Thermodynamic Description of Interfaces Applying the 2PT Method on ReaxFF Molecular Dynamics Simulations

Abstract

The interface between liquid water and the Pt(111) metal surface is characterized structurally and thermodynamically via reactive MD simulations within the ReaxFF framework. The formation of a distinct buckled adsorbate layer and subsequent wetting layers is tracked via the course of the water’s density and the distribution of the H2O molecules with increasing distance to the metal surface. Hereby, also the two-phase thermodynamics (2PT) method has been utilized for studying the course of entropy as well as the translational, rotational, and vibrational entropic contributions throughout the Pt(111)|H2O interface. A significant reduction of the entropy compared to the bulk value is observed in the adsorbate layer (S = 31.05 ± 2.48 J/mol K) along with a density of 3.26 ± 0.06 g/cm3. The O–O interlayer distribution allows for direct tracing of the water ordering and a quantified comparison to the ideal hexagonal adlayer. While the adsorbate layer at the Pt surface shows the occurrence of hexagonal motifs, this near-order is already weakened in the wetting layers. Bulk behavior is reached at 15 Å distance from the Pt(111) metal. Introducing an electric field of 0.1 V/Å prolongs the ordering effect of the metal surface into the liquid water.