Influence of adsorption of ionic liquid constituents on the stability of layered double hydroxide colloids

Performance of ionic liquid functionalized metal organic frameworks in the adsorption process of phenol derivatives

The growth of industrial activities has produced a significant increase in the release of toxic organic pollutants (OPs) to the environment from industrial wastewater. On this premise, this study reports the use of metal organic frameworks (MOFs) impregnated with various ionic liquids (ILs) in the adsorption of phenol derivatives, i.e., 2,6-dimethylphenol and 4,4'-dihydroxybiphenyl. MOFs were prepared starting from 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) with divalent (Co, Ni, Cu) and trivalent (Ce) metal salts in mild hydrothermal conditions using water as a green solvent. Imidazolium base ionic liquids, namely 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium nitrate, 1-butyl-3-methylimidazolium chloride, and 1-hexyl-3-methyl-imidazolium chloride, were used to modify MOFs, leading to composite materials (IL@MOF), which show the structural characteristics of MOFs, and complement the advantages of ILs. SEM, EDX images, and TG data indicate that the IL is just attached on the surface of the adsorbent material, with no changes in crystal size or morphology, but with slightly altered thermal stabilities of IL@MOF composites compared to the original ILs and MOFs, pointing to some interionic interaction between IL and MOF. This research consists of equilibrium experiments, studying the effect of the initial concentration of OPs on the adsorption efficiency of the as-prepared MOFs and IL@MOF, in order to determine the influence of the nature of the adsorbent on its developed adsorption capacity and to investigate the performance of both ILs and MOFs. To determine the maximum adsorption capacity, several empirical isotherms were used: Langmuir, Freundlich, Redlich-Peterson, and Dubinin-Radushkevich. The characteristic parameters for each isotherm and the correlation coefficient (R2) were identified. The IL modification of MOFs increased the adsorption capacity of IL@MOF for the removal of phenol derivatives from aqueous solution. The adsorption capacity function of the MOF structure follows the trend CeHEDP > CoHEDP > NiHEDP > CuHEDP. The best performance was achieved by adsorbent materials based on Ce.

Colloidal Interactions of Microplastic Particles with Anionic Clays in Electrolyte Solutions

Homoaggregation of polystyrene microplastics (MPs) and heteroaggregation of MPs with anionic clay minerals, namely, layered double hydroxide (LDH), in different salt (NaCl, CaCl2, and Na2SO4) solutions were systematically investigated using light scattering techniques. The salt type and ionic strength had significant effects on the stability of both MPs and LDH particles individually and the results could be explained by DLVO theory and the Schulze-Hardy rule. However, once stable colloidal dispersions of the individual particles were mixed, heteroaggregation occurred between the oppositely charged MPs and LDH, which was also confirmed by transmission electron microscopy and X-ray scattering. Adsorption of the LDH particles resulted in neutralization and reversal of MPs surface charge at appropriate LDH doses. Once LDH adsorption neutralized the negative charges of the MP spheres, rapid aggregation was observed in the dispersions, whereas stable samples formed at high and low LDH concentrations. The governing interparticle interactions included repulsive electrical double-layer forces, as well as van der Waals and patch-charge attractions, the strength of which depended on the mass ratio of the interacting particles and the composition of the aqueous solvent. Our results shed light on the colloidal behavior of MPs in a complex aquatic environment and, in the long term, are also useful for developing LDH-based approaches for water remediation to remove contamination with MP particles.

Surface modification of two-dimensional layered double hydroxide nanoparticles with biopolymers for biomedical applications

Layered double hydroxides (LDHs) are appealing nanomaterials for (bio)medical applications and their potential is threefold. One can gain advantage of the structure of LDH frame (i.e., layered morphology), anion exchanging property towards drugs with acidic character and tendency for facile surface modification with biopolymers. This review focuses on the third aspect, as it is necessary to evaluate the advantages of polymer adsorption on LDH surfaces. Beside the short discussion on fundamental and structural features of LDHs, LDH-biopolymer interactions will be classified in terms of the effect on the colloidal stability of the dispersions. Thereafter, an overview on the biocompatibility and biomedical applications of LDH-biopolymer composite materials will be given. Finally, the advances made in the field will be summarized and future research directions will be suggested.

Suppressing the Hofmeister Anion Effect by Thermal Annealing of Thin-Film Multilayers Made of Weak Polyelectrolytes

Thin films made of weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) have been fabricated on silicon wafers using the layer-by-layer (LbL) method. To study the influence of counteranion type on the growth and properties of PAH/PAA multilayers, the nature of the supporting sodium salt was varied from cosmotropic to chaotropic anions (F^–, Cl^–, and ClO₄^–). Results of ellipsometry and AFM measurements indicate that the film thickness and surface roughness systematically increase on the order F^– < Cl^– < ClO₄^–. Furthermore, we found that the hydrophobicity of the PAH/PAA multilayer also follows the described trend when a polycation is the terminating layer. However, the heating of PAH/PAA multilayers to 60 °C during the LbL assembly suppressed the influence of background anions on the multilayer formation and properties. On the basis of the obtained results, it could be concluded that thermal annealing induces changes at the polymer–air interface in the sense of reorientation and migration of polymer chains.

Hard-Cation-Soft-Anion Ionic Liquids for PEDOT:PSS Treatment

A promising conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) experiences significant conductivity enhancement when treated with proper ionic liquids (ILs). Based on the hard-soft-acid-base principle, we propose a combination of a hydrophilic hard cation A⁺ (instead of the commonly used 1-ethyl-3-methyl imidazolium, EMIM⁺) and a hydrophobic soft anion X^- (such as tetracyanoborate, TCB^-) as the best ILs for this purpose. Such ILs would decouple hydrophilic-but-insulating PSS^- from conducting-but-hydrophobic PEDOT⁺ most efficiently by strong interactions with hydrophilic A⁺ and hydrophobic X^-, respectively. Such a favorable ion exchange between PEDOT⁺:PSS^- and A⁺:X^- ILs would allow the growth of conducting PEDOT⁺ domains decorated by X^-, not disturbed by PSS^- or A⁺. Using density functional theory calculations and molecular dynamics simulations, we demonstrate that a protic cation- (aliphatic N-alkyl pyrrolidinium, in particular) combined with the hydrophobic anion TCB^- indeed outperforms EMIM⁺ by promptly leaving hydrophobic TCB^- and strongly binding to hydrophilic PSS^-.

Effect of Water and Salt on the Colloidal Stability of Latex Particles in Ionic Liquid Solutions

The colloidal stability of sulfate (SL) and polyimidazolium-modified sulfate (SL-IP-2) latex particles was studied in an ionic liquid (IL) of ethylammonium nitrate (EAN) and its water mixtures. Aggregation rates were found to vary systematically as a function of the IL-to-water ratio. Repulsive electrostatic interactions between particles dominated at low IL concentrations, while they were significantly screened at intermediate IL concentrations, leading to destabilization of the dispersions. When the IL concentration was further increased, the aggregation of latex particles slowed down due to the increased viscosity and finally, a striking stabilization was observed in the IL-rich regime close to the pure IL solvent. The latter stabilization is due to the formation of IL layers at the interface between particles and IL, which induce repulsive oscillatory forces. The presence of the added salt in the system affected differently the structure of the interfaces around SL and SL-IP-2 particles. The sign of the charge and the composition of the particle surfaces were found to be the most important parameters affecting the colloidal stability. The nature of the counterions also plays an important role in the interfacial properties due to their influence on the structure of the IL surface layers. No evidence was observed for the presence of long-range electrostatic interactions between the particles in pure ILs. The results indicate that the presence of even low concentrations of water and salt in the system (as undesirable impurities) can strongly alter the interfacial structure and thus, the aggregation mechanism in particle IL dispersions.