Line tensions of galena (001) and sphalerite (110) surfaces: A molecular dynamics study
MH Anvari and QX Liu and ZH Xu and P Choi, JOURNAL OF MOLECULAR LIQUIDS, 248, 634-642 (2017).
We used molecular dynamics (MD) simulation to determine the line tension of two sulfide mineral surfaces, namely galena (PbS)(001) and sphalerite (ZnS)(110) at 298 K. Density functional theory (DFT) calculations were used to estimate the partial charges, and the force field parameters were derived based on the nature of the surfaces and the governing potential terms. The computed bulk structures, surface energies and water adsorption energies of the minerals using such partial charges and force field parameters agree well with experiment or DFT results. Contact angles of a series of nano -sized water clusters with different sizes were determined from the MD simulations. Line tensions and macroscopic contact angles of the two mineral surfaces were then determined using the modified Young's equation. The macroscopic contact angles (-84 degrees and 40 degrees for galena and sphalerite surfaces) agree well with the experimental data. The resultant line tensions are on the order of 10(-11)J/m for both surfaces but with different signs. Here, the line tension for PbS(001) with very low wettability is positive, and for the hydrophilic ZnS(110) surface is negative. Examination of the orientation of the water molecules on the two mineral surfaces showed that the tetrahedron structure present in bulk water (order parameter (q) = 0.61) was significantly perturbed on the ZnS(110) surface (q = 0.11) compared to the PbS(001) surface (q = 0.47). In particular, the average angle between water dipole moment and the surface normal and the radial distribution functions of the water molecules show that the oxygen in water molecules tends to orient itself to the cationic sites (Zn atom) in the sphalerite surface, while such orientation to the Pb atom in the galena surface is minimal. The above results explain that the favorable interaction between water molecules and sphalerite causes the microscopic contact angles to be smaller than the macroscopic value, which in turn yields a negative line tension. On the contrary, stronger affinity of water molecules to the bulk region rather than to galena surface results in bigger microscopic contact angles compared to the macroscopic value and a positive line tension. (C) 2017 Elsevier B.V. All rights reserved.
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