Decoupling heats of immersion and dissolution of mineral powders in solution to assess wettability

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Wettability is an important property of powders in many applications such as inks and paints, the food and pharmaceutical industries, as well as flotation separation of mineral particles. It is known that the enthalpy of immersion is a useful method to quantify the hydrophobicity or wettability of a powder since it measures the heat evolved when water molecules interact with the powder surface. However, in the present investigation it was found that for the powders investigated, viz. the minerals, galena, quartz, pyrite, pyrrhotite, chalcopyrite, sphalerite, quartz and albite, an initial endotherm was repeatedly observed, and it was hypothesized that this may be due to the endothermic dissolution of ions from the minerals as a primary step. Various methods of supressing dissolution were investigated, from using saturated solutions of the minerals, to coating of the mineral surface with surfactant, to the use of a non-solvating wetting liquid. The last two of these procedures were found to be most successful in terms of inhibiting the presence of an endotherm. These results provide strong evidence to support the hypothesis that the endotherm is due to the dissolution of ions. In the case of using hexanol, a non-solvating wetting liquid, the resulting exothermic responses provide a relative scale of hydrophobicity values for different mineral surfaces even though it is not directly measuring the mineral-water interaction which is fundamental to the concept of hydrophobicity. The methodology was extended to the measurement of heats of immersion of binary mixtures of powders and extrapolated to predict the heat of immersion, and by extension the hydrophobicity, of the pure components.