Interfacial processes characterization

Current density — 1. For characterizing interfacial processes -> current, I, flowing through an interface, is usually normalized with respect to the geometric (projected) interfacial area, A, yielding the current density, j (j = I/A), being a scalar quantity of unit Am-2. Since the current does not necessarily flow uniformly across... 

Recently, kinetic models have been combined with the equilibrium data of the interfacial processes, taking into account that soils and rocks are heterogeneous and consequently have different sites. These models are called nonequilibrium models (Wu and Gschwend 1986 Miller and Pedit 1992 Pedit and Miller 1993 Fuller et al. 1993 Sparks 2003 Table 7.2). These models describe processes when a fast reaction (physical or chemical) is followed by one or more slower reactions. In these cases, Fick s second law is expressed—that the diffusion coefficient is corrected by an equilibrium thermodynamic parameter of the fast reaction (e.g., by a distribution coefficient), that is, the fast reaction is always assumed to be in equilibrium. In this way, the net processes are characterized by apparent diffusion coefficients. However, such reactions can be equally well described using Equation 1.126. 

FIGURE 1.23 Communicating with a conducting polymer PPy/Cl in solution (a) cyclic voltammetry—a plot of current flow versus the electrical (potential) stimulus applied (b) the electrochemical quartz crystal mircobalance readout—mass polymer versus electrical (potential) stimulus applied (c) the resistometry readout—resistance of the polymer versus the electrical (potential) stimulus applied. (Printed with permission from Materials Science Forum, Vol. 189-190, Characterization of conducting polymer-solution interfacial processes using a new electrochemical method, A. Talaie, G. G. Wallace, 1995, p. 188, Trans Tech Publications, Switzerland.)... 

The tip-generated interfacial undersaturation is governed by the interplay between mass transport in the tip/substrate gap and the dissolution kinetics. This concept is illustrated in Figures 15 and 16. Figure 15a and b shows the radial dependence of the steady-state concentration and flux at the crystal/solution interface for a first-order dissolution process characterized by K, = 1, 10, and 100. For rapid kinetics (K, = 100), the dissolution process is able to maintain the interfacial concentration close to the saturated value and only a small depletion in the concentration adjacent to the crystal is observed over a radial distance of about one electrode dimension. Under these conditions, diffusion in the z-direction dominates over radial diffusion. As the rate constant decreases, diffusion is able to compete with the interfacial kinetics and consequently the undersaturation at the crystal surface... 

Figure 16a and b shows the effect of L on the radial dependence of the steady-state concentration and flux at the substrate/solution interface for a first-order dissolution process characterized by Ki = 10 and L = 0.1, 0.32, and 1.0. As the tip-substrate separation decreases, the effective rate of diffusion between the probe and the surface increases, forcing the crystal/so-lution interface to become more undersaturated. Conversely, as the UME is retracted from the substrate, the interfacial undersaturation approaches the saturated value, since the solution mass transfer coefficient decreases compared to the first-order dissolution rate constant. Movement of the tip electrode away from the substrate also has the effect of promoting radial diffusion, and consequently the area of the substrate probed by the UME increases. 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

The first chapter in this book deals with the fundamental properties and characterization of the water/oil interface. This outstanding contribution by Miller and coworkers is very central in understanding the basics of emulsions from a thermodynamie point of view. The chapter summarizes the newest findings with regard to interfacial processes, theory, and experimental facilities. 

Concerning eventual interfacial processes, there is an abundance of literature. Various techniques have been used to characterize interfaces/ interphases (Schradder and Block, 1971 Di Benedetto and Scola, 1980 Ishida and Koenig, 1980 Rosen and Goddard, 1980 Ishida, 1984 Di Benedetto and Lex, 1989 Thomason, 1990 Hoh et al, 1990 Schutte et al, 1994). Round-robin tests showed that no analytical method is able to provide unquestionable results (Pitkethly et al, 1993). Even in cases where the interface response to humid ageing has been unambiguously identified from studies on model systems (Kaelble et al., 1975, 1976 Salmon et al, 1997), it seems difficult, at this stage, to build a non-empirical kinetic model of the water effects on interfaces/interphases in composites. 

One of the most important tasks of modern electrochemistry is to develop microscopic pictures of solid-liquid interfaces and thus to provide a basis for the detailed understanding of electrochemical processes. To fulfill this task, the development of surface-specific and structure-sensitive in-situ methods to characterize electrochemical interfacial processes is indispensable. As early as 1970, Professor Martin Fleischmann was one of the pioneers in exploring in-situ methods that included surface-enhanced Raman spectroscopy [1], surface X-ray diffraction [2] and nuclear magnetic resonance [3] to characterize electrochemical interfaces. Nowadays, nontraditional electrochemical methods that include spectroscopic and microscopic as well as diffraction techniques have been extensively applied, and this has promoted an understanding of electrochemical interfaces at both atomic and molecular levels. 

It is well known that the maximum efficiency of electrochemical devices depends upon electrochemical thermodynamics, whereas real efficiency depends upon the electrode kinetics. To understand and control electrode reactions and the related parameters at an electrode and solution interface, a systematic study of the kinetics of electrode reactions is required. When ILs are used as solvents and electrolytes, many oftheelectrochemical processes will be differentandsomenewelectrochemical processes may also occur. For example, the properties of the electrode/electrolyte interface often dictate the sensitivity, specificity, stability, and response time, and thus the success or failure of the electrochemical detection technologies. The IL/electrode interface properties will determine many analytical parameters for sensor applications. Thus, the fundamentals of electrochemical processes in ILs need to be studied in order to have sensor developments as well as many other applications such as electrocatalysis, energy storage, and so on. Based on these insights, this chapter has been arranged into three parts (1) Fundamentals of electrode/electrolyte interfacial processes in ILs (2) Experimental techniques for the characterization of dynamic processes at the interface of electrodes and IL electrolytes and (3) Sensors based on these unique electrode/IL interface properties. And in the end, we wiU summarize the future directions in fundamental and applied study of IL-electrode interface properties for sensor applications. 

The interfacial properties of IL/electrode interfaces are different from other media (i.e., aqueous or traditional nonaqueous media) because of the unique properties of ILs, especially the electrochanical properties. To understand the electrode/electrolyte interface chanistry for sensor research, the mechanisms of the electrochemical reactions, and the essential performance-limiting factors, both in the bulk and at the surface of the electrode materials need to be investigated, preferably in situ. In situ analysis is much desired due to the fact that ex situ measurements are usually not able to follow the fast kinetics at electrode interfaces. The past decades have been characterized by a spectacular development of in sim techniques for studying interfacial processes at metal electrodes. Radioactive tracer [31, 32], pulse potentiody-namic [33, 34], and galvanostatic methods [35] have been applied quantitatively to study the adsorption of organic compounds at solid metals. In the study of complex... 

Starting with general principles, the book emphasizes practical applications of the electrochemical impedance spectroscopy to separate studies of bulk solution and interfacial processes, using of different electrochemical cells and equipment for experimental characterization of different systems. The monograph provides relevant examples of characterization of large variety of materials in electrochemistry, such as polymers, colloids, coatings, biomedical... 

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

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