Metal-water interactions

This equation has been derived only as a reference for a comparative discussion of data for sd- and d-metals later on. However, the meaning of such a line is that there exists a limit to AX values in the sense that after a given top effect, a further increase in metal-water interaction will not produce higher AX values.6,7 An indirect confirmation of this is given by the observation of a top value in the decrease of 0 upon water adsorption on d-metals from the gas phase.35,36... 

While A

thermal desorption spectroscopy (TDS) in which heat is used to detach molecules from a surface. TDS data are in parallel with A (and AX) data. This is illustrated in Fig. 19.35 The spectrum of Ag(110) shows only one peak at 150 K, corresponding to ice sublimation. This means that Ag-H20 interactions are weaker than H20-H20 interactions (although they are still able to change the structure of the... 

MD simulations have been used for water at Pt(100) and (111 ),880-882 as well as at Ag(l 11 ).883 The stmcture of water is predicted to conform to a hexagonal pattern and the metal-water interaction is probably stronger for the (111) than the (100) surface.882 On the basis of the extended Huckel theory, Estiu et a/.884,885 have reached different conclusions in favor of the... 

Quantum chemical calculations have recently been extended to In.891 Adsorption has been found to be nondissociative and the metal-water interaction has been proposed to be in the sequence Hg < Ag(100) < In < Cu(100). Compared with the data in Tables 27 and 28, it appears that the positions of In and Ag(100) are exchanged. 

While contrasting results obtained by different experimental techniques as well as different theoretical methods are not surprising, internal controversies over AX values in electrochemistry are more serious. The controversy referred to here32 is that about the sequence of metal-water interactions for the different faces of fee metals. More recently, a controversy has also arisen about single-crystal faces of Cd. 

It is an experimental fact that the capacitance of an electrode in a given solvent is a function of the nature of the metal. This was pointed out by Frumkin et al,333 and has been discussed several times in the literature.7 349 94 99 999 Trasatti34 901 showed that the reciprocal of the differential capacitance at a = 0 is linearly correlated with the strength of the metal-water interaction. The reader is referred to the original papers for a detailed discussion. 

A term that is widely used (and sometimes abused) in discussions about metal-water interactions is hydrophilicity. By this term is meant the strength of interaction between a metal surface and water molecules in contact with it, and the term usually implies chemical bond strength. However, there is a problem with the way hydrophilicity scales are built up. Various quantities (capacitance, adsorption energy, etc.) are used to rank the metals, and the hydrophilicity scale may differ for different parameters. 

On many metals, such as Ni, Pt, Ag, Cu, and Pd, when a submonolayer of oxygen is present on the surface, the reaction H2O + Oadj 2 OHajs may take place. For some noble metal planes [e.g., Ru(OOl)], adsorbed oxygen is responsible for the dissociation of water. Experimental data on the metal-water interaction have been reported and extensively discussed in a review by Thiel and Madey. ... 

The influence of the surface structure on the metal-water interaction has also been determined for silver electrodes (Table 3). There are discrepancies in the AG° values given by different authors for silver electrodes. For example, Vitanov and Popov obtained the same hydrophilicity sequence as for gold Ag(l 11) > Ag( 100). Another sequence based on the interfacial parameter was given by Trasatti. The interfacial... 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

The simplest reaction on a metal ion in aqueous solution is the exchange of a water molecule between the first and second coordination shells. This reaction is fundamental in understanding not only the reactivity of metal ions in chemical and hiological systems hut also the metal-water interaction. The replacement of a water molecule from the first coordination shell represents an important step in complex-formation reactions of metal cations and in many redox processes (1). 

The capability of the dissociating water model, augmented to include metal-water interactions, to qualitatively describe the process of ion hydrolysis can be applied to the problem of electron... 

Image forces, 819, 924, 921, 946, 964 and metal-water interactions, 896 Imaginary impedance, 1128, 1135, 1160 Imaginary number, 1129 Impedance spectroscopy, 1127, 1160 acanddc, 1134... 

Also, as would be expected, the type of metal (i.e., the electronic structure of the electrode) influences the adsorbility of the organic molecules. For example, Fig. 6.116 shows the free energy of adsorption of amyl alcohol and acetonitrile on different metals. This figure indicates how the adsorption energy of the organic molecule decreases as the strength of metal-water interaction increases (the AX parameter in Fig. 6.116) (Trasatti, 1995). 

The metal-water interaction has also been suggested [373] to play a role in determining the extent and strength of hydrogen adsorption. The metal-water interaction is potential dependent in particular it decreases as the potential is made more cathodic. Thus, pseudo-capacitances were observed at higher overpotential than 0.3 V because of the appearance of a considerable amount of adsorbed hydrogen. 

Other Intermolecular Bondings - Metal-Water-Interaction. 112... 

Some hydrates are supposed to possess non-H-bonded water hydrogen atoms, e.g., Na4SnS4 14 H2O and Na2[Fe(CN)5NO] 2 H20 with the highest - energy OD bands (HDO) at 2679 and 2653 cm respectively. In this case, the shift of the OH bands to lower wavenumbers - all solid hydrates known so far show OH frequencies smaller than those of free H2O molecules (see Table 3) - must be caused by metal water interaction alone. However, the question arises whether weak hydrogen bonds are possible, even for H-bond lengths in the range of the van der Waals distances. Possibly then H-bonds cannot be excluded for any water of crystallization (see also Sects. 3.2 and 4.2.3). 

The high sensitivity of H2O librations to the structural environment and intermolecular bonding was analysed by several authors in spite of the numerous comphcations involved, as mentioned above. Thus, it has been claimed, but only proved for some hmited series of hydrates, that the band frequencies expand with increasing metal-water interaction, as indicated by increasing M-0 , stretching frequencies and decreasing M-O distances by the increase in the strength of H-bonds and by the... 

After the pioneering observation by Hansen [28] that electrodes could be removed from the solution by maintaining their electric double layer it became possible to investigate the water adsorption configuration on metal surfaces in the UHV environment [29]. Although his studies were accomplished under very different circumstances from those in electrochemistry, the information on the energies of metal-water interactions was extremely useful to check the accuracy of the classical double-layer Bockris-Devanathan-Miiller model. The interaction of water with the commonly used electrode metals was much less than previously assumed. 

Hydrates of this type contain metal ion coordinated water, and the major concern with these is the effect of the metal-water interaction on the structure of crystalline hydrates. The metal-water interaction can be quite strong relative to the other bonding in a molecular crystal, so that dehydration takes place only at very high temperatures [13], Drugs with solubility, dissolution, or handling problems are most often recrystallized as Na(I), K(I), Ca(II), or Mg(II) salts and are often hygroscopic to some degree [16],... 

The dynamics of any metal-liquid interface involves interactions both between and among particles in the metal and fluid. For the physisorption of water on metals, where the interaction between water molecules is comparable to the metal-water interaction, it is normally assumed that the metal-water interactions can be treated with model potentials and that a detailed quantum mechanical treatment of the interaction between the two phases is not necessary, provided an adequate model of the interaction is used. Howevei a simple quantum mechanical treatment for the metal, the jellium model, exists, and its role in the simulation of metal-water interactions also is considered below. 

Most of the collective potentials mentioned above are relatively complex because they are directly related to atom-atom potentials. Rather than constructing collective potentials in this manner, it is possible to fit them directly to the overall interactions between a water molecule and the metal, providing a simpler model that is easy to implement and interpret. For example, Spohr and Shelley et al. have employed collective potentials in which the interaction is represented as a Fourier expansion in the metal surface, but the potentials are kept simple and terms due to different physical effects are separated to facilitate systematic optimization and variation of the metal-water interactions. 

As a result of the crude approximation used to represent the nuclear and core structure of the metal, applications of the jellium model are limited. Clearly, this model by itself will not yield accurate results for all properties of the metal or metal surface, nor will it provide an accurate model for the complete metal-water interaction potential. However, the jellium model can be used to represent a component of the metal-water interaction potential, especially the... 

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 related issue is the dependence of the quantum mechanically calculated metal-water interaction on the inclusion of other surface-bound and solvating water molecules. Although current model potentials are suspect, it is clear they can provide a qualitative approximation of the true interactions in most cases. 

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