Effect of Adsorbed Cetyltrimethylammonium at the Alumina–Water Interface on the Luminescence Quenching of Tris(2,2′-bipyridine)ruthenium(II) by 2,6-Dimethylphenolate Ions

The luminescence quenching of tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)2+3] by ion 2,6-dimethylphenolate (2,6-DMP), in the presence of cetyltrimethylammonium chloride (CTAC) micelles or monomers of the surfactant absorbed on alumina at pH 12, was studied. At this pH, the alumina has a superficial excess of negative charge. The quenching rate constants were determined from luminescence measurements, steady-state emission intensity, or luminescence lifetime. The luminescence spectrum of Ru(bpy)2+3did not show any shift in the presence of alumina in suspension with or without CTAC adsorbed on the alumina. The ultraviolet spectrum of 2,6-DMP did not show a spectral shift in the above systems, alumina suspension and CTAC–alumina suspension, either. From these facts, it was concluded that the metallic complex and the anionic 2,6-DMP do not interact with both systems. However, the efficiency of quenching by 2,6-DMP decreased in the presence of dispersed alumina at pH 12. The opposite effect, an increase in quenching efficiency, was observed when CTAC was adsorbed on alumina in concentrations lower than or similar to the critical micelle concentration (CMC). These results are discussed in terms of (1) the electrical interaction of the negative alumina surface with both partners in the quenching pair, Ru(bpy)2+3and 2,6-DMP anion, and (2) the neutralization of the surface charge by CTAC owing to the favorable alumina surface–surfactant electrostatic interaction. At CTAC concentrations higher than the CMC, the observed effect is that corresponding to the decrease in the quenching efficiency because of the micellar binding of 2,6-DMP ions.