Molecular dynamics elucidation of melting and solidification mechanisms in sodium sulfate decahydrate for solar thermal storage

Sodium sulfate decahydrate (Na₂SO₄·10H₂O) is a widely recognized and highly promising phase change material. However, its phase transition process is notably complex and often accompanied by significant phase segregation. To date, most studies have primarily focused on the effects of temperature, pressure, and other external factors on the phase transition process, with limited attention given to systematically exploring the underlying mechanisms and evolutionary characteristics of the phase transition. This study employs molecular dynamics simulations to investigate the microscopic mechanisms of the melting and solidification processes of Na₂SO₄·10H₂O. A comparative analysis of five force field models—CVFF, ClayFF, ReaxFF, Gromos, and Dreiding—identified the Dreiding force field as the most suitable for describing the phase transitions. Melting is driven by dehydration and salt dissolution, where coordinated water around Na⁺ dehydrates first, followed by hydrogen-bonded water, with a melting temperature range of 293 K–313 K, consistent with experimental data (about 305.4 K). Solidification begins with supersaturation, forming small sodium sulfate clusters that hydrate into crystallized Na₂SO₄·10H₂O, with a temperature range of 273 K–293 K and a supercooling degree of approximately 20 K. These findings provide theoretical guidance for optimizing sodium sulfate-based energy storage systems and mitigating phase segregation.