Molecular mechanisms of Na₂SO₄ nucleation in sub-/supercritical water: Ion pair stability and hydrogen bond disruption

Salt precipitation easily occurs in supercritical water oxidation reactors due to the unique properties of supercritical water. Revealing nucleation mechanisms of typical inorganic salts is crucial for avoiding reactor plugging caused by salt precipitation. In this work, molecular dynamics simulation of Na 2 SO 4 nucleation was conducted at 573–1073 K and 22–28 MPa. The results indicate that higher temperatures, lower densities, and higher Na 2 SO 4 concentrations promote ion pairing and nucleation. RDF analysis shows that as temperature increases, the strength of interactions between Na + –SO 4 2- ions significantly increases (1.93 Å). PMF analysis confirms that contact ion pairs (CIPs) are the dominant configuration. Hydrogen bond strength decreases sharply with temperature, leading to weakened solvation ability and promoting ion aggregation. Under conditions of 873 K and 25 MPa, the nucleation process is completed within 250 ps, with a nucleation rate reaching 10 35 m −3 ·s −1 . These results emphasize the importance of controlling temperature, pressure, and concentration to prevent salt deposition and optimize the reliability of SCWO systems.