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.

The growing number of applications of layered double hydroxide (LDH) colloids demands for detailed understanding of particle aggregation processes in such samples. Tuning the colloidal stability in aqueous suspensions is essential to design stable systems or to induce controlled aggregation of these elongated particles. In this review, recent progress in this field is summarized; in particular, the charging and aggregation of LDHs of various compositions and sizes in the presence of different aggregating agents are discussed. The review focuses on the effect of monovalent salts, multivalent ions, and polyelectrolytes on the suspension stability of LDHs. The provided information will help to better understand the origin of interparticle forces responsible for the colloidal stability and to design highly stable or aggregating LDH colloids according to the desired goals in certain applications. Moreover, potential future research directions to obtain a broader picture of LDH aggregation are also suggested.

Highly stable suspensions of titanate nanosheets were designed by surface functionalization with P(AAm-co-DADMAC) copolymer.

Covalent organic frameworks (COFs) are commonly synthetized under harsh conditions yielding unprocessable powders. Control in their crystallization process and growth has been limited to studies conducted in hazardous organic solvents. Herein, we report a one-pot synthetic method that yields stable aqueous colloidal solutions of sub-20 nm crystalline imine-based COF particles at room temperature and ambient pressure. Additionally, through the combination of experimental and computational studies, we investigated the mechanisms and forces underlying the formation of such imine-based COF colloids in water. Further, we show that our method can be used to process the colloidal solution into 2D and 3D COF shapes, as well as to generate a COF ink that can be directly printed onto surfaces. These findings should open new vistas in COF chemistry enabling new application areas.