Probing the organization of fulvic acid using a cationic surfactant

Abstract

Abstract The organization of Suwannee River Fulvic Acid (SRFA) is investigated through the interaction of SRFA with a cationic surfactant molecule (Dodecyl-trimethylammonium chloride (DTAC)). Turbidity measurements, Dynamic light scattering, electrophoretic mobility, surface tension, and cryo-transmission electron microscopy are combined to describe the SRFA/DTAC molecular structures thus obtained. Increasing DTAC concentration, fulvic acid-rich unilamellar vesicles, globules, aggregates of globules, disks, DTAC-rich unilamellar vesicles, and loosely aggregated spheroidal structures, are successively observed. Such sequence of molecular structures is typically found in phase diagrams of catanionic systems (mixture of oppositely charged surfactants). The strong surface activity of fulvic acid/DTAC complexes and the geometrical constraints associated with the formation of vesicles imply that a major component of SRFA is an individual amphiphile negatively charged of molecular size similar to that of DTAC. This soft matter approach supports a supramolecular organization for SRFA.