Molecular insights into the contrasting phase behavior of salts in supercritical water: Distinct ion clustering mechanisms of NaCl and Na2SO4

The distinct Type I (e.g., NaCl, favoring liquid–vapor coexistence) and Type II (e.g., Na 2 SO 4 , prone to direct solidification) phase behaviors of salts in supercritical water (SCW) present fundamental challenges. This study combines experimental observations with multi-scale molecular dynamics (MD) simulations and theoretical analyses to provide a molecular-level rationale for this dichotomy. MD simulations reveal that in near-critical, high-density aqueous environments, NaCl (Type I) primarily forms dynamic, solvated ion pairs and smaller aggregates. These entities significantly perturb the solvent structure and lead to the formation of a concentrated fluid brine phase with a distinct interface. In contrast, Na 2 SO 4 (Type II) is characterized by the early formation of stable and compact clusters that minimally impact the bulk solvent and readily separate from the aqueous phase, consistent with direct precipitation. Density functional theory (DFT) calculations further reveal that reduced water density enhances electrostatic interactions, and a correlation between lattice energy and phase behavior was also identified. This work provides a comprehensive mechanistic understanding of Type I/II salt behavioral dichotomy, attributing it to the ions’ varying abilities to form either extensive self-aggregates or dynamic solvated clusters, governed by a balance of fundamental energetic and kinetic factors.