The thermodynamic stability of lignite humic acids (sodium salt) aggregates was studied by high resolution ultrasonic spectroscopy
within the temperature interval from 5 to 90°C. The changes in differential ultrasonic velocity (U12) showed strong differences
among humic solutions within the concentration range from 0.005 to 10 g L−1. Measurement revealed several transitions which were attributed to the weakening of humic secondary structure. Concentration
around 1 g L−1 seemed to be a limit under which the change of the prevalence and importance of hydration occurred. Above this concentration
the difference in U12 decreased following the temperature increase which was explained as a dominance of hydrophilic hydration.
In contrast, below this concentration, the temperature dependence of U12 resulted in increasing tendency which was attributed
to the prevalence of hydrophobic hydration, i.e. uncovering of apolar groups towards surrounding water. Additional experiments
in which the humic sample was modified by hydrochloric acid resulted in a slight structural stabilization which lead to the
conclusion that humic micelle-like subaggregates form an open-layer assemblies easily accessible for interaction with an extraneous
molecule. That was partly verified by addition of propionic acid which brought about even larger reconformation of humic aggregates
and exhibition of polar groups towards hydration water.
The reversible changes in humate solutions induced by elevated temperatures provided the evidence about the existence of significant
physical interactions among humic molecules resulting in formation of various kinds of aggregates. The nature of aggregates,
mainly the stability and conformation, strongly depends on the concentration. Evidently, the changes observed in this work
cannot be simply explained as expansions or conformational changes of macromolecular coils.