Water-metal potentials

A very useful development of water/metal potential energy functions, which takes into account the anisotropic nature of the water/metal interactions, has been recently presented by Zho and Philpott." They used a fit to the ab initio binding energy of water on several metal surfaces and applied some simplifying assumptions to develop potentials for the inter-... 

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 average root-mean-square displacements of first layer water molecules from potential minima for the water-metal potential functions along the metal surface for the 100 and 111 rigid Hg-water interface are roughly 0.5 This small value indicates that water molecules spend much of their time in the vicinity of the potential minima, as is evident from the images of the interface in Figure 9. 

When strips of reactive metals such as zinc are placed in water a potential difference, die electromotive force (emf), is set up die metal becomes negatively charged due to die transfer of zinc ions to die solution and die build-up of electrons on die metal. The metal strips or rods are termed die... 

The dipole density profile p (z) indicates ordered dipoles in the adsorbate layer. The orientation is largely due to the anisotropy of the water-metal interaction potential, which favors configurations in which the oxygen atom is closer to the surface. Most quantum chemical calculations of water near metal surfaces to date predict a significant preference of oxygen-down configurations over hydrogen-down ones at zero electric field (e.g., [48,124,141-145]). The dipole orientation in the second layer is only weakly anisotropic (see also Fig. 7). 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

It is clear that not all possible contaminants can be tested, but sources of contamination must be considered and tests run on the reaction in the presence of the most likely occurring ones. An approach to evaluating the problem of contamination is in the setting-up of a plant material matrix [1]. An example of potential contaminants to be considered, and sometimes overlooked, includes the heat transfer fluids to evaluate the consequences of heat exchanger, coil, or jacket failures. Contaminants which are introduced by other sources, for example, air (oxygen), carbon dioxide, water, metals, lubricants, and greases must also be considered. Also, the effects of chemicals which are used elsewhere in the plant and which could be introduced by mistake should be evaluated and perhaps tested. The possible contaminants in the reactor feeds must also be considered. 

One of the important electrochemical interfaces is that between water and liquid mercury. The potential energy functions for modeling liquid metals are, in general, more complex than those suitable for modeling sohds or simple molecular liquids, because the electronic structure of the metal plays an important role in the determination of its structure." However, based on the X-ray structure of liquid mercury, which shows a similarity with the solid a-mercury structure, Heinzinger and co-workers presented a water/Hg potential that is similar in form to the water/Pt potential described earlier. This potential was based on quantum mechanical calculations of the adsorption of a water molecule on a cluster of mercury atoms. ... 

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. ... 

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. 

What about the hydration sheet of the electrode Some of the solvent molecules adsorbed on the metal have to be removed in order to make room for the ion to adsorb (Fig 6.90). Thus the work needed to remove water molecules from the electrode depends basically on the bonding energy of water molecules to the electrode. Also, we should not forget that water in the interphasial region changes its orientation in response to the variation of the electrode potential (see Section 6.7.5). Thus, the water-metal bond would be dependent also on the potential of the electrode. 

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

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-... 

Fig. 9.5. Acid cooler, courtesy Chemetics www.chemetics.com Cool water flows through 1610 internal 2 cm diameter tubes while warm acid flows counter currently (and turbulently) between the tubes. The tubes are 316L stainless steel. They are resistant to water-side corrosion. They are electrochemically passivated against acid-side corrosion by continuously applying an electrical potential between the tubes and several electrically isolated metal rods. Details shell diameter 1.65 m shell material 304L stainless steel acid flow 2000 m3/hour water flow 2900 m3/hour acid temperature drop 40 K. (Green pipes = water metallic pipes = acid.) Fig. 24.6 gives an internal view.

Discussion Point DP2 Water is potentially an ideal solvent for metal catalyzed reactions. It is plentiful and cheap it is polar and dissolves many polar and ionic... 

The CHARMM code, version c25bl, was chosen for integration with the metal potential. CHARMM is a multi-purpose molecular dynamics program [35], which uses empirical potential energy functions to simulate a variety of systems, including proteins, nucleic acids, lipids, sugars and water. The availability of periodic boundary conditions of various lattice types (for example cubic and orthorhombic) makes it possible to treat solids as well as liquids. 

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