Research on urea-ammonium chloride cocrystal nitrogen fertilizer and the solubility based on terahertz and multi-technology joint characterization

Urea is a nitrogenous fertilizer characterized by high water solubility, yet it suffers from a relatively low nitrogen utilization efficiency. It not only pollutes the environment but also reduces agricultural productivity. Developing the physicochemical properties of nitrogen fertilizers is the key to improve the nitrogen utilization efficiency. In the present investigation, a novel 1:1 urea-ammonium chloride co-crystal system was synthesized through solvent evaporation methodology. This co-crystallization approach aimed at optimizing the physicochemical characteristics of urea. The formation of cocrystal was confirmed by a variety of characterization techniques, including PXRD, and DSC, TGA, SEM, Raman spectroscopy, and the crystal structure and purity were verified. The spectral properties of the material were measured using the THz-TDS system, which can effectively characterize the formation of cocrystal. The crystal structures and theoretical THz spectra of urea, ammonium chloride and the corresponding cocrystal were simulated by using density functional theory calculations. The solubility of the synthesized cocrystal was evaluated under greenhouse conditions, and the cocrystal's urea solubility was 36.15 g/100 mL, a ~ 66% reduction from pure urea's 100–120 g/100 mL. This work provides valuable methodological insights for agricultural cocrystal development and highlights the potential of cocrystal urea as a slow-release nitrogen fertilizer.