Metal Speciation Dynamics in Colloidal Ligand Dispersions

Abstract

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.

Metal speciation dynamics in colloidal ligand dispersions. Part 3: Lability features of steady-state systems

“…This feature has important consequences for the dynamic properties of the metal species. For ligands located on the surface of colloidal particles it has already been shown that the dynamic metal speciation features arise from the coupling between the chemical kinetics of complex formation/dissociation and the local diffusion-controlled transport of metal ions to/from the particle [1]. That led us to introduce the concept of 'generalized' rates of association and dissociation for colloidal metal complexes, k à a and k à d [1], which may differ by several orders of magnitude from their homogeneous solution counterparts, k a and k d .…”

“…For ligands located on the surface of colloidal particles it has already been shown that the dynamic metal speciation features arise from the coupling between the chemical kinetics of complex formation/dissociation and the local diffusion-controlled transport of metal ions to/from the particle [1]. That led us to introduce the concept of 'generalized' rates of association and dissociation for colloidal metal complexes, k à a and k à d [1], which may differ by several orders of magnitude from their homogeneous solution counterparts, k a and k d . The conventional approach of taking a colloidal ligand dispersion as a smeared-out homogeneous solution [2][3][4] overestimates both the dynamics and the lability of the colloidal metal complexes.…”

Electrochemical Methods for Speciation of Trace Elements in Marine Waters. Dynamic Aspects

“…14 The basic Eigen mechanism was kept virtually unchanged in the electroanalytical field since the introduction of the first lability criteria 15 (Figure 2) until 2005. 16 With the advent of the stripping chronopotentiometry in the early 2000s, and especially the scanned deposition potential mode in 2003, the direct determination of the kinetic parameters using that technique clearly showed that the basic Eigen mechanism could not explain more involved systems like the metal ion complexation in colloidal systems and/or with multidentate ligands.…”

“…Pinheiro et al 16 pointed out that, in colloidal systems, the ligands are localized within the vicinity of the particles, whereas in true solutions the ligand distribution is taken as homogeneous over the solution volume (Figure 3). In this case the local diffusion-controlled transport of metal ions to/ from the particle must be taken into account.…”

“…The development of the theory of metal speciation in colloidal dispersions 16 effectively modifies the finite rates of association/dissociation of the colloidal metal complexes, thus invoking the consideration of two basic dynamic criteria: the association/dissociation kinetics of the volume complexation reaction (the "dynamic" criterion) and the interfacial flux of free metal to a macroscopic surface due to dissociation of complex species (the "lability" criterion). Pinheiro et al demonstrated 35 that the conventional approach for homogeneous systems that assume a smeared-out ligand distribution, overestimates both the dynamics and the lability of metal complexes when applied to colloidal ligands.…”