In situ visualization of salt crystallization in sub-/supercritical water environments

Elevated temperature and pressure conditions in sub-/supercritical water environments hinder the direct observation of salt crystallization phenomena, resulting in indeterminate salt crystal morphologies. In this study, an innovative experimental apparatus has been developed to conduct in situ investigations into the crystal morphology of various types of inorganic salts. The results indicate that the critical temperature of the water-salt system is higher than that of pure water systems, with phase transition thresholds for saturated NaCl-H2O and KCl-H2O systems 671.4 K and 728.1 K, respectively. A distinct "flash crystallization" phenomenon is observed during the crystallization of inorganic salts, accompanied by different crystal morphologies. KCl crystals exhibit a dispersed particle form, NaCl crystals manifest in a finely fragmented accumulation form, and Na2SO4 crystals present transparent feather-like structures. The size of NaCl crystals formed on the surface of a Ni wire at 622.8 K through heterogeneous nucleation ranges from 200.4 mu m to 450.9 mu m. The crystallization characteristics of salt in sub-/supercritical water are influenced by various factors, including electric dipole moment interactions, common ion effect, the effect of type I salts on type II salts, and homo/heterogeneous nucleation.