Estimation of surface free energy and solubility parameters of Mg Al layered double hydroxides

Correlations of Surface Free Energy and Solubility Parameters with Dielectric Constant, Refractive Index, and Density for Liquids

There should be some intrinsic correlations between the surface free energy (γ) and solubility (δ) parameters, called characteristic parameters here, of substances with their fundamental physical properties such as the refractive index (nD), relative dielectric constant (εr), and density (ρ) because they are all related to intermolecular interactions. Understanding the correlations between characteristic parameters and fundamental physical quantities is of great fundamental and practical importance. In the current work, possible relationships between the characteristic parameters (γ and δ) and the physical quantities (nD, εr, and ρ) were explored by a trial-and-error fitting method based on the data of 92 liquids (including 14 nonpolar, 37 polar, and 41 hydrogen-bonded liquids). The γ parameters include total (γt), dispersive (γd), and polar (γp) ones, and the δ parameters include the Hildebrand parameter (δt) and the Hansen-dispersive (δd), polar (δp), and hydrogen-bonded (δh) ones. For each characteristic parameter, its relationship with the physical quantities was established. The applicability of the so-obtained relationships was confirmed using the data of another 66 liquids as an external data set. The empirical relationships can be used to estimate the characteristic parameters of liquids from their easily measurable physical quantities.

Correlations of Surface Free Energy and Solubility Parameter with Dielectric Constant and Density for Inorganic Solids

There should be some intrinsic correlations between the surface free energy (γ) and solubility (δ) parameters, called characteristic parameters here, of substances with their basic physical properties such as the relative dielectric constant (εr) and density (ρ), because they are all related to intermolecular interactions. Several correlations have been proposed empirically (or semiempirically) for liquids, but not for solids. It is essential to establish such correlations for solids because the estimation of γ and δ for solids is difficult and/or time-consuming. In the current work, the γ, δ, εr, and ρ data of 34 inorganic solids were chosen, and possible relationships between the characteristic parameters (γ and δ) and the physical quantities (εr and ρ) were explored by a trial-and-error fitting method based on the data of the solids. Six equations relating γ and δ to εr and δ were established. The γ parameters include total (γt), dispersive (γd), and polar (γp) ones, and the δ parameters include the Hildebrand parameter (δt) and the Hansen-dispersive (δd), polar (δp), and hydrogen-bonding (δh) ones. The empirical equations can be used to estimate the characteristic parameters of inorganic solids from their easily measurable physical quantities.

Nickel-Iron Layered Double Hydroxide Dispersions in Ethanol Stabilized by Acetate Anions

This work reports a method to obtain stable dispersions of nickel-iron layered double hydroxide (NiFe-LDH) nanosheets in ethanol by exposing the as-synthetized bulk NiFe-LDH to a sodium acetate solution or by adding acetate and citrate anions inside the reaction mixture. In the case of citrate-containing NiFe-LDH, the formation of single-layer nanosheets is confirmed by X-ray diffraction and atomic force microscopy measurements. Lastly, the effect of acetate ions on the electrocatalytic activity of NiFe-LDH is discussed for the oxygen evolution reaction. Our results provide useful information to improve the existing LDH exfoliation routes based on the use of green solvent alternatives to the mostly used formamide.

Evaluating the Exfoliation Efficiency of Quasi-2D Metal Diboride Nanosheets Using Hansen Solubility Parameters

Non-van der Waals (non-vdW) solids are emerging sources of two-dimensional (2D) nanosheets that can be produced via liquid-phase exfoliation (LPE), and are beginning to expand our understanding of 2D and quasi-2D materials. Recently, nanosheets formed by LPE processing of bulk metal diborides, a diverse family of layered non-vdW ceramic materials, have been reported. However, detailed knowledge of the exfoliation efficiency of these nanomaterials is lacking, and is important for their effective solution-phase processing and for understanding their fundamental surface chemistry, since they have significant differences from more conventional nanosheets produced from layered vdW compounds. Here in this paper we use Hansen solubility theory to investigate nanosheets of the metal borides CrB₂ and MgB₂ derived from LPE. By preparing dispersions in 33 different solvents, we determine Hansen solubility parameters (δ_D, δ_P, δ_H) for both these metal diborides. We find that they exhibit notably higher δ_P and δ_H values compared to conventional vdW materials such as graphene and MoS₂, likely as a result of the types of bonds broken in such materials from exfoliation which allows for more favorable interactions with more polar and hydrogen-bonding solvents. We apply the solubility parameters to identify cosolvent blends suitable for CrB₂ and MgB₂ that produce dispersions with concentrations that match or exceed those of the top-performing individual solvents for each material and that have markedly higher stability compared to the constituent solvents of the blends alone. This work provides insight into the exfoliation effectiveness of different solvents for preparation of nanosheets from metal diborides and non-vdW materials in general. Such knowledge will be crucial for developing liquid-phase exfoliation strategies for incorporating these materials in applications such as nanocomposites, inks, and coatings.

Role of Co in the Electrocatalytic Activity of Monolayer Ternary NiFeCo-Double Hydroxide Nanosheets for Oxygen Evolution Reaction

Monolayer nanosheets have gained significant attention as functional materials and also in photo/electrocatalysis due to their unique physical/chemical properties, abundance of highly exposed coordination sites, edges, and corner sites, motivating the pursuit of highly active monolayer nanosheets. NiFe-based layered double hydroxide (NiFe-LDH) nanosheets have been regarded as the most efficient electrocatalysis for oxygen evolution. However, the limited catalytic active site and the stacking layer limited the performance. Therefore, by introducing highly electroactive Co ions into monolayer NiFe-LDH, the obtained ternary NiFeCo-LDH monolayer structure possessed an increased concentration of defect (oxygen and metal vacancies), providing enough unsaturated coordination sites, benefitting the electrocatalytic water oxidation, as also explained by the density functional theory (DFT). This work reported an efficient strategy for the synthesis of ternary monolayer LDH in the application of energy conversion and storage.

Solvothermal synthesis of carbonate-type layered double hydroxide monolayer nanosheets: Solvent selection based on characteristic parameter matching criterion

Monolayer nanosheets of CO₃^2--type layered double hydroxides (LDHs) have many special applications, but their fabrication is challenging. Herein, Co₂Al-CO₃ and Co₂Fe-CO₃ LDH nanosheets were synthesized via a solvothermal method. 31 solvents with different characteristic parameters, including the surface free energy (γ) and solubility (δ) parameters were chosen, to explore the correlation between the formation of monolayer LDHs (ML-LDHs) and the characteristic parameters of solvents. The results reveal that when the solvents used have the characteristic parameters matching to those of the LDHs, CO₃^2--type ML-LDHs with a thickness of ca. 1 nm can be obtained. The mixed-solvent strategy can provide the effective solvents for the synthesis of ML-LDHs. The dispersions of CO₃^2--type ML-LDHs can be stable for at least six months without obvious precipitation. In addition, it is demonstrated that the δ parameters of LDHs can be calculated from the γ parameters via the molar volume-free γ-δ equations developed previously. Furthermore, a new parameter called "surface free energy distance" is introduced, which can be used for screening effective solvents for the synthesis of ML-LDHs. To the best of our knowledge, this is the first time to investigate the applicable of the characteristic parameter matching principle for the bottom-up synthesis of ML-LDHs. This work deepens the understanding on the feature of CO₃^2--type LDHs and provides a solvent selection strategy for the synthesis of CO₃^2--type ML-LDHs.

Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO₃ layered double hydroxide (LDH) single-layer nanosheets dispersed in water were investigated in the presence of K⁺ salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σ_eff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. On the basis of the Stern model and the DLVO theory, the relationship of CCC with σ_eff and the ionic valences of salts (z_i) is theoretically analyzed, which can accurately describe the dependence of CCC on the σ_eff and z_i but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (E_sc) of anions within the Stern layer is developed. Especially, an empirical relationship of E_sc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO₃^2- and SO₄^2-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability but also provides useful information for the potential applications of LDH single-layer nanosheets.