Electrical double-layer effect

Bjerrum and coworkers have assigned the three rate maxima shown in Figs. 10.7 and 10.8 to (starting from the negative potential) (a) destruction of vanadium polymeric chains (b) electric double layer effect at gold working electrode (c) stabilization of V (V) vs V (IV). These explanations are very plausible. 

ELECTRIC DOUBLE-LAYER EFFECTS ON THE ELEMENTARY ACT OF ELECTRON TRANSFER... 

The effect of the phospholipids on the rate of ion transfer has been controversial over the last years. While the early studies found a retardation effect [6-8], more recent ones reported that the rate of ion transfer is either not retarded [9,10] or even enhanced due to the presence of the monolayer [11 14]. Furthermore, the theoretical efforts to explain this effect were unsatisfactory. The retardation observed in the early studies was explained in terms of the blocking of the interfacial area by the phospholipids, and therefore was related to the size of the transferring ion and the state of the monolayer [8,15]. The enhancement observed in the following years was attributed to electrical double layer effects, but a Frumkin-type correction to the Butler Volmer (BV) equation was found unsuitable to explain the observations [11,16]. Recently, Manzanares et al. showed that the enhancement can be described by an electrical double layer correction provided that an accurate picture of the electrical double layer structure is used [17]. This theoretical approach will be the subject of Section III.C. 

Of interest here is the question relating to the value for the slope coefficient, k, from equation (1), when surfactant structures incorporating both ionic (say sulphonate) and nonionic moieties are included together. The Ghanges in electric double layer effects imparted from salt addition might dominate the packing constraints and therefore the phase inversion process, or perhaps oxyethylene dehydration effects from the presence of toluene could also play a role. 

A characteristic feature of colloidal dispersions is the large area-to-volume ratio for the particles involved. At the interfaces between the dispersed phase and the dispersion medium characteristic surface properties, such as adsorption and electric double layer effects, are evident and play a very important part in determining the physical properties of the system as a whole. It is the material within a molecular layer or so of the interface which exerts by far the greatest influence on particle-particle and particle-dispersion medium interactions. 

An important consequence of (2) is the extension of the Boltzmann distributions to the non-equilibrium case provided (I> is replaced by the relative potential Tp = — ip. Thus, unlike the streaming potential w which only appears at non-equilibrium conditions, the excess Tp plays the role of a potential purely related to electrical double layer effects. Using the above change of variables one can rephrase (1) in terms of cb and Tp as follows... 

The "electrical double layer" effect, i.e. the orientation of electrical charge on each film surface due to the use of ionic emulsifiers, is generally more important in aqueous foams than in organic polymeric foams. The stability effect arises from the repulsion of the electrical charges as the two surfaces approach each other, thus limiting the thinning of the film (cell walls) (3). 

Richards (1980, p. 209) presents a description of the ion atmosphere for a polyelectrolyte in solution. He gives a clear interpretation of the electrical double-layer effects and of the mechanisms by which the presence of excess salts can depress the electrical potential and cause the highly charged polyanions to have many of the physicochemical properties of neutral molecules. 

Other aspects of surface catalysis, including electrical double-layer effects, mist also he taken into account fora complete kinetic description (Stone, 1989). 

Because of the limited magnitude of surface tension gradients and absence of electric double-layer effects, the stabilization of foams in nonpolar liquids requires other ways of retarding the thinning of foam lamellae. These include the high liquid-phase viscosity that has been discussed earlier and increased surface viscosity because of presence of highly viscous or even rigid liquid-crystal films. 

Kuznetsov and Ulstrup initialized recently a discussion on the importance of conformational dynamics and electrical double layer effects on charge transport characteristics in single metal/redox-molecule/metal tunneling junctions [237,253]. Tao et al. explored the stabihty and break down of Au-thiol contacts, and addressed quantitatively local ionic and electron heating in single junctions composed of redox-inactive molecules [254]. 

The most useful attempt to describe the stability of concentrated systems has been made by Levine -, who concluded that electric double-layer effects are unable to provide the necessary energy barrier for dispersions in hydrocarbon media. A similar conclusion was reached from experimental observations of the stability/opacity of dispersions of titanium-dioxide pigments in alkyd resin solutions. ... 

Image forces play a significant role in electric double-layer effects. The excess surface charge density is... 

We have restricted fliis presentation to involve only oil-continuous emulsions. The reason for excluding water-continuous systems is that the OAV emulsions are usually stabilized by means of ionic surfactants (or surfactant mixtures) and consequently the electric double-layer effects can be very large. The electrode polarization will normally also be very strong in many 0/W systems. For the TDS technique water-in-oil emulsions stabilized by means of non-ionie surfactants are very good model systems. 

The concept of viscosity has been introduced and developed with homogeneous fluids, Newtonian or not. In case of an emulsion some phenomena can take place at the scale of a drop size, and the homogeneity assumption is no longer valid. For instance, it is known that in most cases of fluid transport in the presence of surfactant, there is some slipping velocity resulting from the electrical double-layer effect called the streaming potential (86). 

In the future it will be very important to study the role played by the main scaling effects (i.e.,viscous dissipation, flow work, compressibility) and micro-effects (rarefaction, electric double-layer effects) on the convective heat transfer for single-phase flows in microchannels with the aim to explain that many experimental results published in the open literature can be justified by using correctly the conventional theory. 

Consider a thin liqmd Aim in air for which the Hamaker constant Ah is 5 x K)- erg and electrical double-layer effects are negligible. If viscosity is 1 cp, density is 1 g/cm. Aim thickness is 50 nm, and surface tension is 40 mN/m, And the wavenumbers and of the critical and fastest growing disturbances and the time factor (3 for growth of the latter using the inextmisible interface results given above. Compare with the corresponding results for a free interface in the inviscid approximation. 

In the phenomenological theory, the basis of the electrical double layer effect on the elementary act of an electrochemical reaction is the application of the Br0nsted relation which connects the activation energy, Ea, of the process to the heat of the reaction (or reaction free energy) ... 

Bohinc K, Kralj-Iglic V, Iglic A (2001) Thickness of electrical double layer. Effect of ion size. Eiectrochim Acta 46 3033-3040... 

The material used for the electrode also had a strong influence on E° (Figure 7). For Mb in DDAB films, F° values ranged from -50 to +120 mV versus NHF at pH 5.5 in the order lTOapparent rate constant depended weakly on electrode material. Clearly, proteins in snrfactant films do not give the same F° -valnes as in solntion. The influence of surfactant type and electrode material suggests a possible electrical double-layer effect at the electrode-film interface on the potential felt by the protein. Surfactant-protein interactions may also be important. 

The end result of the establishment of the electrical double-layer effect and the various types of ion contact adsorption, is directly to influence the real (actual) concentration of electroactive species at an electrode surface and indirectly to modify the potential gradient at the site of electron transfer. In this respect it is important to understand the influence of the electrical double layer and allow for it where and when appropriate. 

Example 3.1.3 Calculate the force exerted on ovalbumin molecules exposed to an electrical field of constant strength 30 volts/cm, given Z, = 4.5 for ovalbumin at the solution pH. Neglect the electrical double-layer effect. 

Concerning the degradation of the ORR, the electric double layer effect and the platinum oxide effect are thought to be most plausible reasons. The fact that... 

Electrical Double-Layer Effects on Electron Transfer and Ion Transport at the Nanoscale... 

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