Effects at charged interfaces

In the last chapter we learned how to describe electric double layers mathematically. In this chapter we focus on the question, how surfaces become charged. This is obviously an important question because surface charges are the reason for the formation of electric double layers. In addition, we discuss experimental aspects of charged interfaces. This includes techniques to analyze the properties of electric double layers but also applications and effects of charged surfaces. 

There are various techniques to measure different properties of electric double layers. A wide range of information was obtained from electrocapillary experiments. In an electrocapillary experiment the surface tension versus potential of a metallic surface is measured. From this the capacitance and the surface charge can be calculated. For technical reasons this is routinely only possible for mercury. 

Surface charge density and surface potential are of primary interest. For insulating surfaces, charge can be determined by potentiometric or conductometric titration, though this is a tedious procedure. For metals, the relationship between surface charge and potential can be determined by measuring the capacitance. Finally we discuss electrokinetic effects. Elec-trokinetic experiments yield the potential at the outer Helmholtz plane. 

In addition, there are techniques developed in other fields of colloid science, which are not directly related to classical electrochemistry. In surface force experiments, for instance, the distance dependence of the electric double layer is measured precisely. This will be discussed later. 

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Another determination of the surface equilibrium entails the use of the coupling of the DC electric field present at charged interfaces with the electromagnetic field, as described in the theoretical section. Integration of the nonlinear polarization over the whole double layer leads to the following expression of the effective susceptibility tensor ... 

The interfacial tension decreases with increasing amount of surface potential. The reason is the increased interfacial excess of counterions in the electric double layer. In accordance with the Gibbs adsorption isotherms, the interfacial tension must decrease with increasing interfacial excess. At charged interfaces ions have an effect similarly to surfactants at liquid surfaces. 

Molecular and macroscopic models can be effectively tested by Monte-Carlo (MC) or molecular dynamics (MD) computer simulations. These techniques are a valuable source of data for the clarification of the properties of a system and the evaluation of a theory. For this reason they have been used extensively in studies at charged interfaces. 131-133,146,155 157 (jjg majority of this work concerns the properties... 

The study of the electric device (Chapter 6) has revealed a large capacitive effect at the interface between the soUd electrolyte and the metallic electrode. Such a capacitive effect necessarily results in the presence of charges Q at the level of the electrode, which could induce the appearance of a potential V, so that ... 

The electrochemical DNA-biosensor is a complementary tool for the study of biomolecular interaction mechanisms of compounds with DNA, enabling the screening and evaluation of the effect caused to DNA by health hazardous compounds and oxidising substances. The characterization of a DNA-electrochemical biosensor provides very relevant information because the mechanisms of DNA-hazard compound interaction at charged interfaces mimic better the in vivo situation, opening wide perspectives using a particularly sensitive and selective method for the detection of specific interactions. 

PEM research is a multidisciplinary, hierarchical exercise that spans scales from Angstrom to meters. It needs to address challenges related to (i) to ionomer chemistry, (ii) physics of self-organization in ionomer solution, (iii) water sorption equilibria in nanoporous media, (iv) proton transport phenomena in aqueous media and at charged interfaces, (v) percolation effects in random heterogeneous media, and (vi) engineering optimization of coupled water and proton fluxes under operation. Figme 1.13 illustrates the three main levels of the hierarchical structure and phenomena in PEMs. 

A similar approach to the boundary condition for the potential at the metal-solution interface has been applied by Biesheuvel et al., in consideration of diffuse charge effects in galvanic cells, desalination by porous electrodes, and transient response of electrochemical cells (Biesheuvel and Bazant, 2010 Biesheuvel et al., 2009 van Soestbergen et al., 2010). However, their treatment neglected the explicit effect of In principle, the PNP model could be modified to incorporate size-dependent and spatially varying dielectric constants in nanopores, as well as ion saturation effects at the interface. However, in a heuristic fashion, such variations could be accounted for in the Helmholtz capacitance of the Stern double layer model. 

It might be possible to elucidate the detailed mechanism from measurements of the primary isotope effect. Micellar catalysis offers the possibilities both of new methods of synthesis (in hydrophilic environments in aprotic solvents), and of the study of catalytic action at charged interfaces, and so opens up an exciting new area in the study of proton-transfer processes in aprotic solvents. 

Charged interfaces in electrolyte solutions cause counterions to adsorb. A cloud with net opposite charge hovers around the interface, with a characteristic thickness comparable to the Debye length. At distances much larger than the Debye length, the effect of charged interfaces is essentially absent. 

Fig. 5.109 Left TH-stacking fault phase formed at the /J-Agl/AbOs contact [262, 289]. The enormous conductivity effects at the interface can be understood if we think of the boundary layer phase as a cation-disordered heterostructure in the sub-nm range. Right Ion redistribution occuring at each interface of the heterostructure leads, for small enough spacing, to almost predominant disorder. The charge carrier concentrations (v, i) are much higher than in the bulk [263]. According to Ref. [262].

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