Interface-specific effect

Ardizzone, S., The oxide/solution interface Specific effects of indifferent electrolytes, J. Electroanal. Chem., 239, 419, 1988. 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

MV /MV " (HV is heptyl viologen and MV is methyl viologen). The specific effects of iodide on the electrochemical behavior of the layer-type compounds were compared, and the characteristics of several PEC cells were described. The interface energies for n-MoSe2 in contact with various redox couples were given as in Fig. 5.9. 

Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

Now we consider the relationship between the effective concentration(reff) and the surface pressure(tt) at the air/water interface. Ideally, the surface pressure is directly proportional to the concentration of surfactants. However, as the actual it-A isotherms show several specific effects, such as limiting area and points of inflexion, we shall assume the following relationships ... 

A considerably greater body of work with more emphasis on the CD buffer layer exists for this cell. Much of this involves the specific effects of the CD process at the interface, and this will be discussed in a later section. 

A general rule for concentration techniques in their transit function is that selectivity of the transfer of a specific effect facilitates the identification of responsible compounds. A so-called general concentration procedure with an optimal recovery of all organic compounds turns out to be a utopian scheme in many cases. The interface is too broad and too extensive to transmit clearly distinctive signals. Such a broad interface, capable of transmitting simultaneously many different signals, is of use when toxicities of different environmental systems have to be compared with each other. 

Competition at high pH from the hydrogen ion for the soap car-boxylate anion at the interface can be expected. This would tend to confound investigations, of specific effects of cations other than hydrogen. 

The main objection to the use of CMs to describe the solvent effect of an interfacial environment is that such a model neglects the specific effects arising from the interface, thus preventing a faithful description. It is therefore important to test the model and to compare the results obtained with those from other theoretical methods (e.g. simulations) and experiments. 

Ninham and Yaminsky [26], and Karraker and Radke [18] realized that the van der Waals interactions between the ions and interface are not screened by the electrolyte and hence might become more important than the image force, at large electrolyte concentrations. Recognizing that the hydration of ions might also play a role, Bostrom et al. [16,17] showed that the van der Waals interactions alone (with suitable values selected for the interaction parameters) might account for the ion specific effects. 

The accounting for the van der Waals interactions of ions has the advantage of simplicity, since the determination of the values of the B, coefficients for all the ions could lead to a quantitative treatment of ion specific effects within the Poisson-Boltzmann framework. However, as mentioned above, this treatment provides the debatable prediction that the cations would approach the interface closer than the anions. 

It was suggested recently [18,26] that the van der Waals interactions between ions and two media separated by an interface can account, at least partially, for the order in the Hofmeister series and therefore can explain ion specific effects in the distribution of electrolyte ions in the vicinity of an interface. However, there is no consensus regarding the values of the interaction constants which should be used in the van der Waals interactions. It was expected that, due to the van der Waals interactions, the negative ions should be more strongly repelled by the water/air interface than the positive ions. This seems to be, however, contradicted by experiment and simulations. 

The second difficulty is that it predicts very small changes in the interfacial forces for different ions and, therefore, can hardly account for ion specific effects (Figure 2 shows only a minute difference between the distributions of NaH and I- ions). In Figure 3a, the predictions of the BKN model for ce st 1.2 M are compared with the distributions of Na+ and Cl" ions near the air/water interface, obtained via molecular dynamics simulations in ref 9 note that in the molecular dynamics simulations the water molecules occupy the region from x = 0 to the water/air interface, located at x0 = 13.9 A therefore, the expression... 

The overall electrode process consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a number of physical phases associated in the current path and the change of potential in each phase has a specific effect in relation to surface geometry. Also, a number of different reactions can occur simultaneously on the surface and compete in surface coverage and in reaction rate. Particularly, the anodic reactions of silicon in HF solutions have two parallel paths silicon may react with fluoride species and dissolve directly or may react with water to form oxide. 

This system has been studied by Goldstein [294] and Levine [295], and seems to be an example of very site-specific adsorption in which the Cs atoms occupy four-fold coordination sites above the uppermost Si atoms. There is some similarity with other systems in that the Si dangling bond states are removed by Cs deposition to be replaced by Cs-induced gap states. There is, however, no evidence for interface instability effects. 

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