Does humic acid form a micelle?

Abstract

Surfactants are substances that have the ability to lower the surface tension of a solution. It has been recognized for some time that the addition of humic acid to an alkaline, aqueous solution will lower the surface tension of that solution. Thus, humic acid is a surfactant. The surface tension of this solution will continue to decrease as the humic acid concentration is increased until, at some point, it becomes constant despite the addition of more humic acid. For many surfactants, this indicates the formation of aggregates known as micelles. This point is called the critical micelle concentration (or CMC). In an aqueous solution, micelles have a hydrophobic interior and a relatively hydrophilic exterior surface. The presence of a hydrophobic ‘core’ gives micelles the ability to solubilize otherwise insoluble compounds to levels many times greater than their solubility in that solvent alone or in solutions with surfactant concentrations below the CMC. The similarities between the effect of micelle-forming surfactants and humic acid on the surface tension of aqueous solutions has prompted the suggestion that humic acid forms a micelle. This hypothesis has never been rigorously tested. This paper presents a definitive test of this hypothesis based on the ability of a micelle to solubilize a hydrophobic organic compound. It is shown that humic acid will form a micelle in an alkaline aqueous solution but at extremely high concentrations (7.4 g humic acid/1).