Metal flux in ligand mixtures. 2. Flux enhancement due to kinetic interplay: comparison of the reaction layer approximation with a rigorous approach.
The revisited reaction layer approximation (RLA) of metal flux at consuming interfaces in ligand mixtures, discussed in the previous paper (part 1 of this series) is systematically validated by comparison with the results of rigorous numerical simulations. The current paper focuses on conditions under which the total metal flux is enhanced in the ligand (and complex) mixture compared to the case where the individual fluxes of metal complexes are independent of each other. Such an effect is exhibited only in ligand mixtures and results from the kinetic interplay between the various complexes with different labilities. It is exemplified by the Cu/NTA/N-(2-carboxyphenyl)glycine system (see part 1 paper), in which we show that the flux due to the less labile complex (CuNTA) is increased in the presence of a ligand (2-carboxyphenyl)glycine) that forms labile Cu complexes, even when the latter is in negligible proportion in the bulk solution. This paper first explains how the so-called composite and equivalent reaction layer thicknesses computed by RLA can be determined graphically from the concentration profiles of free metal and its complexes, as obtained by rigorous calculations. This approach allows comparison between the latter and RLA predictions. Comparison between these reaction layer thicknesses is then done using the chemical system mentioned above. The mechanism of flux enhancement with this system is studied in detail by following the change of the concentration profiles and reaction layer thicknesses with the increase of concentration of the ligand forming labile complexes. The mechanism of flux enhancement is well explained by the RLA and is validated by the concentration profiles obtained by rigorous numerical simulations. Based on this validation, the RLA is used to predict the conditions of the individual complex labilities and degree of complexation required to get flux enhancement in a two-ligand system. Due to compensation effects between kinetic and thermodynamic factors, a maximum flux enhancement is observed in a specific range of ratios of the lability indices of the two complexes. Flux enhancement might play a significant role in metal uptake in environmental or biological systems and should be considered in data interpretation.
Zhang
Zeshi
Z
Analytical and Biophysical Environmental Chemistry (CABE), University of Geneva, Sciences II, 30 quai E. Ansermet, 1211 Geneva 4, Switzerland. zeshi.zhang@umontreal.ca
Buffle
Jacques
J
Town
Raewyn M
RM
Puy
Jaume
J
van Leeuwen
Herman P
HP
D003198:Computer Simulation; D003300:Copper / Q000737:chemistry*; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D009571:Nitrilotriacetic Acid / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013816:Thermodynamics; D062367:ortho-Aminobenzoates / Q000737:chemistry*
D008024:Ligands; C543125:N-(2-carboxyphenyl)glycine; D009942:Organometallic Compounds; D062367:ortho-Aminobenzoates; D003300:Copper; D009571:Nitrilotriacetic Acid; D005998:Glycine