Influence of surface topology and electrostatic potential on water/electrode systems

Abstract

We have used the classical molecular dynamics technique to simulate the ordering of a water film adsorbed on an atomic model of a tip of a scanning tunneling microscope approaching a planar metal surface. For this purpose, we have developed a classical model for the water–substrate interactions that solely depends on the coordinates of the particles and does not require the definition of geometrically smooth boundary surfaces or image planes. The model includes both an electrostatic induction for the metal atoms (determined by means of an extended Lagrangian technique) and a site-specific treatment of the water–metal chemisorption. As a validation of the model we have investigated the structure of water monolayers on metal substrates of various topology [the (111), (110), and (100) crystallographic faces] and composition (Pt, Ag, Cu, and Ni), and compared the results to experiments. The modeling of the electrostatic induction is compatible with a finite external potential imposed on the metal. This feature is used to investigate the structural rearrangements of the water bilayer between the pair of scanning tunneling microscope electrodes in response to an applied external voltage difference. We find significant asymmetry in the dependence on the sign of the applied voltage. Another result of the calculation is an estimate of the perturbation to the work function caused by the wetting film. For the conditions typical for operation of a scanning tunneling microscope probe, the change in the work function is found to be comparable to the applied voltage (a few hundred millivolts).

Water at an electrochemical interface—a simulation study

“…Following Siepmann and Sprik [17] each electrode atom i carries a Gaussian charge distribution of fixed width but variable amplitude (q i ). These charges are coulombically coupled to all other charges in the system.…”

“…Recently we introduced a way of incorporating some of the essential physical effects necessary for a realistic description of the interfacial charge-transfer process into a simulation which uses interaction potentials and therefore enables simulations of much larger time and length scales than is possible ab initio [16]. In particular, model metallic electrodes maintained at a constant electrical potential may be introduced into such a simulation, following a technique introduced by Siepmann and Sprik [17]. Because the electrodes behave as ideally polarizable metals they support image-charge interactions between charged species and the electrode; these, as we shall see, have an important influence on the electron transfer process.…”

Simulation of Adsorption Processes at Metallic Interfaces: An Image Charge Augmented QM/MM Approach

“…To model this response, Siepmann and Sprik 15 introduced the induced charge distribution ρ m (r), which in turn generates the potential V m (r) screening V H (r) in the region of the substrate. Siepmann and Sprik proposed to define ρ m (r) as a linear expansion of Gaussian functions centered at the metal atoms,…”

“…The Gaussian charge method developed by Siepmann and Sprik 15 treats the image charges as dynamical variables and realizes the mutual modification of induced and adsorbate charges. The Siepmann-Sprik scheme has been successfully applied to water/platinum interfaces [15][16][17] as well as ionic liquids at platinum 18 and graphene 19,20 electrodes within a purely classical frame.…”

Molecular mechanics models for the image charge, a comment on “including image charge effects in the molecular dynamics simulations of molecules on metal surfaces”

“…This model can be trivially applied in any classical MD code and comes with a 3 very low computational overhead compared to more sophisticated solutions such as the one proposed by Siepmann and Sprik. 6 Indeed, the model of Iori and Corni enjoys quite some success and is used in diverse applications in particular within the GolP force field, 7 which is used to simulate water/gold interfaces, peptides interacting with gold surfaces, 8 ionic-liquids in contact with ruthenium nano-particles 9 or even lubricants with iron surfaces. 10 1 Continuing our effort to describe the solvation energy at the metal/water interface, 11 we intended to use this model for a force field describing the water/Pt interface.…”