The effect of molecular size on humic acid associations

Aqueous solutions of two humic acids were subjected to UV photolysis, resulting in chain scission of the solute. The molecular fragments were found to have diminished detergent properties, indicated by a reduced tendency to associate with small hydrophobic species. This is ascribed to the inability of smaller and stiffer humic acid segments to form structured pseudomicellar domains that can sequester nonpolar species. It was noted that photolysis is more likely to involve covalent bond breakage than dissociation of hydrogen bonded conglomerates. An association index, based on fluorescence anisotropy, provided a quantitative measure of humic acid associations.