EFFECT OF pH ON MOLECULAR ASSOCIATIONS IN FULVIC ACID SOLUTIONS

The effect of alkalinization of fulvic acids (FA) solutions on an increase in their optical density is studied. To explain the observed phenomenon, the investigation of FA particles in solutions with different pH was carried out using scanning electron microscopy (SEM) and scanning tunneling microscopy (STM). It was found that with an increase in pH, there is a noticeable decrease in the size of the observed supramolecular formations from FA from about 200 to 100 nm, and when studying FA samples on STM, it was shown that FA in solutions exist in the form of supramolecular formations several hundred nanometers in size, formed by particles-molecules of FA 10–20 nm. An explanation of the observed phenomenon was proposed based on existing ideas about the supramolecular fractal-cluster organization of humic substances (HS). It consists in the fact that the upper layer of F-clusters disintegrates into particles-molecules of FA with a decrease in their size observed with the help of SEM. At the same time, particles-molecules of FA due to their small size are not visible with the help of SEM, but they are well visualized when using STM.