Structural Organization of Humic Acid in the Solid State

Studies of the structural organization of humic acid in solution have suggested that it is composed of three fractions, two humic acid-like and one lipid-like, that have distinct roles in the process of its self-organization. The present work extends the study of humic acid’s structural organization to the solid state by directly comparing its organization with a physical mixture composed of its three fractions using differential scanning calorimetry. Comparative measurements of the specific heat capacity as a function of temperature reveal differences between the original humic acid and the mixture with the same chemical composition. These differences provide direct evidence that humic acid’s solid state structure is more than just a mixture of components and is determined by specific interactions between its components. This study indicates that humic acid in the solid state has a hierarchical or “structure within a structure” architecture. The lower-level structure is determined by the self-assembly of amphiphilic components of humic acid with lipids into a nanostructured composite material. A higher-level structure is formed by the association of this composite material with the nonamphiphilic components of humic acid.