Polythermal Solubility and Coordination Chemistry in CuSO₄–Hexamethylenetetramine–H₂O and CuSO₄–Monoethanolamine–H₂O Systems for the Development of Low-Phytotoxic Copper-Based Fungicides

The investigation of phase equilibria and solubility in multicomponent aqueous systems is important for understanding crystallization processes, coordination interactions, and the design of functional metal-containing materials. In this study, the phase equilibria and polythermal solubility of the CuSO₄–(CH₂)₆N₄–H₂O and CuSO₄–HOCH₂CH₂NH₂–H₂O ternary systems were investigated by the visual polythermal analysis method over the temperature range from −19.0 to +40.0 °C. The aim of the work was to establish phase relationships and evaluate the potential of hexamethylenetetramine and monoethanolamine as physiologically active ligands for the development of new copper-based fungicidal formulations with reduced phytotoxicity.

Based on the experimental data, polythermal solubility diagrams were constructed and crystallization fields were determined. The phase analysis revealed the crystallization of ice, CuSO₄, CuSO₄·5H₂O, (CH₂)₆N₄, (CH₂)₆N₄·6H₂O, and ligand-containing coordination phases depending on temperature and solution composition. In the CuSO₄–(CH₂)₆N₄–H₂O system, a new coordination compound with the composition [CuSO₄·(CH₂)₆N₄·4H₂O] was identified. Its composition and structure were confirmed by chemical and infrared spectroscopic analyses. The spectroscopic results indicate coordination between Cu(II) ions and nitrogen donor atoms of hexamethylenetetramine, leading to the formation of a stable hydrated complex.

The formation of copper–ligand complexes in the investigated systems may reduce the concentration of free Cu²⁺ ions in solution, which is expected to decrease the phytotoxic effects associated with conventional copper sulfate preparations while maintaining their potential antifungal properties. The obtained results provide new insights into the phase behavior and coordination chemistry of CuSO₄-containing aqueous systems and may serve as a scientific basis for the development of environmentally safer copper-based fungicides and agrochemical formulations