Ion specific effects on the stability of layered double hydroxide colloids

Abstract

Positively charged layered double hydroxide particles composed of Mg(2+) and Al(3+) 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.

Effect of Ionic Compounds of Different Valences on the Stability of Titanium Oxide Colloids

“…The first one is at lower ionic strength, the second one is when the restabilization started and the third one is at high salt level once the charge of the negative particles was screened by the salt constituent ions. Such a behaviour has already been described in other oppositely charged particle-multivalent ion systems too [61]. For positively charged particles (Figure 4, left), well-defined IEP values indicated the adsorption of the anions on the oppositely charged surface.…”

“…Such a charge neutralization was followed by charge reversal at higher ionic strengths and the extent of this phenomenon followed the SO 4 2− < Fe(CN) 6 ions. Such a behaviour has already been described in other oppositely charged particle-multivalent ion systems too [61]. For the negatively charged nanosheets above the PZC (Figure 4, right), the anions were the coions.…”

“…Colloidal stability of titania spheres of 200 nm in radius was investigated in time-resolved dynamic light scattering measurements [57], where the stability ratio values were determined to express the rate of aggregation in the dispersions (Figure 2). In the calculation of the stability ratios, the aggregation rates are normalized to the one measured in unstable dispersions, where the aggregation is controlled only by the diffusion of the particles [40,[60][61][62][63]. Therefore, note that stability ratio values close to one indicate unstable dispersions, while higher values refer to more stable samples.…”

A novel approach for the preparation of nanosized Gd2O3 structure: The influence of surface force on the morphology of ball milled particles

“…An amount of 2g NaOH pellets was added for the following two reasons. First, NaOH at this dilute level provided mild washing of the particle surface from any adsorbed anionic impurities [47][48][49][50]. Second, the alkali pH of this solution enhanced the flocculation of the of Gd2O3 particles as in this pH region the charge density of these particles is low.…”

Opposite counter-ion effects on condensed bundles of highly charged supramolecular nanotubes in water

“…[1][2][3][4] Since the first observation of ion-specific effects by Frank Hofmeister, 5 lots of studies pay attention to the ion-specific effects on properties of colloidal and biological systems. 12,13 Ying Tang et al provided an effective approach for the quantitative evaluation of specific ion effects on the colloidal particle aggregation by determining the activation energies. Also aggregation of charged macro-ions shows specific ion effects in aqueous solutions of salts due to preferential adsorption between small ions and the macro-ions according to the law of matching water affinity.…”