Solution complex formation, effect

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

Figure 10.18. Effect of pH on residual metal concentration in fresh waters. Dissolved zinc is plotted against pH. (a) Zinc in relatively undisturbed major rivers including the Yangtze (Chiang Jiang) and tributaries of the Amazon and Orinoco, (b) Zinc normalized to total dissolved solids for the same set of major rivers, (c) Zinc in pH-adjusted aliquots of Mississippi River water (April 1984, 103 mg liter suspended load, pH 7.7) the adjusted aliquots were allowed to equilibrate overnight before filtration and analysis. (From Shiller and Boyle, 1985.) (d) Zinc in different mountain lakes in the southern parts of the Swiss Alps. These lakes are less than 10 km apait, so that the atmospheric inputs can be considered to be uniform over this scale, but their water composition (pH) is influenced by different bedrocks in their catchments. A similar dependence on pH has also been observed for Cd and Pb but this dependence is less pronounced with Cu(II) when solute complex formation counteracts adsorption (data 1983-1992). (From Sigg et al., 1995, in press.)...

Dautzenberg H, Rother G, Hartmann J. Light scattering studies of polyelectrolyte complex formation effect of polymer concentration. In Schmitz KS, ed. Macroion Characterization From Dilute Solution to Complex Fluids. ACS Symposium Series 548. Washington, DC American Chemical Society, 1994 210-224. 

There is an extensive literature in this area. Helfferich (1962, 1995) has provided a comprehensive description of the effect of chemical reactions on ion exchange equilibria. Three types of chemical reactions of interest are complex formation in solution complex formation with the ion exchanger ionization/dissociation in the solution. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Conformational Adjustments The conformations of protein and ligand in the free state may differ from those in the complex. The conformation in the complex may be different from the most stable conformation in solution, and/or a broader range of conformations may be sampled in solution than in the complex. In the former case, the required adjustment raises the energy, in the latter it lowers the entropy in either case this effect favors the dissociated state (although exceptional instances in which the flexibility increases as a result of complex formation seem possible). With current models based on two-body potentials (but not with force fields based on polarizable atoms, currently under development), separate intra-molecular energies of protein and ligand in the complex are, in fact, definable. However, it is impossible to assign separate entropies to the two parts of the complex. 

The solubility of hydrogen chloride in solutions of aromatic hydrocarbons in toluene and in w-heptane at —78-51 °C has been measured, and equilibrium constants for Tr-complex formation evaluated. Substituent effects follow the pattern outlined above (table 6.2). In contrast to (T-complexes, these 7r-complexes are colourless and non-conducting, and do not take part in hydrogen exchange. 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

A which is not observed in individual solutions of the two enones at the same concentrations and may thus be indicative of a complex formation. However, the ratio of isomeric cyclobutane products resulting from such photocycloadditions is generally seen to be a quite sensitive function of steric effects and of the properties of the reaction solvent, of the excited state(s) involved (in some cases two different excited triplet states of the same enone have been found to lead to different adducts) and of the substituents of the excited enone and substrate. No fully satisfactory theory has yet been put forth to draw together all the observations reported thus far. 

The effect of fluoride was further demonstrated by the increase in plutonium solubility in deionized water from about 11 percent to essentially 100 percent by addition of sufficient NaF to raise the fluoride concentration to that of basalt ground water. It is likely that the enhanced solubility of plutonium in waters containing high fluoride concentrations is the result of stabilization of Pu(IV) in solution by formation of fluoride complexes. Normally Pu(IV) is the least soluble of the four... 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

As has been noted earlier, the solvent usually has little effect on free-radical substitutions in contrast to ionic ones indeed, reactions in solution are often quite similar in character to those in the gas phase, where there is no solvent at all. However, in certain cases the solvent can make an appreciable difference. Chlorination of 2,3-dimethylbutane in aliphatic solvents gave 60% (CH3)2CHCH-(CH3)CH2C1 and 40% (CH3)2CHCC1(CH3)2, while in aromatic solvents the ratio became 10 90. This result is attributed to complex formation between the... 

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

Zinc complex formation with 1,3-diketones in aqueous solution has been investigated with pentane-2,4-dione, l,l,l-trifluoropentane-2,4-dione, and 4,4,4-trifluoro-l-(2-thienyl)butane-l, 3-dione. The buffer dimethylarsinic acid was shown to have a catalytic effect on complex formation with pentane-2,4-dione and the proton transfer reactions were affected.471,472 High-resolution solid state 13C NMR studies of bis(2,4-pentanedionato) zinc complexes have been carried out.473... 

Absorption studies of 2-cyclohexenone-ethoxylethylene solutions failed to reveal evidence of donor-acceptor complex formation. It should be noted, however, that photocycloaddition from ground state 7r-complexes (such as would be observed from absorption studies) does not correctly predict the observed orientational effects. 

Rhodium and ruthenium complexes have also been studied as effective catalysts. Rh(diphos)2Cl [diphos = l,2-bis(diphenyl-phosphino)ethane] catalyzed the electroreduction of C02 in acetonitrile solution.146 Formate was produced at current efficiencies of ca. 20-40% in dry acetonitrile at ca. -1.5 V (versus Ag wire). It was suggested that acetonitrile itself was the source of the hydrogen atom and that formation of the hydride HRh(diphos)2 as an active intermediate was involved. Rh(bpy)3Cl3, which had been used as a catalyst for the two-electron reduction of NAD+ (nicotinamide adenine dinucleotide) to NADH by Wienkamp and Steckhan,147 has also acted as a catalyst for C02 reduction in aqueous solutions (0.1 M TEAP) at -1.1 V versus SCE using Hg, Pb, In, graphite, and n-Ti02 electrodes.148 Formate was the main... 

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