Water-metal interaction potential

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). 

In Ref. 49 the orientational distribution of water near the Pt(lOO) surface was investigated in great detail. In spite of the preference for adsorption of isolated water molecules through the oxygen atom, which is incorporated into the water-metal interaction potential, relatively few configurations were observed in which the dipole moment of the molecule points into the solution. The analysis will not be repeated here the interested reader is referred to Ref. 49. 

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... 

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. 

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. 

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-... 

Looking at water-metal interactions as reflected by the zeta potential, one cannot draw any exact conclusions, though the ordering of the potentials follows the results here. In recent work, values of 49.0, 61.0, and 60.8 mV were reported for Ag, Au, and Pt, respectively, in distilled water. The metals were allowed to remain in water for a considerable length of time, however, and oxide layers could have formed. The low value for Ag is explained by the author as related to the weaker metal-oxide bond and to differences in cleanliness of the metal prior to the experiment. In an older work the zeta potential is reported for suspended Ag, Au, and Pt in water as 34, 32, and 30 mV, respectively. ... 

The density profiles obtained from atomic models (platinum [49] and rigid mercury surfaces [40]), where water-metal interactions are described by pairwise additive atomatom interaction potentials, are similar in shape. Height and width are correlated with the depth and force constant of the interaction potential. A similar correlation between peak height and interaction energy holds for water near a variety of different smooth model surfaces (see Ref. 139 and references therein). 

Rose and Benjamin [194, 187] studied the adsorption of Na+ and CF on the Pt(lOO) surface and concluded that the structure of the solvation complex around Na and Cl is very similar to the structure in the bulk . In this study, the same model for the water-metal interactions was used as in the preceding study of Li+ and F hydration [189]. The ion-surface interactions, however, were described by a modified image charge potential which was smoothly truncated. The authors discussed in some detail the anisotropy of the hydration complex that is induced by the density inhomogeneities at the interface. In a study of Fe + and Fe + hydration [197],... 

In the case of the Pt(lOO) surface the interaction potential is derived from semiempirical quantum chemical calculations of the interactions of a water molecule with a 5-atom platinum cluster [35]. The lattice of metal atoms is flexible and the atoms can perform oscillatory motions described by a single force constant taken from lattice dynamics studies of the pure platinum metal. The water-platinum interaction potential does not only depend on the distance between two particles but also on the projection of this distance onto the surface plane, thus leading to the desired property of water adsorption with the oxygen atoms on top of a surface atom. For more details see the original references [1,2]. This model has later been simplifled and adapted to the Pt(lll) surface by Berkowitz and coworkers [3,4] who used a simple corrugation function instead of atom-atom pair potentials. 

In fact, the orientation of water at the potential of zero charge is expected to depend approximately linearly on the electronegativity of the metal.9 This orientation (see below) may be deduced from analysis of the variation of the potential drop across the interface with surface charge for different metals and electrolytes. Such analysis leads to the establishment of a hydrophilicity scale of the metals ( solvophilicity for nonaqueous solvents) which expresses the relative strengths of metal-solvent interaction, as well as the relative reactivities of the different metals to oxygen.23... 

Various approaches have been used to model the interaction between the metal electrode and the water molecules. They range from simple Lennard-Jones or Morse potentials, which have been adjusted to give good values for experimental porperties like the energy of adsorption of a water molecule, to potentials derived from ab initio calculations performed for a cluster of metal atoms and one water molecule. 

Although one cannot simulate an electrolyte solution, molecular-dynamics studies of an ensemble of water with one or two ions have been performed. The long-range nature of the Coulomb force causes considerable technical difficulties in addition, the interaction potentials are somewhat uncertain. So the results have to be considered with caution. Nevertheless, they seem reasonable, and fit in well with our knowledge of the interface. Figure 17.9 shows the results of a simulation of an ensemble of water molecules with one Li+ and one I"" ion in the presence of a fairly large field between the two metal plates [14]. In... 

The metal-solvent interaction is expected to depend on the donicity of the solvent the higher the donor number of the solvent the stronger the solvent-metal interaction should be. Hence, a correlation between the contact potential difference A>// (a = 0) and the donor number of the solvent should be observed. However, this correlation for the Hg electrode is rather poor, with the most deviant point having been found for water, that is, for the case of the strongest dipole-dipole interaction in the bulk. The correlation is better when acceptor numbers of solvents are taken into account. ... 

Much more detailed information about the microscopic structure of water at interfaces is provided by the pair correlation function which gives the joint probability of finding an atom of type/r at a position ri, and an atom of type v at a position T2, relative to the probability one would expect from a uniform (ideal gas) distribution. In a bulk homogeneous liquid, gfn, is a function of the radial distance ri2 = Iri - T2I only, but at the interface one must also specify the location zi, zj of the two atoms relative to the surface. We expect the water pair correlation function to give us information about the water structure near the metal, as influenced both by the interaction potential and the surface corrugation, and to reduce to the bulk correlation Inunction when both zi and Z2 are far enough from the surface. 

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. 

Guidelli model of, 899 Habib and Bockris, 899 at the interface, importance of, 918 -ion interaction energy, 924 -metal interactions, 896 chemical forces, 897, 972 lateral forces, 897 monomers of, definition, 899 orientation of, 898 Parsons model of, 899 and potential of the electrode. 900. 924 preferential orientation of, 912 and solvent excess entropy, 912 the "three-state water model 898, 899 Wave nature of electrons, 788 Wavenumber, 799 Waves... 

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-... 

Raghavan et al. described a form interaction potential between rigid water and a rigid platinum metal surface.63 Using this potential... 

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