a superficial excess of positive charge. The uncorrected luminescence s pectrum of Ru(bpy)(3)(2+) did not show any spectral shift in alumina suspen sion in the absence of SDS in relation to the spectrum in aqueous solution at pH 2 (lambda(max) = 609 nm). In the presence of 1 g/L alumina and concen trations of SDS between 0.6 and 0.8 mM and the critical micellar concentrat ion (CMC), the surfactant forms surface aggregates-hemimicelles or admicell es, In these surface aggregates, the emission spectrum showed a red shift ( lambda(max) = 637 nm). At SDS concentrations higher than the CMC, the surfa ctant forms micelles in the bulk solution and surface aggregates onto a-alu mina, However, from the spectral shift observed with the increase in SDS co ncentration, it seems that, at high surfactant concentrations in the bulk, the metallic complex prefers to remain in the micellar instead of in the he mimicellar phases. The emission maximum in micelles was in 627 nm. The Ster n-Volmer constants (K-SV) of luminescence quenching were determined from st eady-state emission intensity measurements at the maximum of emission in ea ch case. The luminescence quenching of Ru(bpy)(3)(2+) showed that the NO(2) PhOHs are incorporated into both types of aggregates. On the basis of the K -SV values as a function of the SDS concentration, the process of luminesce nce quenching of Ru(bpy)(3)(2+) by these compounds is more efficient in pre micelles/micelles than in the surface aggregates onto a-alumina. This was o bserved 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 concen trations higher than 5 mM, the plot of K-SV vs SDS concentration in the pre sence 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 expl ained in terms of the surfactant that forms surface aggregates onto alumina which do not contain the metallic complex. (C) 2000 Academic Press.