Molecular Insights into the Precipitation Suppression of Salts in Near-Critical Water: Dielectric Restructuring and Competitive Coordination in NaCl–Na2SO4 Mixtures

The complex aggregation and phase behaviors of inorganic salts in near-critical aqueous environments remain a key challenge for high-temperature fluid technologies. In multicomponent brines where highly soluble and aggregation-prone salts coexist, competitive interactions introduce significant physical and chemical complexities. Using a polarizable force field to capture dynamic polarization and dielectric responses under extreme conditions, this study investigates the microscopic ion aggregation mechanisms in ternary NaCl–Na2SO4–H2O brines. By evaluating the structural, dynamical, and energetic evolution, we elucidate mechanisms governing the suppression of agglomeration. Results indicate that introducing highly soluble NaCl retains macroscopic solvent density via electrostriction. This profound electrostriction resists severe thermal expansion, preserving a resilient, liquid-like dielectric screening capacity. Acting in synergy with the competitive coordination and steric hindrance from abundant chloride ions, this sustained dense fluid environment thermodynamically forces the precipitation-prone sulfate anions to remain in a solvated and dispersed regime. This work provides a molecular-level understanding of anomalous solubility retention in mixed near-critical brines, offering theoretical insights for managing scaling in high-temperature aqueous systems.