Second harmonic generation is one of the most powerful techniques used to selectively probe interfaces of all types. The direct ab initio method developed here allows predicting the signal and highlights the importance of local and non-local effects.

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Density functional theory simulations, including a correction for dispersive interactions, were performed to investigate the adsorption of water on the main cleavage plane of the fluorite, namely, the (111) surface. In the case of a single molecule of water, we observe that the molecular form is preferred over the dissociated one, and absorbs on the surface with an energy of −55 kJ mol −1 , including a significant contribution from the dispersion forces. Also, we show that the substitution of a fluorine atom by a hydroxyl group on the surface of fluorite is not energetically favorable. Then, the hydration of the surface in function of the coverage by water molecules was studied in a systematic way. It was shown that the geometries involving the formation of a cluster of water molecules on the surface, with half of the molecules adsorbed, are the most favorable. Finally, ab initio molecular dynamics conducted at 300 K confirms the trends observed at 0 K, albeit the adsorption energies are reduced by about 10 kJ mol −1 . Also, we observe that once put in the interaction with a large number of water molecules, half of the calcium atoms at the surface are in close interaction with a water molecule, whereas the rest of the molecules are further away but present a relatively well-defined structure showing similarities with the one of water clusters.

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The adsorption of fatty acids with various chain structures on the (111) fluorite surface is investigated using density functional theory, including a correction for dispersive interactions. In the case of the acidic form, we observe that the molecular form is preferred over the dissociated one and the molecule adsorbs on a surface calcium atom with an energy of −78.2 kJ mol −1 . Also, we show that the carboxylate anion adsorbs on the surface under two possible configurations, a bidentate binuclear one or a monodentate one, the bidentate binuclear being favored. At both 0 and 300 K, the chain length does not affect the geometry of the carboxyl group but it strongly impacts the global geometry of the molecule adsorption on the fluorite surface: the "flat" adsorption mode, i.e., when the molecule is parallel to the surface, is favored when the number of carbon atoms is equal to or higher than 6, due to dispersion forces. However, when the molecule is in hydrated condition, the chain folds up by itself to reduce the interactions with water while the carboxylate group adsorbs in monodentate configuration. In aqueous conditions, the chain length does not impact anymore the adsorption energies, the vertical adsorption mode being always favored.

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Recently, tungsten has drawn worldwide attention considering its high supply risk and economic importance in the modern society. Skarns represent one of the most important types of tungsten deposits in terms of reserves. They contain fine-grained scheelite (CaWO 4) associated with complex gangue minerals, i.e., minerals that display similar properties, particularly surface properties, compared to scheelite. Consistently, the froth flotation of scheelite still remains, in the twenty first century, a strong scientific, industrial, and technical challenge. Various reagents suitable for scheelite flotation (collectors and depressants, mostly) are reviewed in the present work, with a strong focus on the separation of scheelite from calcium salts, namely, fluorite, apatite, and calcite, which generally represent significant amounts in tungsten skarns. Albeit some reagents allow increasing significantly the selectivity regarding a mineral, most reagents fail in providing a good global selectivity in favor of scheelite. Overall, the greenest, most efficient, and cheapest method for scheelite flotation is to use fatty acids as collectors with sodium silicate as depressant, although this solution suffers from a crucial lack of selectivity regarding the above-mentioned calcium salts. Therefore, the use of reagent combinations, commonly displaying synergistic effects, is highly recommended to achieve a selective flotation of scheelite from the calcium salts as well as from calcium silicates.