Interface liquid/metal

All corrosion inhibitors in use as of this writing are oil-soluble surfactants (qv) which consist of a hydrophobic hydrocarbon backbone and a hydrophilic functional group. Oil-soluble surfactant-type additives were first used in 1946 by the Sinclair Oil Co. (38). Most corrosion inhibitors are carboxylic acids (qv), amines, or amine salts (39), depending on the types of water bottoms encountered in the whole distribution system. The wrong choice of inhibitors can lead to unwanted reactions. For instance, use of an acidic corrosion inhibitor when the water bottoms are caustic can result in the formation of insoluble salts that can plug filters in the distribution system or in customers vehicles. Because these additives form a strongly adsorbed impervious film at the metal liquid interface, low liquid concentrations are usually adequate. Concentrations typically range up to 5 ppm. In many situations, pipeline companies add their own corrosion inhibitors on top of that added by refiners. 

Cavitation erosion. In certain conditions, a thin layer of liquid, nearly static at the metal-liquid interface can prevent impingement of the surface by the turbulent flow of the liquid. However, due to the turbulence of water, bubbles of air or gas of size larger... 

Another type of hot electron transfer from a metal-covered n-Si electrode to a redox system ([Fe(CN)6] ) was studied by Chen and Freese [170, 171]. The hot electrons were actually produced in the metal by transfer from the conduction band of n-Si during cathodic polarization. In sufficiently thin layers (< 500 A) they were transported to the metal/liquid interface without essential thermalization. 

The large signal enhancements often encountered with SERS have stimulated many investigations of analytical applications, examples of which are listed in Table 13.6. Many of these involve biological or environmental analysis, where low concentrations of analytes preclude the use of unenhanced or even resonance-enhanced Raman spectroscopy witout the added benefit of surface enhancement. Despite the great promise of a technique that increases Raman intensity by 10 or more, SERS has not yet resulted in widely used or routine analysis of real samples. SERS has been a very important and valuable probe of surface structure and has stimulated new discoveries about the behavior of metal-gas and metal-liquid interfaces, but its incursion into practical chemical analysis has been limited. It is worth considering why SERS has encountered formidable barriers to widespread analytical utility (2). 

T. Burgi, ATR-IR Spectroscopy at the Metal-Liquid Interface Influence of Fihn Properties on Anomalous Band-Shape, P/ryi. Chem. Chem. Phys. 3, 2124-2130 (2001). 

Fig. 3.1 Potential profile across a metal-liquid interface...

This is a differential capacity because it depends on the potential A o. The total capacity for a metal-liquid interface can be considered as two capacitors, namely Ch and Cj, in series. We have then... 

Kinetics of Electron Transfer at the Metal-Liquid Interface... 

The redox process at metal electrodes described above, should also be briefly discussed in terms of the Gerischer model (see Section 6.2). Assuming equal concentrations for the reduced and oxidized species of the redox system then the energetics of the metal liquid interface are given in Fig. 7.5 for equilibrium, cathodic and anodic polarization. The anodic and cathodic currents are then given by (see Eq. 6.42) ... 

The above discussion shows that existing literature contains arguments, both theoretical and experimental, in favor as well as against the presence of nanobubbles at the metal/liquid interface. Many more targeted experiments and theoretical works are required to clarify this issue. 

As with scale formation the precipitation of organic compounds will depend on the temperature distribution. Bott and Gudmundsson [1977] proposed a qualitative model of organic precipitation. If a waxy hydrocarbon flowing across a cold surface and the metal/liquid interface is at the cloud point temperature, crystals of wax will form on the surface. If the surface temperature is below the cloud point temperature the cloud point temperature will be located away from the surface towards the bulk liquid. The precise location will depend on the thermal resistance of the laminar sub-layer combined with the resistance of any deposit already on the surface. High thermal resistance between the solid surface and the... 

Applying the methods of ab initio quantum chemistry to electrochemistry has a more recent history than their application to such fields as gas-phase chemistry or organic chemistry. This is undoubtedly related to the inherent complexity of the electrochemical interface. One of the main reasons for the recent upsurge in using ab initio quantum chemistry in modeling electrochemical interfaces is the degree of success that has been achieved in applying ab initio quantum-chemical methods to processes and reactions at metal-gas interfaces (for recent reviews in this area, see Refs.4-7). This has motivated many theoretically inclined electrochemists to use similar methods and ideas to model adsorption and reactions at electrified metal-liquid interfaces, and has also attracted theoreticians from the field of surface science to electrochemistry. 

Modern computer simulation studies, along with other theoretical methods applied to the metal-liquid interface, typically require models for the interaction between water molecules, for the metal, and for the metal surface interacting with the water. Designing realistic models of the interface that can be solved in a reasonable time frame is a primary challenge in the area of theory and computer simulation. 

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. 

Cavitation-erosion is the loss of material caused by exposure to cavitation, which is the formation and collapse of vapor bubbles at a dynamic metal-liquid interface—for example, in rotors of pumps or on traihng faces of propellers. This type of corrosion causes a sequence of pits, sometimes appearing as a honeycomb of small relatively deep fissures (see Uhlig s Corrosion Handbook, 2nd edition, R. W. Revie, editor, Wiley, New York, 2000, Fig. 12, p. 261). 

Harata A, Shen Q, and Sawada T (1999) Photo-thermal applications of lasers Study of fast and ultrafast photothermal phenomena at metal-liquid interfaces. Annual Review of Physical Chemistry 50 193-219. 

Useful atomic and subatomic scale information on hydroxylated oxide surfaces and their interaction with aggressive ions (e.g., Cl ) can be provided by theoretical chemistry, whose application to corrosion-related issues has been developed in the context of the metal/liquid interfaces [34 9]. The application of ah initio density functional theory (DFT) and other atomistic methods to the problem of passivity breakdown is, however, limited by the complexity of the systems that must include three phases, metal(alloy)/oxide/electrolyte, then-interfaces, electric field, and temperature effects for a realistic description. Besides, the description of the oxide layer must take into account its orientation, the presence of surface defects and bulk point defects, and that of nanostructural defects that are key actors for the reactivity. Nevertheless, these methods can be applied to test mechanistic hypotheses. 

This section of complex electrochemical reactions in solution and on electrodes is divided into three parts regarding the following questions. First, how does the solvent interact with the unbiased and biased metal surface Second, how does the oxidation/reduction of a single electrochemical active species work in pure solvents And finally, how does a complex electrochemical reaction proceed in solution and on metal surfaces Therefore, metal-liquid interfaces are discussed at the beginning, followed by half cell reactions in solvents, and finally complex redox reactions in metal-liquid interfaces are reviewed. 

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