Water-metal interface, dynamic simulation

M. L. Berkowitz, I.-C. Yeh, E. Spohr. Structure of water at the water/metal interface. Molecular dynamics computer simulations. In A. Wieckowski, ed. Interfacial Electrochemistry. New York Marcel Dekker, 1999, (in press). 

Most of the new molecular-level results concern the structure and dynamics of water at interfaces. We begin this review with a brief summary of this area. Several recent review articles and books can be consulted for additional information. " We then examine in some detail the new insight gained from molecular dynamic simulations of the structure of the electric double layer and the general behavior of ions at the water/metal interface. We conclude by examining recent developments in the modeling of electron transfer reactions. 

The main goal of the molecular dynamics computer simulation of ionic solvation and adsorption on a metal surface has been to test the above model and to provide more quantitative information about the different factors that influence the structure of hydrated ions at the interface. Unfortunately, most of the experimental information about these issues has been obtained from indirect measurements such as capacity and current-potential plots, although in recent years in situ experimental techniques have begun to provide an accurate test of the above model. For a recent review of experimental techniques and the theory of ionic adsorption at the water/metal interface, see the excellent paper by Philpott. ... 

In the next section a brief layout of simulation methods will be given. Then, some basic properties of the models used in computer simulations of electrochemical interfaces on the molecular level will be discussed. In the following three large sections, the vast body of simulation results will be reviewed structure and dynamics of the water/metal interface, structure and dynamics of the electrolyte solution/metal interface, and microscopic models for electrode reactions will be analyzed on the basis of examples taken mostly from my own work. A brief account of work on the adsorption of organic molecules at interfaces and of liquid/liquid interfaces complements the material. In the final section, a brief summary together with perspectives on future work will be given. 

Two major classical simulation techniques, molecular dynamics and Monte Carlo, have been applied to simulation of water-metal interfaces. We first discuss features common to both methodologies and then describe aspects unique to each. The field of computer simulations is an actively evolving one, despite being more than 40 years old. Even for the particular case of water-metal interfaces, many variations exist on the central theme of how best to carry out these calculations. In this chapter, we limit our discussion to the most significant (in our opinion) techniques in use for metal-water interfaces. 

L. Spohr, K. Heinzinger, Molecular dynamics simulation of a water metal interface, Chem. Phys. Lett. 123 (1986) 218-221. 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

Rose and Benjamin (see also Halley and Hautman ) utilized molecular dynamic simulations to compute the free energy function for an electron transfer reaction, Fe (aq) + e Fe (aq) at an electrodesolution interface. In this treatment, Fe (aq) in water is considered to be fixed next to a metal electrode. In this tight-binding approximation, the electron transfer is viewed as a transition between two states, Y yand Pf. In Pj, the electron is at the Fermi level of the metal and the water is in equilibrium with the Fe ion. In Pf, the electron is localized on the ion, and the water is in equilibrium with the Fe" ions. The initial state Hamiltonian H, is expressed as... 

The microscopic structure of water at the solution/metal interface has been the focus of a large body of literature, and excellent reviews have been published summarizing the extensive knowledge gained from experiments, statistical mechanical theories of varied sophistication, and Monte Carlo and molecular dynamics computer simulations. To keep this chapter to a reasonable size, we limit ourselves to a brief summary of the main results and to a sample of the type of information that can be gained from computer simulations. 

An important aspect of the study of water under electrochemical conditions is that one is able to continuously modify the charge on the metal surface and thus apply a well-defined external electric field, which can have a dramatic effect on adsorption and on chemical reactions. Here we briefly discuss the effect of the external electric field on the properties of water at the solution/metal interface obtained from molecular dynamics computer simulations. A general discussion of the theoretical and experi-... 

The structure of the adsorbed ion coordination shell is determined by the competition between the water-ion and the metal-ion interactions, and by the constraints imposed on the water by the metal surface. This structure can be characterized by water-ion radial distribution functions and water-ion orientational probability distribution functions. Much is known about this structure from X-ray and neutron scattering measurements performed in bulk solutions, and these are generally in agreement with computer simulations. The goal of molecular dynamics simulations of ions at the metal/water interface has been to examine to what degree the structure of the ion solvation shell is modified at the interface. 

One of the first studies of multiple ions at the water/solid interface was by Spohr and Heinzinger, who carried out a simulation of a system of 8 Li" and 81" ions dissolved in 200 water molecules between uncharged flat Lennard-Jones walls.However, the issues discussed in their paper involved water structure and dynamics and the single-ion properties mentioned earlier. No attempt was made to consider the ions distributions and ion-ion correlations. This work has recently been repeated using more realistic water-metal potentials. ... 

The theoretical modeling of electron transfer reactions at the solution/metal interface is challenging because, in addition to the difficulties associated with the quantitative treatment of the water/metal surface and of the electric double layer discussed earlier, one now needs to consider the interactions of the electron with the metal surface and the solvated ions. Most theoretical treatments have focused on electron-metal coupling, while representing the solvent using the continuum dielectric media. In keeping with the scope of this review, we limit our discussion to subjects that have been adi essed in recent years using molecular dynamics computer simulations. 

The complex formation proceeded almost completely at the interface. The rate constant of k=5.3xl02M 1 s 1 was determined by a stopped-flow spectrometry in the region where the formation rate was independent of pH. The conditional interfacial rate constants represented by k[ = k k2 [HL] / (k2 + k i[H + ]) were larger in the heptane-water interface than the toluene-water interface, regardless of metal ions. The molecular dynamics simulation of the adsorptivities of 5-Br-PADAP in heptane-water and toluene-water interfaces suggested that 5-Br-PADAP could be absorbed at the interfacial region more closely to the aqueous phase, but 5-Br-PADAP in the toluene-water... 

A. Kohlmayer, W. Witschel, E. Spohr, Molecular dynamics simulation of water/metal and water/vacuum interfaces with a polarizable water model, Chem. Phys., 213 (1996) 211-216. 

AIMD is still a very time-consuming simulation method and has so far mainly been used to study the structure and dynamics of bulk water [14,15], as well as proton transfer [16] and simple Sjv2 reactions in bulk water [17]. AIMD simulations are as yet limited to small system sizes and real simulation times of not more than a few picoseconds. However, some first applications of this technique to interfacial systems of interest to electrochemistry have appeared, such as the water-vapor interface [18] and the structure of the metal-water interface [19]. There is no doubt... 

A wide variety of different models of the pure water/solid interface have been investigated by Molecular Dynamics or Monte Carlo statistical mechanical simulations. The most realistic models are constructed on the basis of semiempirical or ab initio quantum chemical calculations and use an atomic representation of the substrate lattice. Nevertheless, the understanding of the structure of the liquid/metal surface is only at its beginning as (i) the underlying potential energy surfaces are not known very well and (ii) detailed experimental information of the interfacial structure of the solvent is not available at the moment (with the notable exception of the controversial study of the water density oscillations near the silver surface by Toney et al. [140, 176]). 

Understanding the structure and dynamics of pure water on a molecular level is only the beginning. Simulations of electrolyte solutions near metallic interface are much more demanding in terms of computer time than those of bulk water, because the relatively small number of ions even in a highly concentrated electrolyte solution mandates the treatment of systems with a much larger total number of particles than in pure water for a longer time span. Furthermore, as was discussed in section 3, much less is known from quantum chemistry about nature and strength of the ion-metal interaction than about the water-metal interactions, so that the interpretation of the results obtained from the simulations is less clear. 

A significant limitation in obtaining an accurate representation of the interface concerns our knowledge of the metal-water interactions. Quantum mechanical calculations are the best source of this type of information, especially for obtaining interaction potentials. When adequately parameterized, these potential functions can be used effectively within traditional Monte Carlo and molecular dynamics simulations. Because of the extensive amounts of computer... 

A. Kohlmeyer, W. Witschel, and E. Spohr Chem. Phys., 213, 211 (1997). Molecular Dynamics Simulations of Water/Metal and Water/Vacuum Interfaces with a Polarizable Water Model. 

Halley and Mazzolo l develop>ed a flrst-principles-based direct dynamics method to examine the water/copper metal interface. Previous models on the electrochemical metal/ water interface published in the literature could not straightforwardly describe the asymmetry of the capacitance measured experimentally in the double layer. In approach taken by Halley and MazoUo, the electrons in the metal are modeled quantum mechanically using a jellium-type free electron model where only the s-electrons in copper are treated. Pseudopotentials are used to describe the electron interactions with water. The water solution phase is decoupled from the electronic structure and treated by molecular dynamics simulations with explicit water molecules using classical force fields. Gouy-Chapman theory is used to treat ionic screening. The electronic structure at the interface between the metal and the water is carefully matched by p>erforming electronic structure calculations on the metal substrate after each time step in the water MD simulation. The approach was used to examine the influence of applied potential on the structm-e of the metal-water... 

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