Interfacial structures

I n order to investigate the atomistic structure of the interface and the relaxation-related dislocations, H RTEM analysis was applied. These studies aim to provide information on the relative atomic column positions of the two materials. Since the contrast in HRTEM images depends on defocus, lens aberrations, and foil thickness, [61] it is not possible to directly derive atomic column positions from the images. However, the atomic positions with respect to the 

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Richmond G L, Robinson J M and Shannon V L 1988 Second harmonic generation studies of interfacial structure and dynamics Prog. Surf. Sc/. 28 1-70... 

Toney M F, Howard J N, Richer J, Borges G L, Gordon J G, Melroy O R, Yee D and Sorenson L B 1995 Electrochemical deposition of copper on a gold electrode in sulfuric acid Resolution of the interfacial structure Phys. Rev. Lett. 75 4472-5... 

Godail, L. and Packham, D.E., Adhesion of ethylene-octene copolymers to polypropylene interfacial structure and mechanical properties. J. Adhes. Sci. Technol., 15, 1285-1304 (2001). 

Kinning [20] studied the bulk, surface, and interfacial structures of a series of polyureas containing polydimethylsiloxane segments. In this study, the siloxane segment molecular weight (5000) and content (25 wt%) were kept constant, while... 

Fig. 11. Silicone polyurea interfacial structure against orienting medium (PSA). (From Ref. [20, copyright ownership by Overseas Publishers Association, reprinted with permission from Gordon and Breach Publishers.)...

K. F. Mansfield, D. N. Theodoru. Interfacial structure and dynamics of macromolecular liquids A Monte Carlo simulation approach. Macromolecules 22 3143-3152, 1989. 

Certainly these approaches represent a progress in our understanding of the interfacial properties. All the phenomena taken into account, e.g., the coupling with the metal side, the degree of solvation of ions, etc., play a role in the interfacial structure. However, it appears that the theoretical predictions are very sensitive to the details of the interaction potentials between the various species present at the interface and also to the approximations used in the statistical treatment of the model. In what follows we focus on a small number of basic phenomena which, probably, determine the interfacial properties, and we try to use very transparent approximations to estimate the role of these phenomena. 

In the previous section we saw on an example the main steps of a standard statistical mechanical description of an interface. First, we introduce a Hamiltonian describing the interaction between particles. In principle this Hamiltonian is known from the model introduced at a microscopic level. Then we calculate the free energy and the interfacial structure via some approximations. In principle, this approach requires us to explore the overall phase space which is a manifold of dimension 6N equal to the number of degrees of freedom for the total number of particles, N, in the system. 

Equation (17) expresses the cell potential difference in a general way, irrespective of the nature of the electrodes. Therefore, it is in particular valid also for nonpolarizable electrodes. However, since

polarizable electrodes at their potential of zero charge will be discussed here. It was shown earlier that the structural details are not different for nonpolarizable electrodes, provided no specifically adsorbed species are present. 

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

In principle, a measurement of upon water adsorption gives the value of the electrode potential in the UHV scale. In practice, the interfacial structure in the UHV configuration may differ from that at an electrode interface. Thus, instead of deriving the components of the electrode potential from UHV experiments to discuss the electrochemical situation, it is possible to proceed the other way round, i.e., to examine the actual UHV situation starting from electrochemical data. The problem is that only relative quantities are measured in electrochemistry, so that a comparison with UHV data requires that independent data for at least one metal be available. Hg is usually chosen as the reference (model) metal for the reasons described earlier. 

In purely electrochemical experiments the constant term is unknown. Therefore, from a measure of Ea=0t no information can be derived about the interfacial structure. However, if two metals are compared,... 

While no other value exists for Hg (which testifies to the delicacy of the experimental approach), Farrell and McTigue80 have measured the temperature coefficient of the cpd between Hg and water. This quantity is dX/dTt from which a value of -0.4 meV K 1 has been estimated for dexperimental quantities in molecular terms. 

The role of electrolyte is critical in these nanoscopic interfaces, but is difficult to predict and quantify. For sufficiently large rigid interfacial structures, one can apply the model of electrolyte interaction with a single charged surface in Figure 1(a). The double-layer theories or the recent integral-equation theories have been applied. Reviews of this subject are available in the literature [4,5]. For electrolytes in a nanostructure, the double layers from two surfaces overlap and behave differently from the case of a single surface. Ad-... 

Many important processes such as electrochemical reactions, biological processes and corrosion take place at solid/liquid interfaces. To understand precisely the mechanism of these processes at solid/liquid interfaces, information on the structures of molecules at the electrode/electrolyte interface, including short-lived intermediates and solvent, is essential. Determination of the interfacial structures of the intermediate and solvent is, however, difficult by conventional surface vibrational techniques because the number of molecules at the interfaces is far less than the number of bulk molecules. 

In order to evaluate which of these scenarios leads to the most stable interfacial structure, we have to analyze the relation between the chemical potentials of both reservoirs and the overall energy. Therefore, we begin with the Gibbs free energy of the interface. 

The possibility of determination of the difference of surface potentials of solvents, see Scheme 18, among others, has been used for the investigation of Ajx between mutually saturated water and organic solvent namely nitrobenzene [57,58], nitroethane and 1,2-dichloroethane (DCE) [59], and isobutyl methyl ketone (IB) [69]. The results show a very strong influence of the added organic solvent on the surface potential of water, while the presence of water in the nonaqueous phase has practically no effect on its x potential. The information resulting from the surface potential measurements may also be used in the analysis of the interfacial structure of liquid-liquid interfaces and their dipole and zero-charge potentials [3,15,22]. 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

M. Senda, T. Kakiuchi, T. Osakai, and T. Kakutani, in The Interfacial Structure and Electrochemical Processes at the Boundary Between Two Immiscible Liquids (V. E. Kazarinov, ed.), Springer-Verlag, Berlin, Heidelberg, 1987, pp. 107-121. 

This assumption was supported by the fact that the residual current observed by potential scan polarography at the W/DCE interface was larger in Range C than that in other ranges, indicating the increase of the capacitance due to the change of the interfacial structure. 

The study of electrochemically deposited monolayers poses the strictest experimental constraints since the signals will be necessarily very low. On the other hand, these studies can provide much detail on interfacial structure at electrode surfaces as well as on the effects... 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

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