Surface-Electrolyte Interactions

Oxide formation on Pt surfaces in electrolyte environments has been studied for nearly a century [9] yet still much is being learned about the oxide formation and growth on these surfaces. This is in part due to the complexity of the surface/electrolyte interactions under electrostatic potential as well as the need for accurate approaches to study such systems. The drastic improvement in DFT over the past two decades coupled with the improved surface/ electrolyte models have made it such that theoretical tools can be applied to these systems with an acceptable degree of accuracy. We will here discuss some recent studies on the Pt(lll) surface using DFT and some of the implications of the DFT findings. 

Hence, for two similarly charged surfaces in electrolyte, interactions are determined by both electrostatic doublelayer and van der Waals forces. The consequent phenomena have been described quantitatively by the DLVO theory [6], named after Derjaguin and Landau, and Verwey and Over-beek. The interaction energy, due to combined actions of double-layer and van der Waals forces are schematically given in Fig. 3 as a function of distance D, from which one can see that the interplay of double-layer and van der Waals forces may affect the stability of a particle suspension system. 

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-... 

What one requires is a self-consistent picture of the interface, including both metal and electrolyte, so that, for a given surface charge, one has distributions of all species of metal and electrolyte phases. Unified theories have proved too difficult but, happily, it seems that some decoupling of the two phases is possible, because the details of the metal-electrolyte interaction are not so important. Thus, one can calculate the structure of each part of the interface in the field of the other, so that the distributions of metal species are appropriate to the field of the electrolyte species, and vice versa. 

The electrochemical interface between an electrode and an electrolyte solution is much more difficult to characterize. In addition to adsorbate-substrate and adsorbate-adsorbate interactions, adsorbate-electrolyte interactions play a significant role in the behavior of reactions on electrode surfaces. The strength of the adsorbate-substrate interactions is controlled by the electrode potential, which also determines the configuration of the electrolyte. With solution molecules, ions, and potential variation involved, characterization of the electrochemical interface is extremely difficult. However, by examining solvation, ion adsorption, and potential effects as individual components of the interface, a better understanding is being developed. 

No. Compound Electrode material Aqueous electrolyte AG ads solution (M) kcal mol 1 Estimated" orientation (charge on surface) Assumed interaction... 

We recently considered the effect of the nucleic acid-surface electrostatic interaction on the thermodynamics of the surface hybridization [2-5, 22], This theory used an analytical solution of the linearized Poisson-Boltzmann boundary value problem for a charged sphere-surface interaction in electrolyte solution and corresponds to the system characterized by a low surface density of immobilized probes. To understand the motivation for that work and extensions, we need to consider the physical effects of a surface in solution and the theoretical tools available for their study. 

The idea of an inert (indifferent) electrolyte was coined in the context of electrocapillary studies using the Hg electrode. Grahame [34] found a series of electrolytes that produced the same PZC (determined as the electrocapillary maximum) irrespective of the nature or concentration of the electrolyte. Such behavior suggests that the ions of these electrolytes interact with the surface only by a Coulombic force. In contrast, many other electrolytes induced a shift in the PZC, with the magnitude and direction of this shift depending on the nature or concentration of the electrolyte. A shift in the PZC suggests that ions of these electrolytes can be positively adsorbed in spite of electrostatic repulsion that is, they interact with the surface by a noncoulombic force. This phenomenon is termed specihc adsorption. Thus, an inert electrolyte does not show specific adsorption of either ion. 

Actually, interfacial potential differences can develop without an excess charge on either phase. Consider an aqueous electrolyte in contact with an electrode. Since the electrolyte interacts with the metal surface (e.g., wetting it), the water dipoles in contact with the metal generally have some preferential orientation. From a coulombic standpoint, this situation is equivalent to charge separation across the interface, because the dipoles are... 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a... 

Coalescence frequency J depends on dimensionless parameters k, p, Sa, Sr, t, y, a. The parameter k characterizes relative sizes of interacting drops p is the viscosity ratio of drops and ambient liquid Sa and Sr are the forces of molecular attraction and electrostatic repulsion of drops r is the relative thickness of electric double layer, which depends, in particular, on concentration of electrolyte in ambient liquid y is the electromagnetic retardation of molecular interaction a is relative potential of surfaces of interacting drops. Let us estimate the values of these parameters. For hydrosols, the Hamaker constant is F 10 ° J. For viscosity and density of external liquid take m /s, 10 kg/m. ... 

The interaction forces and potentials between two charged surfaces in an electrolyte are fundamental to the analysis of colloidal systems and are associated with the formation of electrical double layers (EDLs) in vicinity of the solid surfaces. The charged surfaces typically interact across a solution that contains a reservoir of ions, as a consequence of the dissociation of the electrolyte that is already present. In colloid and interfacial sciences, the EDL interaction potential, coupled with the van der Waals interaction potential, leads to the fimdamental understanding of inter-siuface interaction mechanisms, based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1]. In practice, the considerable variations in the EDL interaction, brought about by the variations in electrolytic concentration of the dispersing medium, pH of the medium, and the siuface chemistry, lead to a diverse natiue of the colloidal behavior. A fundamental understanding of the physics of EDL interactions, therefore, is of prime importance in... 

As mentioned previously, electrolytes can be differentiated in the manner in which their ionic components interact with the alloy components. Table 4.4 shows interaction energies that were determined for various notional electrolytes interacting with a binary MNxLNi x alloy. For every alloy atom that was on an exposed surface, electrolyte components within... 

A vast amount of research has been carried out in the area of activated carbon adsorption during the past four or five decades, and research data are scattered in different journals published in different countries and in the proceedings and abstracts of the International Conferences and Symposia on the science and technology of activated carbon adsorbents. This book critically reviews the available literature and tries to offer suitable interpretations of the surface-related interactions of the activated carbons. The book also contains consistent explanations for surface interactions applicable to the adsorption of a wide variety of adsorbates that could be strong or weak electrolytes. 

The adsorption process is affected by the presence of any background electrolyte in solution. An increase in ionic strength will shield both surface and polyelectrolyte charges with the consequence that the surface-polymer interaction will decrease. However, and more importantly, there will also be a decrease in the lateral interactions between segments in the adsorbed layer. This allows the adoption of a more compact adsorbed conformation with the result that the adsorbed amount will increase. In general, the adsorbed amount almost always increases with increasing ionic strength. 

When two surfaces in an electrolyte environment approach one another, their double layers overlap and several situations may arise. In the case of oxide surfaces, the interaction may itself influence the degree of dissociation of surface groups, such that neither the surface potential nor the surface charge remains constant. A charge regulation model may then be more appropriate [23]. The constant charge and the constant potential model allow, however, both upper and lower limits for the strength of the interaction to be estimated. For most of the model calculations performed, it is assumed that... 

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