Fluorescence of Zn(II) 8-hydroxyquinoline complex in the presence of aqueous micellar media: The special cetyltrimethylammonium bromide effect

Mixtures of Zn(II) and 8-hydroxyquinoline (8QOH) in a 1:2 proportion, in aqueous solutions, result in fast complexation, followed by precipitation. Addition of 0.05 M sodium dodecyl sulfate (SDS), N  -dodecyl-N,NN,N-dimethylammonio-1-propanesulfonate (SB3-12), N  -hexadecyl-N,NN,N-dimethylammonio-1-propanesulfonate (SB3-16) or Triton X-100 results in considerable retardation of precipitation. In the presence of SDS, SB3-12, SB3-16 and Triton X-100 the 8QOH chelates are only kinetically stable in solution and after 24 h, the precipitation is almost quantitative. Conversely, upon addition of the cationic surfactant hexadecyltrimethylammonium bromide (CTABr), the absorbance of the complex remains constant even after at least six months. The interaction of the ligand 8QOH (and of the (8QO)2Zn(II) complex) with the cationic surfactant was studied by ultraviolet and NMR spectroscopy and 8QOH has a pKa=9.05pKa=9.05 in the presence of the cationic surfactant and the ligand intercalates in the micelle, being preferentially located near the headgroup of the micelle. Although the solubilization site of the (8QO)2Zn(II) complex is similar to that of 8QOH, the interaction of the aromatic moiety with the CTA+ headgroup is much stronger, due to the increased electron density in the aromatic ring of the ligand. As a consequence of this interaction, sphere to rod transition and an increase in microscopic and macroscopic viscosity are observed.

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