The chemical and structural consequences of the interaction
between aggregates made of Al13 polycations
and salicylate ligand were examined using adsorption isotherms,
electrophoresis, infrared spectroscopy,
Fraunhofer light diffraction, and static light scattering spectroscopy.
Drastic changes were observed for
a pseudo equilibrium ligand concentration equimolar to
Al13. Below this concentration, salicylate
forms
monodentate complexes with the charged aluminum sites, decreasing the
electrostatic repulsion between
Al13 particles. A densification of the aggregates is
evident by an increase of the apparent fractal dimension
from 1.8 to 2.9. No dissolution was observed in this concentration
range. Above this ligand concentration,
excess adsorption of salicylate causes charge reversal, and dissolution
of the aggregates, in the form of
soluble complexes and small clusters was observed. The
consequences were a size reduction and loosening
of the aggregates. In aquatic media, such radical in situ
changes are able to strongly influence the transport
of colloids and associated species.
The chemical, textural and structural transformations of Al13-intercalated montmorillonite, resulting from the depolymerization of the interlamellar Al13 polycations by Na salicylate solutions, were studied. Nitrogen gas adsorption shows a dramatic decrease in specific surface area from 493 to 39 m2g–1, due to the loss of microporosity. Modelling of small-angle X-ray scattering (SAXS) curves shows that the final product contains two phases: a Na-exchanged swelling phase accounting for 40% of the clay, and a fixed interlayer distance (20.8 Å ) phase accounting for 60% of the clay. The Al remaining in the clay galleries (45% of the initial Al) is composed of 13% Al13– and 87% hexacoordinated Al, probably Al(OH)03 and oligomeric Al-salicylate complexes, as shown by 27Al NMR. The instability of Al13-intercalated montmorillonite towards organic ligands is related to the hydration shell of the tridecamer, revealed by SAXS. These results focus on the necessity to take into account the presence of organic ligands in natural media when using materials such as Al13-intercalated clays.
In the present work, we examine the transformations of aggregated Al13 polycations interacting in suspension with low-molecular-weight organic acids at near neutral pH. The consequences of the interaction strongly depend on the complexing power and concentration of ligands. The most striking feature is the existence of two domains separated by a characteristic adsorbed ligand concentration, corresponding to charge neutralisation by ligands. In the lower concentration domain, ligands interact with the surface aluminum atoms by forming complexes, decreasing the surface charge. In this domain, no soluble aluminum is released from the aggregates. At higher concentration, ligands are adsorbed in excess by ligand exchange and hydrogen bonding, which results in charge reversal. Simultaneously, ligand promoted dissolution of the aggregates occurs, in the form of soluble complexes. It is assumed that dissolution originates from the initially neutral sites. In aquatic media, such radical insitu changes are able to strongly influence the transport and sedimentation of colloids and associated species.
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