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

Abstract

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 NaNO 3 and KNO 3 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.

Molecular Dynamics Simulations of Mineral Surface Wettability by Water Versus CO2: Thin Films, Contact Angles, and Capillary Pressure in a Silica Nanopore

“…These timescales necessary for equilibration have also been observed in previous MD simulation studies 96 but have not been employed in most reference MD studies of quartz− water−scCO 2 systems to date. A more detailed analysis of equilibrium time scales is provided in Figures S2 and S3.…”

“…Equilibrium configurations were then reached by running the simulation in the canonical ( NVT ) ensemble at 318 K for 20 ns, after which bulk fluid properties were isotropic and the water film between the quartz and CO 2 reached an equilibrium thickness (Figure ). These timescales necessary for equilibration have also been observed in previous MD simulation studies but have not been employed in most reference MD studies of quartz–water–scCO 2 systems to date. A more detailed analysis of equilibrium time scales is provided in Figures S2 and S3.…”

Molecular Dynamics Simulations of Nitrate/MgO Interfaces and Understanding Metastability of Thermochemical Materials

“…It is noteworthy that the parameter mixing approach was experimentally verified by Anagnostopoulos and resulted in correct contact angle for a NaNO 3 droplet on MgO slabs. 26 …”

“…The pronounced liquid/solid interface plays an important role in these systems . It was hypothesized that the reactivity enhancement could be explained quantitatively by accounting for the interfacial energy of molten salt/solid MgO in reaction thermodynamics. , While the MD simulations for nitrates on MgO were performed, it is yet unclear how the nitrate/MgO interfaces looks like, what are the interface energies, and how they change with composition of the nitrate melt, especially for the most relevant nitrates such as LiNO 3 , or eutectic mixtures. This knowledge is crucial for the in-depth understanding of the metastability in nitrate/MgO with respect to charging and discharging reactions (Figure ).…”

Curvature-Dependent Interfacial Dielectric Efficiencies of PDMS@SiO2 Nanocomposites

“…Such a curvature-dependent surface tension has been well studied and evaluated in terms of Tolman length. − Generally, smaller nanoparticle yields larger Tolman length, while high temperature reduces Tolman length. Numerous theoretical − and experimental − studies have shown that Tolman length satisfies thermodynamic consistency, providing a precise correction of interfacial curvature free energy. , In a polymer nanocomposite system, it is conceivable that the dielectric permittivity also exhibits a similar tendency at the curved interface. Although the concept of Tolman length is defined for understanding the curvature-dependent surface tension, it would be of great scientific value to extend this definition to dielectric materials.…”