The proposed pseudomicellar character of five dissolved humic acids has been investigated by monitoring the fluorescence behavior of a pyrene probe added to the solutions. Fluorescence quenching by bromide in the presence of these humic acids was found to be consistent with a hydrophobic microenvironment. The presence of metal ions, with either quenching or nonquenching counterions, increased the pyrene fluorescence of several of the humic acids. The results indicate that cations have the property of increasing intramolecular micellization in these humic acids, thereby minimizing quenching interactions. The proposed mechanism involves enhanced shielding of the probe rather than its expulsion into the aqueous bulk as has been suggested previously. The humic acids that show this micelle-like microorganization also display increased pyrene fluorescence upon heating, which can work in conjunction with metal ion interactions. The observations suggest that a series of events take place that are comparable to the clouding behavior of nonionic detergent solutions. The microorganization progresses through the equivalent of "intramolecular clouding" to the customary intermolecular analogue, leading to a separate humic acid phase.

The cation-enhanced formation of hydrophobic domains in aqueous humic acids has been shown to be a slow process, consistent with the evolution and disintegration of humic acid configurations over periods lasting from days to weeks. After the addition of a magnesium salt to a humic acid solution containing pyrene, the probe fluorescence gradually increased and reached a maximum in 2−10 h. The trend then reversed, and the emission intensity decreased until it came close to its original value after an extended period. In the model proposed here, this self-reversing process is ascribed to a slow migration of metal ions within the humic structure. When the salt was added, the cations first associated with anionic groups on the humic acid polymer, partially neutralizing them and engaging in bridging interactions between different parts of the chain. This drew the humic acid molecule together and enhanced the formation of hydrophobic pseudomicelles. These structures were however of a transitory nature, and they disintegrated as the metal ions gradually migrated to locations where they formed stable inner-sphere complexes with functional groups on the humic acid molecule.

An association index based on the fluorescence anisotropy of a diphenyloxazole probe is introduced as a quantitative parameter for the estimation of the associations of humic acid with nonpolar organic compounds. The variations of the index were consistent with the behavior of dissolved 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 the formation of humic pseudomicelles that have the ability to sequester hydrophobic species in the solution environment. The molecular parameters governing this behavior include size and flexibility. The association indices of humic acid size fractions suggest the existence of an intermolecular component in the association process, leading to pseudomicellar behavior that depends on the polydispersity of the humic acid solution.

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