Ion specific effects on the stability of layered double hydroxide colloids.
Résumé
Pos. charged layered double hydroxide particles composed of Mg2+ and Al3+ layer-forming cations and NO3- charge compensating anions (MgAl-NO3-LDH) were synthesized and the colloidal stability of their aq. suspensions was investigated in the presence of inorg. anions of different charges. The formation of the layered structure was confirmed by X-ray diffraction, while the charging and aggregation properties were explored by electrophoresis and light scattering. The monovalent anions adsorb on the oppositely charged surface to a different extent according to their hydration state leading to the Cl- \textgreater NO3- \textgreater SCN- \textgreater HCO3- order in surface charge densities. The ions on the right side of the series induce the aggregation of MgAl-NO3-LDH particles at lower concns., whereas in the presence of the left ones, the suspensions are stable even at higher salt levels. The adsorption of multivalent anions gave rise to charge neutralization and charge reversal at appropriate concns. For some di, tri and tetravalent ions, charge reversal resulted in restabilization of the suspensions in the intermediate salt concn. regime. Stable samples were also obsd. at low salt levels. Particle aggregation was fast near the charge neutralization point and at high concns. These results, which evidence the colloidal stability of MgAl-NO3-LDH in the presence of various anions, are of prime fundamental interest. These are also crit. for applications to develop stable suspensions of primary particles for water purifn. processes, with the aim of the removal of similar anions by ion exchange.