Size Separation and Characterization of Fractions Involved in Humic Acid Self-Assembly.

Humic acids are important natural organic matter components of soils. Our previous studies have shown that the lipid-like (L₁), the amphiphilic (HA₂) and nonamphiphilic fractions (HA₁) of humic acid have different contributions to the formation of molecular aggregates. In this work we characterized the amphiphilic and nonamphiphilic fractions isolated from two humic acid samples (Guanella Peat Pass and Leonardite) with different origins and chemical compositions. Their size distribution was investigated by ultrafiltration, the carbon-type distributions of fractionated samples was measured by solid-state ¹³C DPMAS NMR, the morphology of the solid samples was determined by scanning electron microscopy, and the surface activity of the size fractions were determined using tensiometric measurements.

Data show that the size distribution of the amphiphilic fraction (HA₂) shifts towards lower values for both Guanella Peat Pass and Leonardite samples compared to the nonamphipilic fraction (HA₁). Under the same conditions the SEM images show formation of fractal aggregates on the substrate surface in case of HA₁, while only small particles can be seen for the HA₂ fraction. It was also found that not all the size-fractionated samples show surface activity. The relationship between the chemical nature and the surface activity was investigated by NMR techniques.