Adsorbed ions, locations

During an extended period the structure of the two-dimensional (2D) ensemble formed by adsorbed ions located in the inner Helmholtz plane was postulated on intuitive grounds. 

Fig. 3.3 Schematic ic aesentation of the electric double layca with immobilised ions in the Stem layer and mobile ions in the diffuse layer, a further subdivision of the Stem layer is indicated by specifically adsorbed ions located at the inner Helmholtz plane (DIP) and immobifised hydrated counter-ions at the outer Helmholtz plane (OHP) the OHP marks the beginning of the diffuse layer, where the electric potential exponentiaUy decays with surface distance the regions between the surface, the IHP and the OHP are free of charge the shear plane or slip plane (SP) marks the transition from stagnant to mobile solvent...

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

Fig. 2. Schematic diagram of a suspended colloidal particle, showing relative locations of the Stem layer (thickness, 5) that consists of adsorbed ions and the Gouy-Chapman layer (1 /k) which dissipates the excess charge, not screened by the Stem layer, to 2ero ia the bulk solution (108). In the absence of a...

The issue of contact vs. noncontact adsorption and the location of the adsorbed ion relative to the surface is best handled by an examination of the ion potential of mean force. If we denote by P(z) the probability density for finding an ion at a position z relative to a planar interface, then the free energy profile (or the potential of mean force) is given by... 

In the triple layer model, the potential determining ions are located at the oxide surface with the specifically adsorbing ions and the ion pairs in the inner Helmholz... 

The four layer model (Bowden et ah, 1980 Bousse and Meindle, 1986) also locates different adsorbing ions in different planes. It has been used to model adsorption of phosphate, citrate and selenite (Bowden et ah, 1980) and borate (Bloesch et al., 1987) on goethite and competitive adsorption of Ca and Cd on ferrihydrite (Cowan et al., 1991). 

The variation of the electric potential in the electric double layer with the distance from the charged surface is depicted in Figure 6.2. The potential at the surface ( /o) linearly decreases in the Stem layer to the value of the zeta potential (0- This is the electric potential at the plane of shear between the Stern layer (and that part of the double layer occupied by the molecules of solvent associated with the adsorbed ions) and the diffuse part of the double layer. The zeta potential decays exponentially from to zero with the distance from the plane of shear between the Stern layer and the diffuse part of the double layer. The location of the plane of shear a small distance further out from the surface than the Stem plane renders the zeta potential marginally smaller in magnitude than the potential at the Stem plane ( /5). However, in order to simplify the mathematical models describing the electric double layer, it is customary to assume the identity of (ti/j) and The bulk experimental evidence indicates that errors introduced through this approximation are usually small. 

AHa for the Adsorption of Alkali Metals. If an alkali metal atom is located at an infinite distance from a metal surface at zero potential, then the heat of adsorption comprises the work done in (1) transferring an electron from the atom to the metal, and (2) bringing the positive ion to its equiUbrium distance from the metal surface (127). In the first step, the energy change is (e0 — el), where is the work function of the metal and I is the ionization potential of the alkali metal atom. In the second, the force of attraction on the positive ion at a distance d from the metal surface, i.e., the electrostatic image force, is e /4d hence, the heat Uberated is e /4do, where do is the equilibrium distance of the adsorbed ion from the metal surface. This distance is often assumed to be equal to the ionic radius, which is 1.83 A. for the Na ion. The initial heat of adsorption, therefore, is... 

Ions with a weak solvation shell, anions in general, lose a part of or the complete solvation shell in the double layer and form a chemical bond to the metal surface. The adsorption is termed specific since the interaction occurs only for certain ions or molecules and is not related to the charge on the ion. The plane where the center of these ions are located is called the inner Helmholtz layer. In the specific adsorption, ions are chemically bound to the surface and the interaction has a covalent nature. In the case of non-specific adsorption, in which an electrostatic force binds ions to the surface, the coverage of ions is below 0.1 -0.2 ML due to electrostatic repulsion between the ions. In contrast, the coverage of specifically adsorbed ions exceeds this value, and a close-packed layer of specifically adsorbed ions is often observed. Specifically adsorbed ions are easily observed by STM [22], indicating that the junction between the electrode surface and the inner Helmholtz layer is highly... 

In terms of Equations 7 and 8 the change in adsorption behavior at pH 6 and 10"5M Co (II) shown in Figure 4 can be described as being caused by the operation of a specific adsorption potential (+ cal./mole). Since the hydrolysis products are unlikely to contribute a sufficiently large potential to account for the increased adsorption, it must be concluded that above 10"5M at pH 6, and above pH 6.5-7.5 at 10 4M Co (II), the Co2+ ion is specifically adsorbed and located within the Stem plane. It is probable that these conditions correspond to the fact that an activity ratio of Co2+ and surface O" or OH" sites has been exceeded. 

This point may be located analytically by virtue of the fact that a semiconductor powder in a solution of adsorbing ions acts as a buffer for those ions everywhere but at the PZZP. Thus, potential drift or differential potentiometric titrations( 7,8 ) can be employed to determine the PZZP as illustrated in Figure 2 for CdS. Once the PZZP is determined in this fashion, a direct comparison of EA and is possible and has been done for a variety of semiconductors. (jO Figure 3 illustrates the vs. pH data for p-GaP and shows good agreement between the predicted at the PZZP from electronegativity calculations and the observed value.(9)... 

Fig. 1.10 Schematic view of the electrical double layer in agreement with the Gouy-Chapman-Stem-Grahame models. The metallic electrode has a negative net charge and the solvated cations define the inner limit of the diffuse later at the Helmholtz outer plane (OHP). There are anions adsorbed at the electrode which are located at the inner Helmholtz plane (IHP). The presence of such anions is stabilized by the corresponding images at the electrode in such a way that each adsorbed ion establishes the presence of a surface dipole at the interface...

Specifically adsorbed ions are those which are attached (albeit temporarily) to the surface by electrostatic and/or van der Waals forces strongly enough to overcome thermal agitation. They may be dehydrated, at least in the direction of the surface. The centres of any specifically adsorbed ions are located in the Stern layer - i.e. between the surface and the Stern plane. Ions with centres located beyond the Stern plane form the diffuse part of the double layer, for which the Gouy-Chapman treatment outlined in the previous section, with 0o replaced by (f/d, is considered to be applicable. 

The difference patterns for deuterium absorbed on Co304 at room temperature (Figure 1) show axially symmetric 200 and 420 peaks which indicate that the deuterium atoms adsorb at Co3+ ions located in tetrahedral sites. The presence of such ions was unexpected in a normal spinel and the authors suggested that this may be a product of the special preparation required of Co304 to obtain a high surface area specimen. 

As is shown in fig. 1 a layer of hydrated ions is assumed adsorbed on the negatively charged solid surface. The plane SH, taken as the origin, is located a distance jA from the midplane, and is determined by the (average position of) the centers of those water molecules in the hydration sheaths of the adsorbed ions that lie on the water side of the ions. The minimum distance 6 j of the counterions in the diffuse layer from the plane SH is about the radius S of the hydrated ion, while the minimum distance of the co-ion from the same plane, 0. Expression (15)... 

The inner part of the double layer may include specifically adsorbed ions. In this case, the center of the specifically adsorbed ions is located between the surface and the Stem plane. Specifically adsorbed ions (e.g., surfactants) either lower or elevate the Stem potential and the zeta potential as shown in Figure 4.31. When the specific adsorption of the surface-active or polyvalent counter ions is strong, the charge sign of the Stem potential will be reversed. The Stem potential can be greater than the surface potential if the surface-active co-ions are adsorbed. The adsorption of nonionic surfactants causes the surface of shear to be moved to a much longer distance from the Stem plane. As a result, the zeta potential will be much lower than the Stem potential. 

Using this model, one cannot forecast the adsorption of the background electrolyte ions because this model do not consider the reactions responsible for such a process. Zeta potential values, calculated on the basis of this model, are usually too high, nevertheless, because of its simplicity the model is applied very often. In a more complicated model of edl, the three plate model (see Fig. 3), besides the mentioned surface plate and the diffusion layer, in Stern layer there are some specifically adsorbed ions. The surface charge is formed by = SOHJ and = SO- groups, also by other groups formed by complexation or pair formation with background electrolyte ions = SOHj An- and = SO Ct+. It is assumed that both, cation (Ct+) and anion (A-), are located in the same distance from the surface of the oxide and form the inner Helmholtz plane (IHP). In this case, beside mentioned parameters for two layer model, the additional parameters should be added, i.e., surface complex formation constants (with cation pKct or anion pKAn) and compact and diffuse layer capacities. 

Conversely, according to the description of the electrical double layer based on the Stern-Gouy-Chapman (S-G-C) version of the theory [24], counter ions cannot get closer to the surface than a certain distance (plane of closest approach of counter ions). Chemically adsorbed ions are located at the inner Helmholtz plane (IHP), while non-chemically adsorbed ions are located in the outer Helmholtz plane (OHP) at a distance x from the surface. The potential difference between this plane and the bulk solution is 1 ohp- In this version of the theory, Pqhp replaces P in all equations. Two regions are discernible in the double layer the compact area between the charged surface and the OHP in which the potential decays linearly and the diffuse layer in which the potential decay is almost exponential due to screening effects. 

All of this requires the caveat that the situation becomes different when the specifically adsorbed Ion Itself Is also titrated. This Is. for Instance, the case with phosphate Ions. By potentlometry these Ions are then counted as If they were part of the solid. In that case, anions would also reduce the p.z.c. whereas cations increase It. For this reason. SlOj dispersions containing traces of alumina have a p.z.c. above that of S102. and alumina dispersions with S102 contamination have a p.z.c. below that of AI2O3. Doping of the solid gives rise to similar trends. The difference between the two sets of directions of p.z.c. shifts helps to establish the location and binding of the added Ion. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

In conclusion, the combined experimental and theoretical study of methanol adsorbed on MgO films with different defect densities allows for a better identification of the surface sites responsible for the MgO reactivity. On the inert terrace sites only physisorption is observed. Molecular chemisorption, activation, and heterolytic dissociation occur on irregular sites. The low-coordinated Mg-O pairs of ions located at edges and steps can lead to strongly activated and even dissociated methanol molecules. Adsorption of CHsO" and H+ fragments seems to be preferred over dissociation into and OH ... 

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