Double ionic adsorption

In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

If the solid diaphragm material adsorbs both hydrogen and hydroxyl ions it is evident that electric endosmose will cease when equal ionic adsorption has taken place, the double layer potential or electrokinetic potential being at this point zero and the diaphragm is at the isoelectric point. 

The simplest effect of pure electrostatic ionic adsorption on electrode reaction rates of ions is the Frumkin double layer effect already been discussed in Sect. 3.5. 

Cantwell, F.R Retention model for ion-pair chromatography based on double-layer ionic adsorption and exchange. Pharm. Biomed. Anal. 1984, 2, 153-164. 

DOUBLE-LAYER IONIC ADSORPTION AND EXCHANGE ON POROUS POLYMERS Frederick F. Cantwell... 

In summary, since the surface excesses are obtained by integrating over the whole diffuse layer, the GC estimates are assumed to give reasonable values for these quantities. The estimates are often used in the analysis of double layer data involving ionic adsorption in the inner layer. 

An important aspect of analyzing the double layer data in the presence of specific adsorption is the determination of the dielectric properties of the irmer layer. In the Grahame model for ionic adsorption [Gl], the adsorbed ions are assumed to have their charge centers located on the inner Helmholtz plane (iHp). Furthermore, the iHp is closer to the electrode surface than the oHp. This is due to the fact that the adsorbed ions replace solvent molecules on the electrode surface, whereas the counter ions on the oHp do not. Another feature of the following treatment is that the charge on the adsorbed ions is assumed to be located on the iHp. Accordingly, the potential drop across the inner layer is given by... 

Any portion of a dynamic ionic adsorption layer leads to an electrical double layer out of electroneutrality. The adsorbed layer acquires the charge of the fast diffusing ion, while the diffusion layer takes the charge of the slow diffusing ion. It is possible to describe qualitatively the adsorption layer interactions and their kinetics without rigorous mathematical analysis. The initial adsorption of siuface active ions is followed by the adsorption of the counter ions which reside in the diffuse double layer. Macroscopically equivalent numbers of oppositely charged ions are involved to preserve overall electric neutrality, each ion is transported by diffusion. 

The effect of ionic adsorption on the kinetics of the electrode reactions is mainly due to the change of the surface properties of the metal (e.g., work function) and of the potential profile in the double layer. The decrease in the available current of reaction sites on the electrode owing to the adsorption of ions is small, since strong electrostatic... 

ELECTRICAL DOUBLE-LAYER CAPACITY AND IONIC ADSORPTION... 

This result was taken as an evidence that ionic adsorption is not a major rate-determining factor in the studied system. The comparison of the diffusion currents produced by the egress of cations and anions from the water-filled nanopipettes (fl 11 nm) to IL showed that the mass transfer inside the pipette shaft is not significantly affected by migration and other electrostatic effects. No correlation was found between the interfacial size and IT kinetics, which would be indicative of double-layer effects. 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

For a metal/solution interface, the pcz is as informative as the electron work function is for a metal/vacuum interface.6,15 It is a property of the nature of the metal and of its surface structure (see later discussion) it is sensitive to the presence of impurities. Its value can be used to check the cleanliness and perfection of a metal surface. Its position determines the potential ranges of ionic and nonionic adsorption, and the region where double-layer effects are possible in electrode kinetics.8,10,16... 

Althongh van der Waals forces are present in every system, they dominate the disjoining pressnre in only a few simple cases, such as interactions of nonpolar and inert atoms and molecnles. It is common for surfaces to be charged, particularly when exposed to water or a liquid with a high dielectric constant, due to the dissociation of surface ionic groups or adsorption of ions from solution, hi these cases, repulsive double-layer forces originating from electrostatic and entropic interactions may dominate the disjoining pressure. These forces decay exponentially [5,6] ... 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

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