RESUMO
The luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) by 2-, 3-, and 4-nitrophenol (NO(2)PhOH) was studied in the absence and presence of aggregates of sodium dodecyl sulfate (SDS) in the bulk solution and in the adsorbed state on alpha-alumina at pH 2. At this pH, the alumina has a superficial excess of positive charge. The uncorrected luminescence spectrum of Ru(bpy)(3)(2+) 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 (lambda(max)=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 (lambda(max)=637 nm). At SDS concentrations higher than the CMC, the surfactant forms micelles in the bulk solution and surface aggregates onto alpha-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 (K(SV)) 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)(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 luminescence quenching of Ru(bpy)(3)(2+) by these compounds is more efficient in premicelles/micelles than in the surface aggregates onto alpha-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 K(SV) 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. Copyright 2000 Academic Press.
RESUMO
Binding interactions of alpha-diimine-chromium(III) complexes with sodium dodecyl sulfate (SDS) were studied in air-saturated solution and in N2-purged solution using lifetime measurements. The lifetime titration curves consisted of two regions with different slopes: one of them negative and more pronounced at low concentrations of SDS until reaching a minimum and the other, at a later stage, with a less pronounced positive slope till a plateau was reached. This biphasic behavior of lifetime-SDS concentration data revealed the presence of premicellar aggregates at low SDS concentration and the formation of normal micelles at high surfactant concentration. The results were analyzed with a model that includes binding of the sensitizer to micelles and to small premicellar aggregates. Binding constants obtained by fitting of lifetime titration curves are in agreement with the electrostatic interactions between the complexes and the surfactant and with the hydrophobic interactions between the ligands of the complexes and the formed micelle. Copyright 1999 Academic Press.
RESUMO
The luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) by 2-, 3-, and 4-nitrophenol (NO2PhOH), in their protonated forms, has been studied by static and time-resolved techniques in water and sodium dodecyl sulfate (SDS) aqueous solution. In water, the bimolecular rate constants were obtained separately from both techniques and they were coincident. The values were close to the diffusional limit. In the presence of SDS micelles at a fixed detergent concentration, the Stern-Volmer plots of tau0/tau vs quencher total concentration ([Q]t) were linear in the whole range of [Q]t used. However, the Stern-Volmer plots of the ratio I0/I vs [Q]t in SDS micelles showed an upward deviating curve. From these plots of I0/I, the concentrations of the NO2PhOHs in the micellar pseudophase were measured. The treatment of the data of both types of measurements let us determine the association constants (K) of the NO2PhOHs to SDS micelles and the exit (k-) and entrance (k+) rate constants of NO2PhOHs to the micelles. The K values obtained from either static or time-resolved luminescence techniques were in agreement within experimental error. An electron-transfer mechanism was also proposed for the quenching reaction of the excited state Ru(bpy)32+ by nitrophenols in which the metallic complex is oxidized. Copyright 1997 Academic Press. Copyright 1997Academic Press