Electrochemical interfaces, molecular

M. R. Philpott, J. N. Glosli. Molecular dynamics simulation of interfacial electrochemical processes electric double layer screening. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 2, pp. 13-30. 

The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed HgO ions the solvation stoichiometries of ions in the first monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. 

Ultrahigh vacuum surface spectroscopies can provide far greater breadth and depth of information about surface properties than can yet be achieved using in situ spectroscopies at the aqueous/metaI interface. Application of the vacuum techniques to electrochemical interfaces is thus desirable, but has been plagued by questions of the relevance of the emersed, evacuated surfaces examined to the real electrochemical interfaces. This concern is accentuated by surface scientists observations that in UHV no molecular water remains on well-defined surfaces at room temperature and above (1). Emersion and evacuation at room temperature may or may not produce significant changes in electrochemical interfaces, depending.on whether or not water plays a major role in the surface chemistry. 

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. ... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

The electrochemical behavior of a modified electrode ultimately depends on structural details at the molecular level. For example, the molecular-level interaction between the redox site in the film and the solvent from the contacting solution phase might play an important role in the electrochemical response. Molecular-level details are often difficult to infer from electrochemical methods alone, but do lend themselves to spectroscopic analyses. In recent years there has been an explosion of new spectroscopic techniques for characterizing modified electrodes and the electrode-solution interface in general [44,45]. In this section, we review some of these spectroelectrochemical methods. 

There has recently been much activity in developing molecular spectroscopic probes of electrochemical interfaces, as for other types of heterogeneous systems. The ultimate objectives of these efforts include not only the characterization of adsorbate molecular structure interactions under equilibrium conditions, but also the extraction of mechanistic and kinetic information from spectral detection of reactive adsorbates. 

Two problems are inherent, however, in applying such techniques to electrochemical interfaces. Firstly, the extremely small quantity of material present within the molecular thin interfacial region can present severe challenges in analytical detection. Secondly, this difficulty is exacerbated by the usual need for the incoming and outgoing photons to traverse the bulk solution, so that under normal... 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

The use of single crystal surfaces, their chemistry, and their influence on various surface chemistry types is exemplified in the first five chapters which represent a well-chosen selection of the diversity of surface chemistry and catalysis studied on metal-based single crystal model systems, and the depth of molecular information obtained from such studies. Metallic single crystals can also serve as model surfaces of electrochemical surface science, and these in-situ electrochemical interfaces can be similar to the interfaces encountered in ultrahigh vacuum surface science studies but with some significant differences as summarized by Stamenkovic and Markovic. 

Molecular assemblies and redox reactions of zinc(ll) tetraphenylporphyrin and zinc(ll) phthalocyanine on Au(lll) single crystal surface at electrochemical interface. Chemical Physics, 319,147-158. 

Two-step electronic in situ STM processes rest on the notion of two sequential single-electron transfer steps, at the substrate-molecule and molecule tip interfaces. Analogous multiple-step processes would, similarly involve multiple hopping along a chain of molecular groups. The latter was addressed in Chapter 6 and will be disregarded here. The formalism of electron transfer at a single electrochemical interface is known from electron... 

The understanding of electrochemical systems thus far has been based principally on the use of measurements that do not directly yield information at the molecular level. Until very recently, scientists have not had access to information about chemical species at electrochemical interfaces of the type that has played, for example, such an important role in understanding the chemistry of molecules in the bulk phases. Recent advances in instrumental techniques, however, promise access to molecular-level information about electrochemical systems that heretofore has been unavailable. This exciting development opens up important new opportunities in fundamental and applied science. 

Each generation of textbooks of electrochemistry shows a sketch of the interface like Fig. 1 in its introductory chapters. The complexity of the sketch increases with our increasing knowledge of the molecular detail of the electrochemical interface. The characteristic difference between Fig. 1 and similar ones in, e.g., the books by Bockris and Reddy [14, 15], Brett and Brett [12], or by Schmickler [2] is the fact that... 

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. 

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