Wettability of NaNO3 and KNO3 on MgO and Carbon SurfacesUnderstanding the Substrate and the Length Scale Effects

Inorganic salt-based ceramic composites have extended thermal energy storage applications particularly when the salts are used as phase change materials. The wetting behavior of interacting materials in such cases can significantly affect the formulation and manufacturing processes of the composites. Carbon is often employed in these composites, which takes the form of nanotubes or flakes, for either or both shape stabilization and heat transfer enhancement. We studied the wettability of molten NaNO3 and KNO3 on MgO (1 0 0) and several carbon surfaces both experimentally and by molecular dynamics modeling. The contact angle was found to be well predicted by atomistic simulations. Density, viscosity, and surface tension of the molten salts were also studied by the molecular modeling, and results were in good agreement with experiments. The contact angle of both salts was found to be significantly higher on the carbon surface than that on the MgO surface. A carbon nanotube was also studied to examine the confinement effects. In all cases, nonwetting behavior was observed. A sensitivity analysis was performed, and the results showed a significant increase in the contact angle with increasing droplet size up to 10 nm, which was higher than that predicted by the approximation of the Tolman length. The line tension was found to be positive for polar ceramic surfaces and negative for nonpolar carbon ones. Its absolute value was in the range of 10–11 N/m, independent of polarity. Furthermore, the density of the salt inside the nanotube was observed to increase with the tube diameter, reaching the bulk value when the tube diameter was above 6 nm. Conclusively, in this work, it is demonstrated that the wetting trend of molten nitrate salts can be satisfactorily simulated by a small-scale molecular dynamics system and that polarity and roughness effects can be also accounted for correctly.