Colloidal properties of humic acids and spontaneous changes of their colloidal state under variable solution conditions

Humic acids (HAs) can be dissolved or precipitated in aqueous media, can accumulate at interfaces, can form self-assemblages, can solubilize organic compounds, and can exist in different colloidal states depending on the solution conditions. Because of the randomness of formation, uniform behavior cannot be expected for the composite, polydisperse product, and only trends can be predicted for the properties. I summarize the essence of our previous results related to surface activity, micelle formation, conformational changes, and fractal character with a theoretical approach to explain the joint effect of pH and neutral salts on the colloidal stabilities of humate solutions for HAs with different origins. I present here some newer results based on CPMAS C-13-NMR, surface tension, acid-base titration, and colloidal stability investigations of a polydisperse HA sample prepared from peat and its ultrafiltered fractions. By comparing the chemical and colloidal properties of fractions and whole HA, the structural, charge, and aggregation diversities of components have been identified. Characteristic trends have been found in changes of the colloidal state of humic materials and the way in which these changes correlate with the composition of the materials.