Adsorbate ions

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]. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

Eigure 3 schematically depicts the stmcture of the electrode—solution interface. The inner Helmholtz plane (IHP) refers to the distance of closest approach of specifically adsorbed ions, generally anions to the electrode surface. In aqueous systems, water molecules adsorb onto the electrode surface. 

The outer Helmholtz plane (OHP) refers to the distance of closest approach of non specifically adsorbed ions, generally cations. The interactions of the ions of the OHP with the surface are not specific and have the character of longer range coulombic interactions. Cations that populate the outer Helmholtz plane are usually solvated and are generally larger in size than the anions. 

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...

Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

Even in the absence of Faradaic current, ie, in the case of an ideally polarizable electrode, changing the potential of the electrode causes a transient current to flow, charging the double layer. The metal may have an excess charge near its surface to balance the charge of the specifically adsorbed ions. These two planes of charge separated by a small distance are analogous to a capacitor. Thus the electrode is analogous to a double-layer capacitance in parallel with a kinetic resistance. 

The spectroscopic evidence suggests that on adsorption of CO, an adsorbed ion and a polymeric form of the CO molecule are formed thus... 

Elution The stripping of adsorbed ions from an ion-exchange material by the use of solutions containing other ions in concentrations higher than those of the ions to be stripped. 

I.H.P. and O.H.P. (the diffuse layer being disregarded), and adistinction is made between contact-adsorbed ions and ions electrostatically adsorbed by long-range coulombk forces, Wroblowa, Kovac and Bockris showed that when = 0(pzc). the B,D.M. isotherm can be expressed in the form... 

The inner layer (closest to the electrode), known as the inner Helmholtz plane (IHP), contains solvent molecules and specifically adsorbed ions (which are not hilly solvated). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholtz plane (OHP), reflects the imaginary plane passing through the center of solvated ions at then closest approach to the surface. The solvated ions are nonspecifically adsorbed and are attracted to the surface by long-range coulombic forces. Both Helmholtz layers represent the compact layer. Such a compact layer of charges is strongly held by the electrode and can survive even when the electrode is pulled out of the solution. The Helmholtz model does not take into account the thermal motion of ions, which loosens them from the compact layer. 

In the case of ionic adsorbates, the variation in WS50is normally unable to provide a clue to the molecular structure of the solvent since free charge contributions outweigh dipolar effects. In this case UHV experiments are able to give a much better resolved molecular picture of the situation. The interface is synthesized by adsorbing ions first and solvent molecules afterward. The variation of work function thus provides evidence for the effect of the two components separately and it is possible to see the different orientation of water molecules around an adsorbed ion.58,86,87 Examples are provided in Fig. 6. 

Figure 5.55. (a) CuJ4 cluster used to model the Cu(100) surface. Oxygen has been adsorbed on the central 4-fold hollow site, (b) Pt25 cluster used to model the Pt(111) surface. Oxygen has been adsorbed on the central 3-fold hollow site. The position of the adsorbed ions (or point charges) is also shown.83,84 Reprinted with permission from the American Chemical Society. 

To this end quantum mechanical calculations have been carried out83 84 using cluster models of the Pt(lll) surface, as well as of the Cu(100) surface, with various representations of the adsorbed ions on the catalyst surface (Fig. 5.55). 

The analysis of a pharmaceutical tablet (6) requires sample preparation that is little more complex as most tablets contain excipients (a solid diluent) that may be starch, chalk, silica gel, cellulose or some other physiologically inert material. This sample preparation procedure depends on the insolubility of the excipient in methanol. As the components of interest are both acidic and neutral, the separation was achieved by exploiting both the ionic interactions between the organic acids and the adsorbed ion exchanger and the dispersive interactions with the remaining exposed reverse phase. 

As the solvent mixture also contained 225 mg of tetramethyl ammonium hydroxide pentahydrate per liter at a high water content (75%), the surface of the reverse phase would have been largely covered with the tetramethyl ammonium hydroxide pentahydrate. This would have acted as an adsorbed ion exchange stationary phase. It is clear that the free acids, salicylic acid, acetylsalicylic acid (aspirin) and benzoic acid were retained largely by ionic interactions with adsorbed basic ion exchanger and partly by dispersive interactions with the exposed reversed phase. The acetaminophen and the caffeine, on the other hand, being unionized substances, were retained only by dispersive interactions with the exposed reversed phase. 

Complete resolution was not achieved due to the carryover of interfering substances which frequently occurs when separating the components of biological samples. The column carried a reverse phase, but as the mobile phase contained low concentrations of lauryl sulfate, some would have adsorbed on the surface of the stationary phase and significantly modified its interacting properties. The retention mechanism is likely to have involved both ionic interactions with the adsorbed ion exchanger together with dispersive interactions with any exposed areas of the reverse phase. 

The peak capacity is not pertinent as the separation was developed by a solvent program. The expected efficiency of the column when operated at the optimum velocity would be about 5,500 theoretical plates. This is not a particularly high efficiency and so the separation depended heavily on the phases selected and the gradient employed. The separation was achieved by a complex mixture of ionic and dispersive interactions between the solutes and the stationary phase and ionic, polar and dispersive forces between the solutes and the mobile phase. The initial solvent was a 1% acetic acid and 1 mM tetrabutyl ammonium phosphate buffered to a pH of 2.8. Initially the tetrabutyl ammonium salt would be adsorbed strongly on the reverse phase and thus acted as an adsorbed ion exchanger. During the program, acetonitrile was added to the solvent and initially this increased the dispersive interactions between the solute and the mobile phase. 

A correlation between the amount of adsorbed ions and the electrode potential, in particular E. , has been identified apparently for the first time by Frumkin and Obrutschewa [26Fru]. A minimum of ionic adsorption was found at E, this is equivalent to the absence of specific adsorption at Ep c- The measurement of the amount of adsorbed ions was performed by measuring the ionic concentration in the solution as a function of the electrode potential or by measuring the surface concentration of adsorbed ions by e.g. radiotracer techniques (see also 4.2). (Data obtained with this method are labelled lA). 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

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