An association index based on the fluorescence anisotropy of a dipheny
loxazole probe is introduced as a quantitative parameter for the estim
ation of the associations of humic acid with nonpolar organic compound
s. The variations of the index were consistent with the behavior of di
ssolved humic acids in response to solution conditions. These included
salt concentration, pH, humic acid concentration, probe concentration
, and type of humic acid. The evidence provided further support for th
e formation of humic pseudomicelles that have the ability to sequester
hydrophobic species in the solution environment. The molecular parame
ters governing this behavior include size and flexibility. The associa
tion indices of humic acid size fractions suggest the existence of an
intermolecular component in the association process, leading to pseudo
micellar behavior that depends on the polydispersity of the humic acid
solution.