Interlayer Ions Research Articles

Plasma-grafted polyamine/hydrotalcite as high efficient adsorbents for retention of uranium (VI) from aqueous solutions

The high efficient adsorbent of polyamine/hydrotalcite (PANI/HT) composites has been fabricated by plasma-grafting techniques. In this study, the effect of environmental factors on the adsorption of U(VI) on PANI/HT was investigated by EXAFS, modeling and theoretical calculations. The characteristic results indicated that the large layer spacing of PANI/HT provided the massive interlayer ion exchange capacity for U(VI). The batch results illustrated that no effect of ionic strength on U(VI) adsorption at pH > 5.0 was observed, moreover the U-N and U-Si shells were observed at pH 5.0 and 7.0 by the EXAFS analysis, respectively, indicating that inner-sphere surface complexation dominated U(VI) adsorption at pH > 5.0. The desorption results showed that release of U(VI) from PANI/HT was irreversible, which indicated that the PANI/HT presented the large layer spacing to fix uranyl into cage space of the adjacent octahedral. The adsorption behaviors of U(VI) on PANI/HT can be well fitted by double layer modeling with ion exchange and inner-sphere surface complexation. According to theoretical calculations, the binding energies of U(VI) with carbonate (15.49 kcal/mol) were significantly higher than that of U(VI) with hydroxyl (8.35 kcal/mol), indicating that U(VI) was easily combined with carbonate compared to hydroxyl groups at high pH. These observations revealed that PANI/HT can be used as an excellent adsorbent for retardation of uranium species into sub-environments.

Pressure-induced coherent sliding-layer transition in the excitonic insulator Ta2NiSe5.

The crystal structure of the excitonic insulator Ta2NiSe5 has been investigated under a range of pressures, as determined by the complementary analysis of both single-crystal and powder synchrotron X-ray diffraction measurements. The monoclinic ambient-pressure excitonic insulator phase II transforms upon warming or under a modest pressure to give the semiconducting C-centred orthorhombic phase I. At higher pressures (i.e. >3 GPa), transformation to the primitive orthorhombic semimetal phase III occurs. This transformation from phase I to phase III is a pressure-induced first-order phase transition, which takes place through coherent sliding between weakly coupled layers. This structural phase transition is significantly influenced by Coulombic interactions in the geometric arrangement between interlayer Se ions. Furthermore, upon cooling, phase III transforms into the monoclinic phase IV, which is analogous to the excitonic insulator phase II. Finally, the excitonic interactions appear to be retained despite the observed layer sliding transition.

Functional reduced graphene oxide-based membranes with selective ion transport channels for zwitterionic ions separation based on the pH gradient.

In this work, we report a method for the fabrication of a functional free-standing graphene membrane (FFGM) with high mechanical strength, enlarged interlayer spacing and ion channels for zwitterionic ions separation. To obtain the FFGM, the anionic dye Eosin Y (EY) was introduced into a graphene oxide (GO) and hydroquinone (HQ) mixture to prepare functional graphene-based membranes on Cu foil using simply a drop-casting method. In comparison with a GO membrane, the molar flux and the mechanical strength of the FFGM were dramatically increased. The FFGM was then equipped on custom-built glass reservoirs for zwitterionic amino acids (AAs) separation based on the inner pH gradient, which was formed by controlling H+ and OH- (in the feed and receiver solution) migration in rGO/GO sheets via an external electric field. With the help of the inner pH gradient and external electric field, AAs could change their charge behaviors. The ionized AAs transport through the FFGM and finally separation was realized.

Layer charge robust delamination of organo-clays.

To date delamination of organo-clays is restricted to highly charged, vermiculite-type layered silicates (e.g. n-butylammonium vermiculites) while – counterintuitively – low charged, smectite-type layered silicates do not delaminate although their Coulomb interactions are much weaker. Guided by previous findings, we now identified organo-cations that allowed for extending the delamination of organo clays to charge densities in the regime of low charged smectites as well. Upon intercalation of protonated amino-sugars like N-methyl-d-glucamine (meglumine) robust delamination of 2 : 1 layered silicates via repulsive osmotic swelling in water is achieved. This process is stable over a wide range of charge densities spanning from smectites (layer charge x ∼ 0.3 charges per formula unit Si4O10F2, p.f.u.) to vermiculites (x ∼ 0.7 p.f.u.). It is evidenced that a combination of first, a sufficiently large charge equivalent area (bulkiness) of meglumine with second, a significant hydrophilicity of meglumine leads to swelling above a threshold d-spacing of ≳17.5 Å in moist air (98% r.h.). Hereby, electrostatic attraction is critically weakened, causing the onset of repulsive osmotic swelling which leads to utter delamination. Moreover, meglumine renders delamination tolerant to charge heterogeneities typically found in natural and synthetic clays.

Effects of Interlayer Ion in Montmorillonite on Appearance of Decaffeinated Tea Beverage

Effects of Interlayer Ion in Montmorillonite on Appearance of Decaffeinated Tea Beverage

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.

Catalytic Activity of Maghnite-H+ in the Synthesis of Polyphenylmethylsiloxane under Mild and Solvent-free Conditions

In this study, a new and easy strategy was discussed for the purpose of synthesizing of a polymer of phenylmethylcyclosiloxane type (PPMS). The cationic ring opening polymerization of triphenyltrimethylcyclotrisiloxane (D3Ph,Me) was initiated by a solid, efficient and environmentally-friendly catalyst called Maghnite-H+. Maghnite-H+ is a natural clay composed essentially of montmorillonite, it is activated with an acid treatment by replacing the interlayer ions by protons, that results to the increase of the basal space, this may be confirmed by XRD analysis. The reaction was carried out without solvent at different temperatures and for different periods of time, using also several catalyst contents. Subsequently, the operating conditions were opted in order to obtain a maximum yield of the linear polymer and a high average molecular mass as well. The structure of polymers obtained was confirmed by IR analysis. 1H NMR and 13C NMR analyzes were used to follow the crosslinking of polymer chains over time. The thermal behavior was investigated by DSC analysis. The average molecular mass and the polydispersity indices were determined by GPC.

Cationic Ring Opening Polymerization of Octamethylcyclotetrasiloxane Using a Cost-Effective Solid Acid Catalyst (Maghnite-H+)

The polymerization of octamethylcyclotetrasiloxane (D4) catalyzed by Maghnite-H+, a nontoxic and green solid catalyst, is studied. The Maghnite-H+ is a montmorillonite type 2:1 dioctahedral phyllosilicates whose interlayer ions are exchanged by hydronium ions after activation with sulfuric acid which gives it its catalytic appearance. D4 was polymerized cationically by ring opening at 60 °C without solvent using Maghnite-H+ contents less than 5% by weight. The molecular structure of the polymer obtained was identified by IR, 1H NMR and 13C NMR. The DSC was used to study the thermal properties. The operating conditions were optimized so that we can achieve best performance for obtaining a linear polymer with high average molecular mass. The variation of the molecular mass distribution was verified by GPC. Finally, a reaction mechanism was proposed to show the role of the Maghnite-H+ during the different steps of the reaction.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.

Structural and vibrational studies of tunnel-like titanate nanoribbons with good ion exchange capacity

Potassium titanate nanoribbons were synthesized by hydrothermal method treating TiO2 anatase in KOH aqueous solution. Titanates with different contents of remnant potassium were produced by treating the precursor sample with HCl. After acid treatment, a solution of NaOH was used to wash the sample with less potassium in order to promote the H-Na exchange. The morphology of the nanoribbons has been confirmed by transmission electron microscopy and the ion exchange by atomic absorption spectroscopy. X-ray diffraction, infrared and Raman spectroscopies showed that the samples are octatitanates with the ion content depending on the acid/basic washing process. XRD patterns revealed a decrease in the interlayer distance during the K-H exchange, indicating that the hydrogen ions are located in interlayer space. This distance has an intermediate value in the sample with sodium. FT-IR and Raman measurements showed that the less energetic modes are more affected by interlayer ions, implying that displacements of interlayer ions are involved, while the more energetic ones are related to vibrations mainly involving the TiO6 octahedra.

Effect of chemical and geometrical parameters influencing the wettability of smectite clay films

The wettability of three swelling clays (beidellite, nontronite and montmorillonite) exchanged with different interlayer ions (Li+, Na+, K+ and Ca2+) was investigated by using the static sessile drop method for contact angle measurements. The results show that water contact angles on clay films vary as a function of the nature of the ion with a specific behaviour of K+-exchanged clays. The effect of various parameters influencing contact angle measurements such as relative humidity (RH), particle size and surface roughness has also been examined. RH has only a limited effect on contact angles even if drop stabilisation was observed. As far as the effect of particle size is concerned, the results obtained on montmorillonite of four different sizes show that the smallest angles are obtained on films of particles of the smallest sizes for a given exchangeable ion. This is clearly linked to differences in roughness that were deduced by analysing Atomic Force Microscopy experiments. In contrast, the nature of the exchangeable cation influences roughness in the order Li+<Na+<K+<Ca2+ but this order does not correspond to that deduced from the evolution of contact angles, which confirms the influence of the nature of the cation on wettability.

Changes in the Interlayer Structure and Thermodynamics of Hydrated Montmorillonite Under Basin Conditions: Molecular Simulation Approaches

AbstractInterlayer swelling of hydrated montmorillonite is an important issue in clay mineralogy. Although the swelling behavior of montmorillonite under ambient conditions has been investigated comprehensively, the effects of basin conditions on the hydration and swelling behaviors of montmorillonite have not been characterized thoroughly. In the present study, molecular dynamics simulations were employed to reveal the swelling behavior and changes in the interlayer structure of Na-montmorillonite under the high temperatures and pressures of basin conditions. According to the calculation of the immersion energy, the monolayer hydrate becomes more stable than the bilayer hydrate at a burial depth of 7 km (at a temperature of 518 K and a lithostatic pressure of 1.04 kbar). With increasing burial depth, the basal spacings of the monolayer and bilayer hydrates change to varying degrees. The density-distribution profiles of interlayer species exhibit variation in the hydrate structures due to temperature and pressure change, especially in the structures of bilayer hydrate. With increasing depth, more Na+ ions prefer to distribute closer to the clay layers. The mobility of interlayer water and ions increases with increasing temperature, while increasing pressure caused the mobility of these ions to decrease.

Phase Changes of Monosulfoaluminate in NaCl Aqueous Solution.

Monosulfoaluminate (Ca4Al2(SO4)(OH)12∙6H2O) plays an important role in anion binding in Portland cement by exchanging its original interlayer ions (SO42− and OH−) with chloride ions. In this study, scanning transmission X-ray microscope (STXM), X-ray absorption near edge structure (XANES) spectroscopy, and X-ray diffraction (XRD) were used to investigate the phase change of monosulfoaluminate due to its interaction with chloride ions. Pure monosulfoaluminate was synthesized and its powder samples were suspended in 0, 0.1, 1, 3, and 5 M NaCl solutions for seven days. At low chloride concentrations, a partial dissolution of monosulfoaluminate formed ettringite, while, with increasing chloride content, the dissolution process was suppressed. As the NaCl concentration increased, the dominant mechanism of the phase change became ion exchange, resulting in direct phase transformation from monosulfoaluminate to Kuzel’s salt or Friedel’s salt. The phase assemblages of the NaCl-reacted samples were explored using thermodynamic calculations and least-square linear combination (LC) fitting of measured XANES spectra. A comprehensive description of the phase change and its dominant mechanism are discussed.

Capture and sequestration of CO2 in the interlayer space of hydrated calcium Montmorillonite clay under various geological burial depth

We perform, at nanoscale level, the structure and dynamics of carbon dioxide molecules in hydrated Ca-montmorillonite clays. The swelling behaviour of hydrated Wyoming-type Montmorillonite including CO2 molecules and counterions is presented and analysed. In addition, the atom density profile, diffusion behaviours and radial distribution functions of CO2, interlayer water molecules and Calcium ions have been investigated at different geological burial depth of 0 km, 3 km and 6 km, which correspond to various temperature and pressure of simulation conditions. Furthermore, the influence of different hydration state on the dynamical behaviours of carbon dioxide is also explained. The calculated self-diffusion coefficient shows that the carbon dioxide species diffuse more freely with the increase of depth and water content. We also found that the presence of interlayer CO2 inhibits the diffusion of all the mobile species. These results mainly show that the hydrated clay system is an appropriate space capable of absorbing CO2 molecules.

A Series of Unique Architecture Building of Layered Zinc Hydroxides: Self-Assembling Stepwise Growth of Layered Zinc Hydroxide Carbonate and Conversion into Three-Dimensional ZnO

A series of uniformly unique three-dimensional (3D) structures of layered zinc hydroxides (LZHs) were synthesized via a hydrothermal method. During the process, two kinds of LZHs with different crystal structures could be formed, depending on the ionic groups intercalated in zinc hydroxide layers: when carbonate anions were the only interlayer ions, the corresponding LZH compound was assigned as LZHC or zinc hydroxide carbonate; when the interlayer ions contained both carbonate anions and long-chain quaternary ammonium groups, the corresponding LZH compound was denominated as LZHDC. Interestingly, the LZHDC is transformed into LZHC though a structure and compositional transformation involving an exchange of long-chain quaternary ammonium groups by carbonate anions. We propose a stepwise growth and anion exchange mechanism involving conversion from zinc precursor to LZHDC to LZHC. We demonstrated the utility the LZHs obtained in the process as self-sacrificial templates for formation novel 3D ZnO structure...

Electrode Models for Ionic Liquid-Based Capacitors

We investigate the influence of different electrode models on properties of a BMIM PF6 ionic liquid-based capacitor system using molecular dynamics simulations. We compared atomic graphene electrodes against smooth walls of equal average interaction energy. Furthermore, two different methods for simulating a constant potential drop between the electrodes were employed and compared to constant charge electrode models. In the evaluation of the voltage dependence of the ion configuration in the interfacial electrode regions, the influence of the electrode structure was determined by analyzing the in-plane radial distribution of the adsorbed ions, the interlayer ion exchange process, and the differential capacitance. We see similar results for the constant charge and constant potential approach for capacitance and structure of the adsorbed ions. We demonstrate that, due to the underlying ionic models, anions and cations adapt to the structural pattern of the graphene electrodes in different ways. This affects the charging process and leads to distinct asymmetric features in the differential capacitance.

Influence of Tetrahedral Layer Charge on the Organization of Interlayer Water and Ions in Synthetic Na-Saturated Smectites

Smectite hydration strongly influences dynamical properties of interlayer cations and thus the fate of H2O and pollutants in surficial systems where smectite-based materials are often used as a major barrier component. Smectite crystal chemistry is known to rule its hydration, although the influence of specific parameters such as the amount and location of layer charge deficit remains poorly understood. A set of tetrahedrally charged trioctahedral smectites, with a common structural formula inter[Nax]oct[Mg6]tet[Si8.0–xAlx]O20(OH,F)4 and a layer charge (x) varying from 0.8 to 3.0, were thus synthesized to assess the influence of layer charge on smectite hydration and interlayer structure. Both hydroxylated and fluorinated samples were synthesized because of the increasing use of the latter varieties in recent spectroscopic studies aiming at the determination of interlayer H2O dynamical properties. The distribution of charge-compensating cations and of associated H2O molecules was determined both experimen...

Investigation of the structure and ionic conductivity of intercalated kaolinites with potassium acetate in hydrous and anhydrous phases.

The intercalated kaolinite with potassium acetate (K-KAc), with ca. 91.9% intercalation ratio, was prepared. Thermogravimetric and variable-temperature X-ray powder diffraction analyses disclosed that a small amount of water is easily absorbed into the interlayer space of the K-KAc. The previously reported phase with the 14.2 Å interlayer distance is actually the hydrous K-KAc, which has an approximate formula of Al2Si2O5(OH)4·0.5KAc·0.25H2O. The crystal structures of hydrous and anhydrous phases of K-KAc were simulated in the density functional theory framework, demonstrating that the interactions between the K(+) and acetate ions and the inner surface of kaolinite are significantly strengthened in the anhydrous phase with regard to the hydrous phase. The ionic conductivity of K-KAc indicated that the mobility of the interlayer ions is strongly improved by thermal activation and the conductivity increased by four orders of magnitude from 363 to 423 K.

Characterization of urea-modified Palabora vermiculite

Replacing the exchangeable interlayer ions in Palabora vermiculite with magnesium, aluminum or iron (III) ions does not affect the exfoliation onset temperature (456±11°C). However, co-intercalation of minor amounts of urea lowers the exfoliation onset temperature to 209±35°C. This significant decrease is attributed to the low thermal stability of urea which releases ammonia and water as blowing agents. Thermo-mechanical analysis indicates that the expanded vermiculite exhibits spring-like properties. Part of the original expansion is recovered when the applied force is removed. The observed hysteresis is attributed to the progressive flattening of warped/buckled sheets that make up the expanded worm-like structure. Provided the expanded flake has not been heated beyond 450°C, it can be reconstituted by applying a sufficiently high compressive force followed by equilibration with distilled water. Such flakes show the conventional exfoliation behavior when heated again.

Reversibly tunable upconversion luminescence by host-guest chemistry.

Tuning upconversion (UPC) luminescence using external stimuli and fields, as well as chemical reactions, is expected to lead to novel and efficient optical sensors. Herein, highly tunable UPC luminescence was achieved through a host-guest chemistry approach. Specifically, interlayer ion exchange reactions reversibly tuned the emission intensity and green-red color of Er/Yb-codoped A2La2Ti3O10 layered perovskite, where A corresponds to proton and alkali metal ions, enabling the visualization of host-guest interactions and reactions.