Ionic interfaces

Figure 17.14 The two possible tautomers for assembly of amidines with carboxylic acids. The neutral interface is favored for basic carboxylates in low dielectric solvents. The ionic interface is favored for electron-poor carboxylates and in polar solvents.

This relationship was used effectively by Coxon and Binder (54), and Lim and Franses (55) to solve electrophoretic systems in which flow was not applied. Lim and Franses used ionic reactions at the electrodes as boundary conditions to model apparently decreasing electrophoretic mobilities in an electrophoretic mass transport analysis. They showed that increasing ionic strength at one electrode decreased the local potential, thus decreasing mobility. Coxon and Binder specified concentrations at electrodes to arrive at a model for ionic interfaces in isotachophoretic processes. Each team investigated different systems, thus resulting in different boundary conditions. 

The distribution of a reactive counterion is described by ion exchange constant because substantial experimental work has demonstrated that micellar and other aqueous ionic interfaces act as selective ion exchangers. For example, reactive counterions, for example, H+ in anionic micelles exchange with other cations such as Li" or Cs" " and reactive anions, for example, OH or CN in cationic micelles are displaced by nonreactive counterions. Their distributions are generally described by ion exchange constants, (14) ... 

The mechanism of electrode polarization varies slightly for dc and ac currents. (The development which follows makes reference to Figure 2.2.) For the dc case, ions are attracted to the electrodes. An ionic interface layer is established between each electrode and the electrolyte. This process results in an apparent impedance for the solution which is different from the true impedance. The apparent impedance is somewhat higher than the true impedance, and one may consider that the polarization effect at the electrode surfaces contributes a series polarization impedance to the true impedance of the electrolyte. Other effects are discussed in Chapter 3. Electrode boundary... 

Figure 2 presents the ionic interface profiles of the 1-M NaNOs salt of Fig. 1 Rf 30 A is sufficient). At that concentration, the ion-ion correlations have a non-negligible effect on the shape of the interface distributions. The addition of the air-ion dispersion potentials greatly modifies the bare profiles obtained with pure hard sphere + image force contributions.

Fig. 3. KCl 0.1 M + KI 0.1 M mixture at air-water interface. Top PM interface-ions potentials. Bottom HNC ionic interface distributions. Dotted lines image force + dispersion potentials of Ref 7. Thick lines an extra attraction must be added for the anions in order to reproduce the experimental adsorption. (From Ref. 10.)...

Thus, this novel acid-alkaline hybrid capacitor has a larger voltage window and higher specific energy, when compared to that in a single electrolyte of either acid or alkaline. However, the maximum output power is still limited by the ionic interface/membrane that results in higher equivalent series resistance (ESR). Based on the bipolar membrane (BPM) we used, the ESR of acid-alkaline hybrid capacitor is around 4 times higher than that of either acidic or alkaline capacitor, in which no separator was used. 

The challenge of acid-alkaline hybrid power sources lies in how to separate the acid and alkaline electrolytes effectively while maintaining ion transport between these two electrolytes. As shown in Table 11.1, some conventional electrochemical power sources do operate in two chambers with two electrolytes separated by a membrane or an ionic interface, such as the Daniell cell and most flow batteries. In these well-known electrochemical cells with two electrolytes, the pEt value of both anolyte and catholyte are almost the same and there usually exists a common species to function as a charge carrier between the anolyte and catholyte. Examples include S04 ion for Daniell battery, and H+ ion for vanadium redox flow battery, in which there is a separator to avoid electrolyte mixing. An ion-exchange... 

The low current density of this acid-alkaline hybrid electrochemical system still restricts its further development. The key limiting factor is the ionic interface. Therefore, this technique will be widely applied when there is a specific functional membrane, like selective and improved ionic-conduction. 

Electrochemistry is concerned with the study of the interface between an electronic and an ionic conductor and, traditionally, has concentrated on (i) the nature of the ionic conductor, which is usually an aqueous or (more rarely) a non-aqueous solution, polymer or superionic solid containing mobile ions (ii) the structure of the electrified interface that fonns on inunersion of an electronic conductor into an ionic conductor and (iii) the electron-transfer processes that can take place at this interface and the limitations on the rates of such processes. 

Since taking simply ionic or van der Waals radii is too crude an approximation, one often rises basis-set-dependent ab initio atomic radii and constnicts the cavity from a set of intersecting spheres centred on the atoms [18, 19], An alternative approach, which is comparatively easy to implement, consists of rising an electrical eqnipotential surface to define the solnte-solvent interface shape [20],... 

Migration is the movement of ions due to a potential gradient. In an electrochemical cell the external electric field at the electrode/solution interface due to the drop in electrical potential between the two phases exerts an electrostatic force on the charged species present in the interfacial region, thus inducing movement of ions to or from the electrode. The magnitude is proportional to the concentration of the ion, the electric field and the ionic mobility. 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

Manne S 1997 Visualizing self-assembly Force microscopy of ionic surfactant aggregates at solid-liquid interfaces Prog. Colloid Polym. Sol. 103 226-33... 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

A potential that develops at the interface between two ionic solutions that differ in composition, because of a difference in the mobilities of the ions ( ij). 

Liquid Junction Potentials A liquid junction potential develops at the interface between any two ionic solutions that differ in composition and for which the mobility of the ions differs. Consider, for example, solutions of 0.1 M ITCl and 0.01 M ITCl separated by a porous membrane (Figure 11.6a). Since the concentration of ITCl on the left side of the membrane is greater than that on the right side of the membrane, there is a net diffusion of IT " and Ck in the direction of the arrows. The mobility of IT ", however, is greater than that for Ck, as shown by the difference in the... 

Most ionic nitrations are performed at 0—120°C. For nitrations of most aromatics, there are two Hquid phases an organic and an acid phase. Sufficient pressure, usually slightly above atmospheric, is provided to maintain the Hquid phases. A large interfacial area between the two phases is needed to expedite transfer of the reactants to the interface and of the products from the interface. The site of the main reactions is often at or close to the interface (2). To provide large interfacial areas, a mechanical agitator is frequently used. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

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