Ion specific effects on the stability of layered double hydroxide colloids

Abstract

Positively 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 aqueous suspensions was investigated in the presence of inorganic 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⁻ > NO₃⁻ > SCN⁻ > 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 concentrations, 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 concentrations. For some di, tri and tetravalent ions, charge reversal resulted in restabilization of the suspensions in the intermediate salt concentration regime. Stable samples were also observed at low salt levels. Particle aggregation was fast near the charge neutralization point and at high concentrations. 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 critical for applications to develop stable suspensions of primary particles for water purification processes, with the aim of the removal of similar anions by ion exchange.