Ion exchange, surfaces

Figure 2. TEM images of PtBi/C catalysts prepared by (a) ion exchange/surface redox reaction, and (b) coimpregnation.

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

The purpose of the present chapter is to introduce some of the basic concepts essential for understanding electrostatic and electrical double-layer pheneomena that are important in problems such as the protein/ion-exchange surface pictured above. The scope of the chapter is of course considerably limited, and we restrict it to concepts such as the nature of surface charges in simple systems, the structure of the resulting electrical double layer, the derivation of the Poisson-Boltzmann equation for electrostatic potential distribution in the double layer and some of its approximate solutions, and the electrostatic interaction forces for simple geometric situations. Nonetheless, these concepts lay the foundation on which the edifice needed for more complicated problems is built. 

There is debate over whether the ion pair forms in the mobile phase to form a new species, which then sorbs to the stationary phase, or whether the agent sorbs to the stationary phase by itself, forming in effect an ion exchange surface (18-20). In any case, the effect is roughly the same - an increase in retention of solute species that are charged oppositely to the ion-pairing agent. 

FIGURE 19.3 Schematic of a cation resin bead showing the polystyrene chains with sulfonic groups, the divinylbenzene cross-links, and the active sulfonic groups where the ion exchange occurs. Because the void spaces between the polymer chains and crosslinks contain water, the effective ion-exchange surface area of the bead is not limited to its outside diameter. 

Ion-interaction chromatography is an intermediate between reversed-phase and ion-exchange chromatography. Introduction of amphiphilic and Uo-philic ions into the mobile phase causes their adsorption on the hydrophobic surface of packing material with subsequent transformation into a pseudo ion-exchange surface. Ionic interactions with charged analytes can occur in the mobile phase and with counterions that may be adsorbed on the stationary-phase surface. 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

A wide range of interactions including ion exchange, surface complexation, and precipitation contribute to the removal of arsenic from aquatic solution by soil and sediments. The majority of arsenic present in soils is sorbed onto the surface of the solid matrix. Adsorption processes. 

The first model proposed an ion-exchange mechanism where the free Upophilic charged counter-ion is adsorbed onto the non-polar surface of the stationary phase to constitute an ion-exchange surface. The sample ions of opposite charge to the counter-ions are then partitioned between the stationary phase and the mobile phase by an ion-exchange process. If the concentration or lipophiUcity of the counter-ion is increased, the surface coverage of the stationary phase by counter-ion will be increased. 

Continued penetration of the glass structure by some ions could lead to its decomposition. The thickness of the ion-exchanged surface layer is related to the durability of the glassInitial reactions are rapid... 

Rigidity and mechanical stability of the substrate under conditions of high-performance liquid chromatography are important and microspherical particles of macroporous highly crosslinked PS-DVB or poly(metacrylate) and silica are well suited for CIC. There are three main methods of producing a chelating ion-exchange surface. 

The deposition of vinyl pyridine polymers initiated by the action of dihalides in the pores of hollow fibers having less than 100 A radius leads to ion exchange surfaces which can readily transfer ions according to the Donnan formulations i.e., the concentration gradient of a pump ion can be used to transfer another ion across the membrane wall against the latter s concentration gradient. 

Wersin ([53] and references therein) used PHREEQC to include a combination of ion exchange and surface complexation processes to describe a bentonite backfill pore water. Vico [54] also used the ion exchange-surface complexation combination to model the sorption process of Cu on sepioUte. 

An assemblage or multisurface model is a description of the soil solid phases as a set of surfaces where ions are bound by different mechanisms (i.e., ion exchange, surface complexation) the total amount bound of a species i is simply the sum over all the representative surfaces, such as... 

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