Polyelectrolytes, dissociation kinetics

A Comparison of the Dissociation Kinetics of Rare Earth Element Complexes with Synthetic Polyelectrolytes and Humic Acid... 

Here, we report on dissociation kinetic studies of the rare earth elements Sm(III) and Eu(III) from PAA and HA. The solution chemistry of both metals is essentially identical, and they are frequently used as analogs for trivalent actinide elements. We have studied the rare earth dissociation kinetics from different size fractions of PAA and HA using ion exchange and ligand exchange methods. The differences in metal labilities for complexes of the two polyelectrolytes and the effect of polyelectrolyte molecular weight on binding times are discussed in the context of possible polyion-cation interactions in these systems. 

Sm(III) dissociation from the polyelectrolytes occurred by multiple first-order processes. To determine the number of first-order processes and estimate rate constants and the concentration of metal dissociating by each process in the reaction, a kinetic spectrum method was used (75). The kinetic spectrum H(k,t) is defined as the following distribution function ... 

Figure 1. Kinetic spectrum for Sm(III) dissociation from humic acid as measured by a, stopped-flow and b, conventional spectrophotometry. T = 25" C, I = 1.10 M (NaClO ), pH 4.2, 5% metal loading of the polyelectrolyte.

NickeI(Il).— The rate constants have been measured for complex formation and dissociation with neutral and anionic ligands in the presence of polyelectrolytes [sodium polystyrene sulphonate, sodium poly(ethylene sulphonate), and poly-(ethylenimine) hydrochloride]. In the reaction with the murexide anion, the formation rate constant kt was decreased by the presence of either anionic or cationic polyelectrolyte, whereas the dissociation rate constant remained unchanged. With the neutral ligands (phen, bipy, and pada), however, the polyelectrolytes with hydrophobic residues enhanced kt (the value of k again being unchan ). It appears that the increase in kt is associated with a reduction in the activation enthalpy e.g. 13.5 kcal mol for bipy with no added polyelectrolyte but 11.2 kcal mol with 1.5x 10 equivl of sodium polystyrene sulphonate). The authors discuss their results in terms of the formation of polyelectrolyte-metal complexes but it is quite clear that the presence of innocent species can have large effects on the kinetics of metal complex formation - even with neutral ligands which apparently follow the normal mechanism. 

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