Effects Of Surface Energy Of Substrate On Water Density Profile At A Solid Interface

Abstract

Achieving a proper knowledge of water/substrate interfaces at the molecular-scale is essential in numerous areas of science. Recently, several experimental studies have been performed on the water density profile at a solid interface. The uncertainties associated with experimental studies on water at a solid interface calls for numerical simulations, which can provide a detailed and reliable understanding of how varying parameters at molecular scales affect the water density profile at a solid interface. The aim of this work is to investigate the microscopic structure of water at a solid surface, using molecular dynamics simulations. In particular, the water density profile at a solid substrate as a function of the hydrophobicity or hydrophilicity of the surface, which is often associated with the surface energy of the substrate ( 𝑆𝐸 𝑠 ), is investigated. The substrate material chosen is silver, which is originally a high energy material with surface energy equal to 𝑆𝐸 𝐴𝑔 . The surface energy of the substrate is modified to adjust the strength of interaction energy between water and surface. The results show that the water density profile as a function of the vertical distance from the substrate (+ z ) depends on the surface energy of the substrate. There are two peaks in the water density profile at z = 2.5 Å and z = 5.5 Å for the unmodified silver substrate ( 𝑆𝐸 𝑠 = 𝑆𝐸 𝐴𝑔 ) indicating that the water molecules form a double-layer structure. The double-layer structure and the vertical distance of each layer from the substrate remains unchanged but the density of both layers decreases when 𝑆𝐸 𝑠 decreases to 40% of its initial value. The second layer at z = 5.5 Å disappears when 𝑆𝐸 𝑠 is reduced to 20% of its original value. The layered structure disappears when 𝑆𝐸 𝑠 reduces to 10% of its original value, and the water density becomes lower that its bulk value for z < 3 Å. The results indicate that on both hydrophobic and hydrophilic surfaces, interfacial water layers possess properties different from those of the bulk water. On hydrophobic surfaces, weakening of water-surface interaction strength results in dewetting and a water-vapor interfacial