Ion Preference

Ion preference or selectivity is defined as the potential of a charged surface to demonstrate preferential adsorption of one ion over another. Such ion preference is described by the lyotropic series and an example, from highest to lowest preference, is Ba2+ Pb2+ Si2 Ca2+ Ni Cd2+ Cu2+ Co2+ Mg2+ Ag+ Cs+ Rb+ K+ NHJ Na+ Li+ (Helfferich, 1972). Note, however, that this lyotropic series is not universally applicable. The series often depends on the nature of the adsorbing surface (Sullivan, 1977). Some rules of cation selectivity are listed below  

Selectivity Rules Based on the Physical Chemistry of Cations 

Generally, for two cations with the same valence (e.g. Na+ vs. K+), the cation with the smaller hydrated radius, or least negative heat of hydration (Tables 4.1 and 4.4), is preferred (e.g., K+). 

For two cations with the same valence, but one a stronger acid than the other (e.g., Cu2+ versus Ca2+), the cation with stronger acid behavior or higher hydrolytic constant would be preferred if the surface behaved as a relatively strong base. 

For any cation with any valence, when in the presence of two different anions, the anion with the highest potential to form neutral pairs with the cation controls the latter s adsorption potential, assuming that the anions do not react with the surface. For example, Ca2+ in the presence of Cl- exhibits greater adsorption potential than Ca2+ in the presence of SO4- due to the latter s greater potential to form neutral CaS04 pairs. 

One of the most important aspects of ion exchange is the definite preference for particular ions of a given sample, either of some soil components or of whole natural soils. There has been many studies on ion exchange on soils (Sparks 2001), but this issue is not yet settled (Teppen and David 2006 Rotenberg et al. 2009). It is sometimes, but not always, observed that ion preference is described by the lyotropic series, and an example, from highest to lowest preference, is (Helfferich 1995) 

Actually, the series often depends on the nature of the adsorbing snrface, as in the examples given in Section 5.3.4.I. Several models have been proposed in the literature (Eisenman 1962 Sullivan 1977 Xu and Harsh 1992 Auboironx et al. 1998), but with limited success. A number of factors have been proposed to influence ion 

FIGURE 5.12 K-Ca exchange isotherms and no preference isotherm at seven Aridisol soils from Iquique province, northern Chile, mostly composed of quartz. (From Gacitua, M. et al., Aust. J. Soil Res. 46, 745-750, 2008. With kind permission of CSIRO Publishing, Australia.) 

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It is possible to systematically alter the net magnetic moment of ferrites by chemical substitutions. A very important industrial appHcation is the increase of the magnetic moment in mixed MnZn-ferrites and NiZn-ferrites. When Zn ions are introduced in Mn-ferrite or Ni-ferrite, these ions prefer to occupy A-sites. Because is nonmagnetic, the A-sublattice magnetization is reduced and consequendy the total net magnetic moment is increased. 

The fact that all ligands failed to transfer Fe " ion from the aqueous into the organic phase was not unexpected, since this ion prefers to bind with picric acid more than the other ligands. This property is typical only for the Fe " ion [54]. Yet, our previous observations [49] indicated that, when FelNO.ala was used instead of metal picrate, it was possible to efficiently extract Fe " into the organic phase by utilizing ligands 1, 3, and 4. 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

Large concentrations of halide ions, preferably iodide, favour the formation of /ra/i5-stilbene from benzaldehyde and benzyltriphenylphosphonium halides in methanol with methoxide as base, whereas large concentrations of methoxide ions slightly favour formation of the m-isomer. These effects have been explained by the preferential solvation of P+ by halide ions, leading to greater reversibility of betaine formation. Methoxide ions, on the other hand, are preferentially solvated by methanol. 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

Mixed clusters NH3/H20 (139-141), NH3/MeOH (61), and NH3/Me2CO (142) have been reacted with bare metal ions and in general the transition metal ions preferred coordination to ammonia whereas the non-transition metal ions such as Mg+ and Al+ were nonselective, showing some similarity to condensed-phase systems. 

The observation that the macroscopic proton coefficient is a function of adsorption density and pH has several implications for macroscopic modeling of cation and anion adsorption. The dependency of x on pH and T affects 1) the relationship of the macroscopic partitioning coefficient to pH and adsorption density, 2) the notion of metal ion preferences for a particular surface in systems with multiple solid phases, 3) the accuracy of predictive models when used over a range of adsorption density and pH values, and 4) conclusions about site heterogeneity based upon partitioning expressions which use constant proton coefficients. 

It is evident that the central Cu(I) ion, preferring a tetrahedral geometry, abandons one of the coordinated sulfur atoms. 

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases. Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe " " or Cu " "), and alkali metal ions (preferably Cs" "). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst it also affects the surface acidity. 

Fig. 6.16). Loss of H" from [QHg]" leads to the formation of [C7H7] ions, preferably of tropylium structure, [54] and thus, to the typical fragment ion series described above. 

The structure is tentative, but it is known that (1) europium ions prefer an octahedral geometry, (2) they complex well with ammonia, and (3) (in this case) two naphthalene units are involved (Stevenson et al. 1999). 

A series of ion-selective membrane electrodes based on neutral carrier solvent polymeric membranes has been designed for the potentiometric determination of ion activities (for reviews see Refs. 52, 65). Systems with analytically relevant selectivities for Li+, Na+, K+, NHJ, Ca2+, and Ba2+, are available. In agreement with the treatment given in Sections III and IV, the ions preferred in potentiometric studies may be transported preferentially through the same membranes in electrodialytic experiments. So far, selective carrier transports have been realized for Li+, Na+, K+, and Ca2+. 

Figure 2.8 Redox-driven translocation of a copper center, based on the Cu"/Cu change. The Cu11 ion stays in the tetramine compartment of the ditopic ligand 10, whereas the Cu1 ion prefers to occupy the bis-(2,2 -bipyridine) compartment. The translocation of the copper center between the two compartments is fast and reversible when carried out through the Cun-to-Cu1 reduction with ascorbic acid and Cu -to-Cu" oxidation with H202, in a MeCN solution.

Each Cu1 center (gray ball) is bound to an imine and to a pyridine nitrogen atom from each strand and shows a rather distorted tetrahedral coordination geometry. On the other hand, Fig. 2.15, which displays the molecular structure of the [Cun(16)](CF3S03)2 salt, shows that the Cu11 ion prefers to form a mononuclear complex species. 

For example, phosphines (RjP) and thioethers (R2S) have a much greater tendency to coordinate with Hg24, Pd24, and Pt2+, but ammonia, amines (R N), water, and fluoride ions prefer Be24, Ti4+, and Co3+. Such a classification has proved very useful in accounting for and predicting the stability of coordination compounds. 

Several generalizations emerge from an exhaustive study of these complexes the methoxide ion prefers to attack an a-position in the thiophene nucleus the resulting thiophene complexes are in general more stable than the corresponding Meisenheimer complexes in the benzene series and the stability is increased if the reactive centre already carries an alkoxy substituent. 

In animals and in many bacteria, PEP is formed by decarboxylation of oxaloacetate. In this reaction, which is catalyzed by PEP carboxykinase (PEPCK), a molecule of GTP, ATP, or inosine triphosphate captures and phosphorylates the enolate anion generated by the decarboxylation (Eq. 13-46).252 The stereochemistry is such that C02 departs from the si face of the forming enol.253 The phospho group is transferred from GTP with inversion at the phosphorus atom 254 The enzyme requires a divalent metal ion, preferably Mn2+. 

Although they often share little sequence similarity and have quite different specificiities, many restriction enzymes have similar three-dimensional structures as well as mechanisms of action. This is true for the EcoRI, BamHl (Fig. 26-5),83/90 EcoRV,91/91a and C/r 101 enzymes,84 and presumably many others. The specifically shaped and tightly packed active sites in the enzyme-substrate complexes ensure specificity. For example, the EcoRV endonuclease cleaves DNA at its recognition site at least a million times faster than at any other DNA sequence.91 As mentioned in Chapter 12, restriction endonucleases require a metal ion, preferably Mg2+, and probably act via a hydroxyl ion generated from Mg2+-OH2 at the active site. Three conserved active site residues, Asp 91, Glu 111, and Lys 113, in the EcoRI endonuclease interact with the DNA near the cleavage site. Lys 113 is replaced by Glu 113 in the BamHl enzyme.83 90... 

Lau et al. (2) have used electrophoresis and 31P NMR to estimate divalent cation adsorption potentials and the fraction of phosphorous nuclei whose magnetic resonance is affected by the presence of divalent cobalt. Although they see the same preference for solid vs. melted chains and the preference of (saturated chain) DPPC and DMPC for Ca2+ over Mg2, they note no similar ion preference by egg lecithin. They used... 

The effect of salts on the CP may be explained in terms of the polarity of the solute and the solvent. Ions prefer a water environment, and thus they increase the polarity of the solvent, which decreases the solubility of L64. CNSmd I- ions have a slight preference to associate with the L64 molecule and increase its polarity, and thus they increase the solubility, resulting in an increase in the CP. 

Fig. 1.1. The relationship between the ligand and metal ion preferences, the resulting molecular structure and the molecular properties.

These variations in the ease of formation of bridgehead cations are attributed to steric effects 9S>105>187l Carbonium ions prefer planarity strongly 187>292). 

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