title: Evidence from surface tension and fluorescence data of a pyrene-assisted micelle-like assemblage of humic substances.
abstract: Surface activity and fluorescence of humic substances (HS) and HS/pyrene solutions were monitored under various pH conditions. For HS alone the surface tension of the solutions decreased with increasing acidity, with a minimum at around pH 4. This effect, which is a consequence of an increase in the amphiphilic character of structures, is much more pronounced in humic (HA) than in fulvic acids (FA). The addition of pyrene (0.1 micromolL(-1)) results, for HA, in a marked reduction in the migration of amphiphilic species to the solution surface. FA profiles are not modified in presence of pyrene at that concentration. A decrease in the pyrene I1/I3 ratio in HS solutions shows that below pH 9 pyrene molecules react progressively to the change to a more hydrophobic environment, the greatest effect being observed at around pH 6 to 7. These signals are followed by a significant increase in the pyrene excimer fluorescence (lambda(exc)/lambda(em)=334 nm/450 nm), which is a consequence of the proximity of pyrene molecules. For FA, the I1/I3 decrease is less significant and no excimers develop. This set of effects is explained in view of conformational adjustments of HS, mainly HA, which become arranged in micelle-like domains in aqueous solution, the aromatic moieties being assembled around the pyrene molecules.
authors.affiliation: Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil. sierra@qmc.ufsc.br
authors.forename: M M D
authors.initials: MM
authors.lastname: Sierra
authors.forename: T G
authors.initials: TG
authors.lastname: Rauen
authors.forename: L
authors.initials: L
authors.lastname: Tormen
authors.forename: N A
authors.initials: NA
authors.lastname: Debacher
authors.forename: E J
authors.initials: EJ
authors.lastname: Soriano-Sierra
mesh_terms: D001578:Benzopyrans / Q000032:analysis*; D004784:Environmental Monitoring; D005453:Fluorescence; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D008823:Micelles; D014865:Waste Disposal, Fluid
chemical_list: D001578:Benzopyrans; D006812:Humic Substances; D008823:Micelles; C005023:fulvic acid