Study on the phase transition characteristics of nano-hydrated salts based on molecular dynamics simulations

• Simulation deviations of melting/solidification temperatures are within 3.24 ℃. • Large electrostatic energy, low van der Waals and binding energies cause supercooling. • Large thermal conductivity of SiO 2 and micro-convection reduce melting temperatures. • Distributions of molecules and ions around SiO 2 affect solidification temperatures. Hydrated salts possess great competitiveness in terms of enhancing utilization efficiency of heat due to large densities of heat storage and low costs. However, the existence of supercooling degree is the common phenomenon for hydrated salts, which might restrict the application prospects in partial fields. This work focuses on hydrated salts with relatively larger latent heats, namely Na 2 CO 3 ·10H 2 O and Na 2 HPO 4 ·14H 2 O in the absence and presence of SiO 2 as well as eutectic hydrated salt prepared with 40 wt% Na 2 CO 3 ·10H 2 O and 60 wt% Na 2 HPO 4 ·12H 2 O. Their melting/solidification temperatures were determined and the supercooling behavior was analyzed based on molecular dynamics simulations. The simulation results of melting/solidification temperatures present small deviations of 0.17–3.24 ℃ in comparison with the experimental data. Additionally, the aggregation phenomenon among salt ions and dispersion phenomena among water molecules and between water molecules and salt ions for supercooled liquid hydrated salts are revealed, which are attributed to stronger electrostatic interactions and weaker van der Waals interactions as well as lower binding energies. Moreover, SiO 2 not only lowers the melting temperatures due to its larger thermal conductivity than hydrated salts and the existence of micro-convection, but affects solidification behavior of hydrated salts. The different influences of SiO 2 on the solidification temperatures of Na 2 HPO 4 ·14H 2 O and Na 2 CO 3 ·10H 2 O were associated with dispersion degree between water molecules and salt ions around SiO 2 , which would be attributed to different strengths of interaction forces between SiO 2 and salt ions. On these bases, this work provides new perspectives for clarifying the phase change characteristics of nano-hydrated salts.