Metal water interface

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. 

To date, the only applications of these methods to the solution/metal interface have been reported by Price and Halley, who presented a simplified treatment of the water/metal interface. Briefly, their model involves the calculation of the metal s valence electrons wave function, assuming that the water molecules electronic density and the metal core electrons are fixed. The calculation is based on a one-electron effective potential, which is determined from the electronic density in the metal and the atomic distribution of the liquid. After solving the Schrddinger equation for the wave function and the electronic density for one configuration of the liquid atoms, the force on each atom is ciculated and the new positions are determined using standard molecular dynamics techniques. For more details about the specific implementation of these general ideas, the reader is referred to the original article. ... 

Nagy et investigated the effect of the external electric field on the dynamics of water molecules at the water/metal interface. They found... 

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 order to study the behavior of ions at the water/metal interface using the molecular dynamics method, the potential energy functions for the interaction between the ions and the water and between the ions and the metal surface must be specified. 

Simulations of Electrolyte Solutions at the Water/Metal Interface... 

These results show that including quantum mechanical electronic rearrangement in dynamics calculations of the configurations of water on a metal surface can reveal effects that are not present in classical models of the water metal interface which treat the interaction of water with the surface as a static, classical potential energy function. For example, in classical calculations of the behavior of models of water at a paladium surface the interaction with one water molecule with the surface had a similar on-top binding site, a clas-... 

Corrosion is an electrochemical process whereby the oxidation of metal(s) or alloys to their (lower energy state) oxides or cations takes place, resulting in loss of mechanical or structural strength and metal wastage. Corrosion takes many forms and includes biocorrosion, which is corrosion taking place at the water-metal interface of a biofilm. 

In real-world applications, the importance of interfaces is hard to overestimate and three chapters are devoted to the effects of radiation at aqueous-solid boundaries. Jonsson focuses on applications within the nuclear industry where basic studies on radiation effects at water-metal interfaces have enabled a proposal for safe storage of spent nuclear fuel. Also with implications for the nuclear industry, Musat et al. document alterations in the radiation chemistry of liquid water confined on the nanoscale. Such nanoconfmed solutions are prevalent in the media proposed and indeed in use for waste storage. In another application, radiation chemistry has successfully been used to produce nanoscale objects such as metallic clusters and nanoparticles, an area summarized by Remita and Remita. 

The above techniques have been used in numerous calculations of solute free energy profiles. Wilson and Pohorille [52] and Benjamin[53] have determined the free energy profiles for small ions at the water liquid/vapor interface and compared the results to predictions of continuum electrostatic models. The transfer of small ions to the interface involves a monotonic increase in the free energy which is in qualitative agreement with the continuum model. This behavior is consistent with the increase in the surface tension of water with the increase in the concentration of a very dilute salt solution, and it represents the fact that small ions are repelled from the liquid/vapor interface. On the other hand, calculations of the free energy profile at the water liquid/vapor interface of hydrophobic molecules, such as phenol[54] and pentyl phenol[57] and even molecules such as ethanol [58], show that these molecules are attracted to the surface region and lower the surface tension of water. In addition, the adsorption free energy of solutes at liquid/liquid interfaces[59,60] and at water/metal interfaces[61-64] have been reported. 

E. Spohr, A computer simulation study of iodide ion solvation in the vicinity of a liquid water metal interface, Chem. Phys. Lett. 207 (1993) 214. 

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. 

For bulk water-metal interfaces, there have been a few studies in which the quantum nature of the metal is treated explicitly. Such studies require... 

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. 

R. R. Nazmutdinov and E. Spohr, /. Phys. Chem., 98,5956 (1994). Partial Charge Transfer of the Iodide Ion Near a Water/Metal Interface. 

Halley, Price and co-workers were the first to apply large-scale very nearly ab initio calculations to the study of the structure of liquid water-metal interface in a slab geometry. " The metal valence electrons were treated in the DFT formalism, but the water molecules were interacting via classical potentials. For the Cu(lOO)-... 

The previous section deals with adsorption of individual water molecules in ultravacuum conditions. Are these considerations still valid at coverage higher than the monolayer, taking into account the solvent effect Water is obviously a solvent that has been widely studied, the structure and chemistry at the water/metal interface being critical for the properties in many biological, chemical, and materials systems. Understanding the complex structure of the water-ion-adsorbate/metal interface and its dependence on potential has posed a major theoretical challenge for over 100 years [101]. 

To simulate constant potential systems, one maps out the free energies between different states of the system over a range of different potentials. The free energy between states can then be calculated at any fixed potential. This approach has been used to examine the structure and reactivity of the aqueous water/metal interface under electrochemical conditions. Some of the results are summarized in the next section. 

A systematic study of physical effects that influence the water structure at the water/metal interface has been made. Water structure, as characterized by the atom density proflles, depends most strongly on the adsorption energy and on the curvature of the water-metal interaction potential. Structural differences between liquid/liquid and liquid/solid interfaces, investigated in the water/mercury two-phase system, are small if the the surface inhomogeneity is taken into account. The properties of a polarizable water model near the interface are almost identical to those of unpolarizable models, at least for uncharged metals. The water structure also does not depend much on the surface corrugation. 

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