Effect of surface oxidation on interfacial water structure at a pyrite (100) surface as studied by molecular dynamics simulation

• A fresh pyrite (100) surface is naturally hydrophobic.
• Contact angles reveal the hydrophilic state of the oxidized pyrite (100) surface.
• SEM found ferric hydroxide formation on the oxidized pyrite (100) surface.
• MDS reasonably predicts contact angles for fresh/oxidized pyrite (100) surfaces.
• Interaction between ferric hydroxide and water makes the surface hydrophilic.

In the first part of this paper, a SEM and contact angle study of a pyrite (100) surface is reported describing the relationship between surface oxidation and the hydrophilic surface state. In addition to these experimental results, the following simulated surface states were examined using molecular dynamics simulation (MDS): a fresh unoxidized (100) surface; polysulfide at the (100) surface; and elemental sulfur at the (100) surface. Crystal structures for the polysulfide and elemental sulfur at the (100) surface were simulated using density functional theory (DFT) quantum chemical calculations. The well known oxidation mechanism which involves formation of a metal deficient layer was also described with DFT. Our MDS results of the behavior of interfacial water at the fresh and oxidized pyrite (100) surfaces without/with the presence of ferric hydroxide include simulated contact angles, number density distribution for water, water dipole orientation, water residence time, and hydrogen-bonding considerations. The significance of the formation of ferric hydroxide islands in accounting for the corresponding hydrophilic surface state is revealed not only from experimental contact angle measurements but also from simulated contact angle measurements using MDS. The hydrophilic surface state developed at oxidized pyrite surfaces has been described by MDS, on which basis the surface state is explained, based on interfacial water structure.

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