Swelling of Na∕Mg-montmorillonites and hydration of interlayer cations: A Monte Carlo study

Critical role of water structure around interlayer ions for ion storage in layered double hydroxides

Water-containing layered materials have found various applications such as water purification and energy storage. The highly structured water molecules around ions under the confinement between the layers determine the ion storage ability. Yet, the relationship between the configuration of interlayer ions and water structure in high ion storage layered materials is elusive. Herein, using layered double hydroxides, we demonstrate that the water structure is sensitive to the filling density of ions in the interlayer space and governs the ion storage. For ion storage of dilute nitrate ions, a 24% decrease in the filling density increases the nitrate storage capacity by 300%. Quartz crystal microbalance with dissipation monitoring studies, combined with multimodal ex situ experiments and theoretical calculations, reveal that the decreasing filling density effectively facilitates the 2D hydrogen-bond networking structure in water around interlayer nitrate ions along with minimal change in the layered structure, leading to the high storage capacity.

Study of Molybdenite Floatability: Effect of Clays and Seawater

Current challenges in froth flotation are the presence of complex gangues and the use of low-quality waters, such as seawater. In this scenario, the recovery of molybdenum minerals is difficult, mainly due to the hydrophobic faces’ physicochemical changes. In the present study, the natural floatability of pure molybdenite was analyzed by using microflotation assays, and hydrophobicity was measured by performing contact-angle measurements. The impact of two clays, kaolin (non-swelling) and Na-montmorillonite (swelling), was studied. The behavior in freshwater and seawater at pH 8 was compared, considering the current condition of the Cu/Mo mining industries, which use seawater in their operations. The presence of clays lowered the natural floatability of molybdenite precisely because they adhere to the surface and reduce its contact angle. However, the intensity with which they cause this phenomenon depends on the type of water and clay. Kaolin strongly adheres to the valuable mineral in both freshwater and seawater. For its part, Na-montmorillonite does it with greater intensity in a saline medium, but in freshwater, a high concentration of phyllosilicate is required to reduce the hydrophobicity of molybdenite. The clays’ adherence was validated by scanning electron microscopy (SEM) analysis.

Clay Swelling: Role of Cations in Stabilizing/Destabilizing Mechanisms

The stepwise hydration of clay minerals has been observed repeatedly in studies, but the underlying mechanism remains unclear. Previous numerical studies confirmed the presence of one-water layer (1W) and two-water layer (2W) hydration states. However, the undisturbed transition between these hydration states has never been captured. Using molecular dynamics simulation, this study (i) simulated for the first time the free 1W-2W transition during clay hydration and (ii) identified the underlying mechanism to be the detachment of cations from the clay surface and the formation of a shell of water molecules around the cation. The swelling dynamics of clay was found to be affected by the clay charge, clay mineralogy, and counterions through complex cation-clay interactions, cation hydration capacity, and cation migration rate.

Hydration and swelling: a theoretical investigation on the cooperativity effect of H-bonding interactions between p-hydroxy hydroxymethyl calix[4]/[5]arene and H2O by many-body interaction and density functional reactivity theory

In order to explore the nature of the hydration and swelling of superabsorbent resin, a theoretical investigation into the cooperativity effect of the H-bonding interactions in the hydrates of four model compounds that can be regarded as the units of hydroquinone formaldehyde resin (HFR) (i.e., O-hydroxymethyl-1,4-dihydroxybenzene, methylene di-O-hydroxymethyl-1,4-dihydroxybenzene, p-hydroxy hydroxymethyl calix[4]arene and p-hydroxy hydroxymethyl calix[5]arene) was carried out by many-body interaction and density functional reactivity theory. The HFR···H₂O···H₂O complexes, in which the H₂O···H₂O moieties are bound with both the hydroxyl groups of HFR, are the most stable. For the HFR(H₂O)_n clusters, the interaction energy per building block is increased as the number of the size n increases, indicating the cooperativity effect. Therefore, a deduction is given that the cooperativity effects of the H-bonding interactions play an important role in the process of the hydration and swelling of HFR, and the swelling behavior is mainly attributed to the cooperativity effects which arised from the interactions between the H₂O molecules. The origin of the cooperativity effect was examined employing several information-theoretic quantities in the density functional reactivity theory. The degree of swelling of HFR was quantitated using a measure of volume. Graphical abstract.

Potential anticancer activity of protocatechuic acid loaded in montmorillonite/Fe3O4 nanocomposites stabilized by seaweed Kappaphycus alvarezii

Superparamagnetic magnetite nanocomposites (Fe₃O₄-NCs) were successfully synthesized, which comprised of montmorillonite (MMT) as matrix support, Kappaphycus alvarezii (SW) as bio-stabilizer and Fe₃O₄ as filler in the composites to form MMT/SW/Fe₃O₄-NCs. Nanocomposite with 0.5 g Fe₃O₄ (MMT/SW/0.5Fe₃O₄) was selected for anticancer activity study because it revealed high crystallinity, particle size of 7.2 ± 1.7 nm with majority of spherical shape, and M_s = 5.85 emu/g with negligible coercivity. Drug loading and release studies were carried out using protocatechuic acid (PCA) as the model for anticancer drug, which showed 19% and 87% of PCA release in pH 7.4 and 4.8, respectively. Monolayer anticancer assay showed that PCA-loaded MMT/SW/Fe₃O₄ (MMT/SW/Fe₃O₄-PCA) had selectivity towards HCT116 (colorectal cancer cell line). Although MMT/SW/Fe₃O₄-PCA (0.64 mg/mL) showed higher IC₅₀ than PCA (0.148 mg/mL) and MMT/SW/Fe₃O₄ (0.306 mg/mL, MMT/SW/Fe₃O₄-PCA showed more effective killing towards tumour spheroid model generated from HCT116. The IC₅₀ for MMT/SW/Fe₃O₄-PCA, MMT/SW/Fe₃O₄ and PCA were 0.132, 0.23 and 0.55 mg/mL, respectively. This suggests the improved penetration efficiency and drug release of MMT/SW/Fe₃O₄-PCA towards HCT116 spheroids. Moreover, concentration that lower than 2 mg/mL MMT/SW/Fe₃O₄-PCA did not result any hemolysis in human blood, which suggests them to be ideal for intravenous injection. This study highlights the potential of MMT/SW/Fe₃O₄-NCs as drug delivery agent.

Interaction of Metamitron and Fenhexamid with Ca2+ -Montmorillonite Clay Surfaces: A Density Functional Theory Molecular Dynamics Study

Metamitron (Meta), an herbicide, and fenhexamid (Fen), a fungicide, are authorized by the European Union to be used in agriculture. This article reports theoretical calculations about Meta and Fen in interaction with a clay surface: a Ca-montmorillonite (Mont). Conformational searches have been performed thanks to Car-Parrinello molecular dynamics simulations from which geometries have been extracted. Interaction and adsorption energies have been calculated for isomers of Meta or Fen in interaction with Mont to understand the relative stability of various kinds of complexation. Substantial adsorption energies are comparable for Meta and Fen: around -40 kcal/mol. For Fen-Mont, the CO monodentate family is surprisingly the lowest in energy. Moreover, the 10 lowest-energy isomers involve complexation on Fen carbonyl oxygens. The Meta-Mont lowest-energy family, N-N, does not involve π delocalization breaking within Meta. At the same time, the stronger the interaction energy is, the larger the structural modifications within Mont are, particularly concerning the interacting cation distance to the surface. The non-negligible charge transfer and the magnitude of the adsorption energy speak in favor of the chemisorption of the pesticide on the surface. © 2019 Wiley Periodicals, Inc.

Swelling of clay minerals: dual characteristics of K+ ions and exploration of critical influencing factors

Clay swelling occurs frequently and is closely relevant to a number of engineering and industrial processes, while the underlying mechanisms remain elusive. In this study, K+-bearing clay systems with different charge amounts and charge locations have been simulated by molecular dynamics, showing that swelling is unfavorable for lower charge amounts (1.00 and 1.25 e uc-1) while it relies on charge locations for higher charge amounts (1.50 and 1.75 e uc-1): inhibited when tetrahedrally charged and favored when octahedrally charged. Accordingly, K+ shows dual characteristics and is not always a swelling inhibitor as generally thought. The various influencing factors are inspected and only the hydration effect interprets satisfactorily the swelling behaviors for all K+-bearing clay systems. The critical role of hydration effect during clay swelling is corroborated by the results of residence time, distribution of interlayer water and divergent swelling behaviors from Na+-bearing clay systems. Although water participates in a wide spectrum of physical and chemical processes, hydration is not necessarily among the most important influencing factors. Hydration effect has been evidenced as critical for clay swelling, and the results provide new insights into unraveling the complex swelling processes and resolving the associated engineering and industrial problems.

Influence of hydrophilicity on adsorption of caffeine onto montmorillonite

Some types of montmorillonite containing different interlayer ions were prepared and the changes in the interlayer spacings, the hydrophilicity, and the characteristics of adsorption of caffeine in solution were observed. Ion exchange treatments were performed using Li, Na, K, Rb, Cs, Mg, Ca, Sr, or Ba. As a result, Li- and Na-type montmorillonite showed larger interlayer distance (1.31–1.53 nm), than K, Rb, and Cs-type montmorillonite (1.23–1.26 nm). In the measurement of hydrophilicity using a pulse NMR-based particle interface analyzer, Li- and Na-type montmorillonite showed higher hydrophilicity. In addition, KLang, which indicates the interaction with caffeine, was 0.25–0.32 l/mmol, which is lower than K-, Rb-, and Cs-type montmorillonite (1.14–1.60 l/mmol). It is possible that adsorption of water molecules inhibits caffeine from adsorbing. Because of the difficulty of exchange between caffeine and water molecules in interlayer of the Li- and Na-type montmorillonite, the interaction with caffeine decreased. Alternatively, another possibility is that when highly hydrophilic montmorillonite retains many water molecules, the caffeine adsorption sites are blocked by water molecules. In either case, hydrophilicity has a large influence on the adsorption of caffeine onto montmorillonite.

Predicting Organic Cation Sorption Coefficients: Accounting for Competition from Sorbed Inorganic Cations Using a Simple Probe Molecule

With the increasing number of emerging contaminants that are cationic at environmentally relevant pH values, there is a need for robust predictive models of organic cation sorption coefficients (K_d). Current predictive models fail to account for the differences in the identity, abundance, and affinity of surface-associated inorganic exchange ions naturally present at negatively charged receptor sites on environmental solids. To better understand how organic cation sorption is influenced by surface-associated inorganic exchange ions, sorption coefficients of 10 organic cations (including eight pharmaceuticals and two simple probe organic amines) were determined for six homoionic forms of the aluminosilicate mineral, montmorillonite. Organic cation sorption coefficients exhibited consistent trends for all compounds across the various homoionic clays with sorption coefficients (K_d) decreasing as follows: K_d^Na+ > K_d^NH₄+ ≥ K_d^K+ > K_d^Ca2+ ≥ K_d^Mg2+ > K_d^Al3+. This trend for competition between organic cations and exchangeable inorganic cations is consistent with the inorganic cation selectivity sequence, determined for exchange between inorganic ions. Such consistent trends in competition between organic and inorganic cations suggested that a simple probe cation, such as phenyltrimethylammonium or benzylamine, could capture soil-to-soil variations in native inorganic cation identity and abundance for the prediction of organic cation sorption to soils and soil minerals. Indeed, sorption of two pharmaceutical compounds to 30 soils was better described by phenyltrimethylammonium sorption than by measures of benzylamine sorption, effective cation exchange capacity alone, or a model from the literature (Droge, S., and Goss, K. Environ. Sci. Technol. 2013, 47, 14224). A hybrid approach integrating structural scaling factors derived from this literature model of organic cation sorption, along with phenyltrimethylammonium K_d values, allowed for estimation of K_d values for more structurally complex organic cations to homoionic montmorillonites and to heteroionic soils (mean absolute error of 0.27 log unit). Accordingly, we concluded that the use of phenyltrimethylammonium as a probe compound was a promising means to account for the identity, affinity, and abundance of natural exchange ions in the prediction of organic cation sorption coefficients for environmental solids.

Connecting the molecular scale to the continuum scale for diffusion processes in smectite-rich porous media

In this paper, we address the manner in which the continuum-scale diffusive properties of smectite-rich porous media arise from their molecular- and pore-scale features. Our starting point is a successful model of the continuum-scale apparent diffusion coefficient for water tracers and cations, which decomposes it as a sum of pore-scale terms describing diffusion in macropore and interlayer "compartments." We then apply molecular dynamics (MD) simulations to determine molecular-scale diffusion coefficients D(interlayer) of water tracers and representative cations (Na(+), Cs(+), Sr(2+)) in Na-smectite interlayers. We find that a remarkably simple expression relates D(interlayer) to the pore-scale parameter δ(nanopore) ≤ 1, a constrictivity factor that accounts for the lower mobility in interlayers as compared to macropores: δ(nanopore) = D(interlayer)/D(0), where D(0) is the diffusion coefficient in bulk liquid water. Using this scaling expression, we can accurately predict the apparent diffusion coefficients of tracers H(2)0, Na(+), Sr(2+), and Cs(+) in compacted Na-smectite-rich materials.

Cetylpyridinium chloride at the mica-water interface: incomplete monolayer and bilayer structures

Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported. Simulation results reveal that monolayer or bilayer aggregates of CP(+) ions at the muscovite-water interface remain incomplete up to a CP(+) coverage compensating the negative charge of muscovite. It is predicted that at this CP(+) coverage only a partial desorption of K(+) ions occurs and the two aggregates can be distinguished with help of the X-ray reflectivity technique. Formation of inner-sphere and outer-sphere adsorption complexes of CP(+) ions at distances of approximately 3 A and approximately 5 A, respectively, from the surface is observed. Despite an increasing adsorption of CP(+) ions, the structure of the adsorbed water film is largely preserved within approximately 5 A from the surface. A strong decrease of water density beyond this distance and formation of "adsorbed K(+)"-Cl(-) ion pairs result in coadsorption of Cl(-) in an amount equivalent to 1/4 of the negative charge of muscovite as close as approximately 4.3-4.8 A to the surface for the incomplete bilayer aggregate. For the incomplete monolayer aggregate, no segregation between K(+) and CP(+) ions and a displacement of K(+) ions into the adsorption sites approximately 1.6 A from the surface are observed.

Interlayer expansion and mechanisms of anion sorption of Na-montmorillonite modified by cetylpyridinium chloride: a Monte Carlo study

To study the change of interlayer structure of a Wyoming-type Na-montmorillonite as a result of the replacement of interlayer Na+ ions by cetylpyridinium (CP+) ions, a series of NPT Monte Carlo simulations of the clay mineral with different contents of CP+, Na+, Cl- ions and water in its interlayer space is carried out. In agreement with conclusions from experimental studies, the simulations show that the CP+ ions form monomolecular, bimolecular, and pseudotrimolecular layers with increasing interlayer contents. Calculated potential energies reveal that clay-organic interactions are stronger than organic-organic interactions in CP+-modified montmorillonite, which is in conformity with observations of earlier thermogravimetric experiments. The simulation results indicate that the pseudotrimolecular arrangement of CP+ ions is a prerequisite for the experimentally observed interlayer sorption of inorganic anions. Furthermore, in the interlayer space with a pseudotrimolecular layer, chloride ions favor the formation of pairs with inorganic rather than organic cations. On the basis of these findings and available experimental data, we propose that the interlayer sorption of inorganic anions from the pore space of an organically modified montmorillonite may occur not only in pairs with organic cations, as suggested earlier, but also in pairs with inorganic cations, which represents a so-far unconsidered and maybe more important mechanism of anion sorption on clay minerals.