Experimental methodologies for assessing the surface energy of highly hygroscopic materials: The case of nanocrystalline magnesia

Measuring the surface energy of highly hygroscopic materials has remained a thorny problem for many years, mainly because obtaining an anhydrous surface state and maintaining this condition during the surface energy assessment has been considered an impractical task. In this work, we developed synthetic and calorimetric approaches that overcome these difficulties and applied them to measure the surface energy of anhydrous nanocrystalline magnesium oxide. Anhydrous MgO with specific surface area of ∼300 m² g ^-1 was synthesized by laser ablation in a controlled oxygen partial pressure environment. High resolution transmission electron microscopy and X-ray diffraction showed cubic nanoparticles with sizes ranging from 5 to 10 nm (as controlled by the partial pressure) and with the periclase crystal structure. The surface energy of the anhydrous state was assessed using high temperature oxide melt drop solution calorimetry and differential scanning calorimetry; the surface energies were 1.2 ± 0.1 and 1.3 ± 0.1 J m^-2, respectively. These values are slightly higher than from previously reported experiments and are consistent with a less hydrated surface.