Interlayer Cations Research Articles

Fabrication of Bragg stack films of clay nanosheets and polycations via co-polymerization of intercalated monomers and functional interlayer cations.

The fabrication of one-dimensional (1D) crystalline, monodomain nanocomposite films (hybrid Bragg stacks) is still limited to a few combinations of polymers and clay. The main reason is the segregation of clay and polymers driven by the entropic loss faced by the polymer confined in a narrow slit between the nanosheets. By exchanging synthetic sodium-fluorohectorite with vinylbenzyltrimethylammonium chloride, we succeeded in delaminating clay via 1D dissolution in N-methylformamide to obtain a liquid crystalline suspension. By combining this with bisphenol A glycerolate diacrylate, 1D crystalline nanocomposites could be obtained via photopolymerization of doctor bladed wet coatings. Infrared spectroscopy confirmed the co-polymerization of monomers and the organic modifier between the hectorite platelets. This single-phase hybrid material shows very low oxygen and water vapor transmission rates. The incorporation of the modified clay into the polymer leads to an oxygen transmission rate of 0.21 cm3 m-2 day-1 atm-1 at 50% r.h. and 23 °C and a water vapor transmission rate of 0.05 g m-2 day-1 for a coating of 3.7 μm, making this material appropriate for challenging packaging applications.

Development of natural sodium montmorillonite as a cathode material

Montmorillonite (MMT) is naturally occurring clay mineral in Sri Lanka. It is an aluminosilicate crystalline compound with layered structure. In this structure an octahedral sheet of aluminum-oxygen (Al-O) is sandwiched between two tetrahedral sheets of silicon-oxygen (Si-O). The MMT structure can accommodate various monovalent and divalent cations such as H⁺, Li⁺, Na⁺, Mg²⁺ within the interlayer spacing. Natural Li-MMT and Na-MMT are found in various parts of the world. The MMT structure has an ability to exchange interlayer cations with various other divalent cations. MMT with interlayer alkali cations show appreciable ionic conductivity. Natural Na-MMT found in Sri Lanka was used for the present study. To improve the ionic conductivity of natural Na-MMT, the structure was further saturated with Na⁺ ions. The Na⁺ ion concentration of saturated Na-MMT was measured by Inductive Couple Plasma Mass Spectroscopy. The structural and electrical properties of Na saturated MMT were characterized by SEM and Complex Impedance Spectroscopy. As a result of Na⁺ ion saturation the bulk conductivity of Na-MMT was increased to 3.41×10^–4 S cm^–1 at 30⁰C . Saturated Na-MMT has negligible electronic conductivity which is less than 0.001%. For using Na-MMT as intercalation cathode materials in rechargeable batteries, an appreciable electronic conductivity must also be present. To enhance the electronic conductivity of saturated Na-MMT, carbon black was mixed. The electronic transference number of saturated Na-MMT pellets was measured by the DC polarization technique. The electronic transference number of saturated Na-MMT with 25% and 30% carbon black were 0.81 and 0.87 respectively.

Interaction mechanism of supercritical CO2 with shales and a new quantitative storage capacity evaluation method

Geochemical interactions between shale and supercritical carbon dioxide (scCO2) may dominate CO2 storage in shale gas reservoirs by changing the brine chemistry and rock properties. Understanding the thermodynamic mechanism of scCO2 with shale is particularly essential for CO2 storage, as it provides an innovative strategy to evaluate the potential of shale gas reservoirs. In this study, ReaxFF molecular dynamics (MD) simulations in combination with scCO2−brine−shale experiments were conducted to gain insight into the storage mechanism. A series of laboratory tests and analyses were used to determine variations in physical and chemical properties. The DAM model was employed to calculate the dissolution, adsorption and mineralization capacities of shale samples. The MD trajectory shows that scCO2/H2O molecules rapidly diffuse to mineral surfaces within 1 ps to initiate adsorption and protonation processes. The interlayer cations that are released more slowly from illite (1.1 ns) and Ca-montmorillonite (1.5 ns) structures consume CO2 via mineralization, resulting in carbonate formation during the reaction. However, due to the release of CO32− during calcite decomposition, Ca2+ leaching at 1.6 ns cannot reduce the amount of CO2 molecules. As evidenced by reaction kinetics from MD and experiments, dissolution, adsorption and mineralization prosesses dominate the CO2 storage in shale gas reservoirs, accounting for 10.0–47.6%, 11.0–28.2% and 21.1–73.5%, respectively. The adsorption capacity is associated with space volume, and the mineralized amount is controlled by the illite and mixed-layer illite/smectite contents. Other factors, such as fluid property and space compressibility may also impact the storage capacity, resulting in a maximum adsorption capacity with a depth of around 800 m and an enhanced mineralized capacity with increasing depth. These findings demonstrate that adsorption and mineralization at storage conditions play a key role in CO2 geo-sequestration.

New laboratory procedure for distinguishing smectites with varying permanent layer charge based on the combination of Cu-trien and ethylene glycol solvation methods and X-ray diffraction

Permanent layer charge of smectites (or smectite layers in mixed-layer minerals) is important for their properties and materials' applicationss. However, distinguishing low- and high-charge smectites by traditional laboratory methods is time-consuming and instrumentally demanding and it can also produce equivocal results. In addition, the use of structural formulae based on chemical analyses combined with knowledge on crystallochemistry is challenging in arbitrary mineral mixtures. The aim of this work was to test a novel laboratory procedure for distinguishing actual multi-mineral specimens of smectites according to their permanent layer charge. This is conducted on interlayer cation exchange by the Cu-trien complex and subsequent solvation with ethylene glycol. This procedure returned more straightforward results than the traditional routine of ethylene glycol-saturated K+-exchanged specimens by producing better-resolved X-ray diffraction (XRD) patterns. The practical application was demonstrated by analysing sauconite samples from Skorpion in Namibia, which was shown to contain more sauconite phases of distinct crystallographic properties present in variable proportions. XRD of the oriented ethylene glycol-saturated specimens was insufficient for this purpose. The procedure was tested by reference clay minerals of the smectite group with varying structural parameters. It was distinguished more smectite phases with distinct layer charges and estimated the layer charge by combining interlayer cation exchange with the Cu-trien complex and ethylene glycol solvation. This was followed by XRD analyses and full-profile diffraction pattern modeling. Further investigation is required for comparison with other methods. Possible reasons for the formation of two populations of sauconites were discussed.

Prediction of Na- and Ca-montmorillonite contents and swelling properties of clay mixtures using Vis-NIR spectroscopy

The aim of this study was to evaluate whether Na- and Ca-montmorillonite and the swell-indicating properties (i.e., free swell index, water uptake capacity, and cation exchange capacity (CEC)) of clay mineral mixtures can be estimated using visible-near-infrared (Vis-NIR) spectral features. The data regarding four types of reference clay minerals (KGa-1b, kaolinite; IYd, illite; SWy-3, Na-montmorillonite; STx-1b, Ca-montmorillonite) and binary and ternary mixtures of the reference clay minerals (SWy-3/KGa-1b/IYd, and STx-1b/KGa-1b/IYd) with specific mass percentage ratios were used as a calibration dataset. The absorption spectral features could be correlated well with changes in the clay mineral content of the mixtures. The leave-one-out cross-validation results showed that the partial least square regression (PLSR) calibration models produced the best prediction in the following order: Na or Ca-montmorillonite, kaolinite, and illite amounts in the mixtures. The calibration models produced better predictions in the ascending order of CEC, free swell index, and water uptake capacity, regardless of the multivariate statistical methods and type of exchangeable cations in montmorillonite. Validation using an independent dataset indicated that the PLSR model predicted the Na- and Ca-montmorillonite content in clay mineral mixtures and bentonite samples. The results of this study provide the possibility of using Vis-NIR absorption spectral features as a screening tool for predicting the montmorillonite content with different interlayer cations in relatively pure clay soils (e.g., bentonite), if the calibration model is developed using the specific montmorillonite contained in the clay soil of interest.

Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays.

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO2) bring wet supercritical CO2 (scCO2) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO2/smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO2 adsorption properties of Pb2+, Rb+, and NH4+ saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na+, Cs+, and Ca2+ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO2 in the interlayer fluid and CO2/cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO2. In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO2. While both independent tests show that cation polarizability is an important factor in smectite-scCO2 systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO2 and in predicting the behavior of caprocks over time.

Molecular Modeling to Predict the Optimal Mineralogy of Smectites as Binders of Aflatoxin

AbstractNumerous experiments have verified that smectites can adsorb aflatoxin B1 (AfB1) effectively and the efficiency of this process depends heavily on the chemical, physical, and mineralogical characteristics of the smectite. Several relationships between these characteristics and AfB1 sorption have been determined experimentally, but the molecular mechanisms underlying these were not investigated. In the current study the effects of charge density, type of exchange cation, and charge origin (octahedral vs. tetrahedral) on AfB1 sorption on smectites were analyzed by a series of molecular simulations. The calculations confirmed the formation of water bridges between carbonyl groups of AfB1 molecules and interlayer cations. Flat orientation of AfB1 molecules on smectite surfaces was also confirmed. For larger amounts of AfB1 molecules in the intercalates, self-association of two AfB1 molecules bound by π–π interaction was shown. The thermodynamics of AfB1 sorption depends heavily on the water content in the structure, being optimal for basal distances corresponding to two layers of water. A clear preference for sorption of AfB1 on smectites with bivalent cations (Ba2+, Ca2+) and an octahedral origin of its layer charge was confirmed and this was explained as steric hindrance between hydrated ions and AfB1 molecules, which tend to lie flat on smectite surfaces devoid of ions. Ba-montmorillonite with a charge of 0.4 per half unit cell was shown to have the smallest and thus the best potential energy of adsorption compared to the other layer charges.

A novel low molecular quaternary polymer as shale hydration inhibitor

Polymer inhibitors for deep shale gas drilling have poor temperature resistance and can influence the rheological attributes of the drilling muds. To overcome this issue, a high temperature resistant and low molecular weight quaternary polymer DVAA was prepared by hydrothermal radical polymerization. The characterizations of the DVAA were evaluated by IR, NMR, TG, and GPC, and the inhibition, rheology, and filtration parameters were examined. The shale inhibition mechanism was evaluated by the SEM, zeta potential, contact angle, and XRD. Initial decomposition temperature of DVAA is 354.83 °C and Mp is 4583. The performance test outcomes suggest that using DVAA could lead to a linear expansion reduction rate of MMT at 73.4%, whereas the rolling recovery rate of cuttings was 86.25%. The API filtration of 4% MMT after hot-rolling is 13.6 ml. The mechanism of DVAA showed that DVAA and MMT form ionic bonds and hydrogen bond through electrostatic attraction and can still maintain a good structure under elevated temperature conditions. DVAA can effectively weaken the surface hydration by entering the crystal layer to replace the inter-layer cations, where it forms a polymer-film on the bentonite surface. In summary, the research and development of the DVAA can provide an effective guarantee for the stability of the wellbore during the development of deep shale gas reservoirs.

Understanding smectite to illite transformation at elevated (>100 °C) temperature: Effects of liquid/solid ratio, interlayer cation, solution chemistry and reaction time

Bentonite (mainly smectite) has been proposed as a buffer material in a nuclear waste repository, due to its low permeability, high swelling capability, and high ion exchange capacity. To evaluate the isolation performance of the material, it is important to understand a possible hydrothermal alteration of smectite to illite. We investigated the hydrothermal alteration of smectite under various physical and chemical conditions to identify key controlling factors of the process. The experiments were performed on <2 μm fractions of Na-rich or K-exchanged smectite in DI water or 1 M KCl solution at 200 °C for various liquid/solid ratios over several time periods up to 112 days. Multiple analytical techniques were used to characterize the solid and liquid products to determine the extent of the conversion and the related mineral structural changes. Our results suggest that the conversion could be relatively fast (i.e., weeks) relative to waste disposal isolation at the optimal conditions (high K+ concentration and high liquid/solid ratio), which is much faster than previously postulated for the temperature used here. A high K+ concentration in the solution is required to trigger the structural transition of a low charge smectite layer to a high charge illite layer. This transition appears to be continuous, thus contradicting the dissolution-precipitation mechanism for smectite to illite transformation. Based on the solution chemistry measurements, with respect to silica solubility, the equilibrium constant for the transformation of K-exchanged SWy-2 smectite to illite is estimated. In addition, the overall conversion is limited, to a large extent, by the dissolved silica concentration in the solution. Given a generally low potassium content, a very low liquid/solid ratio, and an extremely slow diffusion in an actual engineered barrier system, the extent of smectite conversion to illite in a repository environment, if happening at all, would be limited. The conversion can be further inhibited by introducing brucite and amorphous silica as chemical additives to the buffer material.

Difference in expansion and dehydration behaviors between NH4- and K-montmorillonite

Montmorillonite (Mt) expansion and swelling are key factors determining the barrier performance of bentonite in trans-uranic (TRU) and high-level radioactive waste disposal. In the case of co-located geological disposal of TRU and high-level waste, NH4+ ions formed from NO3− ions leached from TRU waste may contact bentonite, exchanging with interlayer cations of Mt. to form NH4-Mt, with a reduction in barrier performance. Because of the similar hydration energies of NH4+ and K+, NH4-Mt may have less expandability or even change to a non-expandable mineral such as K-Mt. An understanding of the expansion and alteration behavior of NH4-Mt, especially in comparison with K-Mt, is thus necessary in waste-disposal safety assessment. Here, the hydration behavior of NH4-Mt was investigated by X-ray diffraction (XRD) analysis and molecular dynamics (MD) simulation and compared with that of K-Mt. XRD profiles under relative humidity (RH) control indicate that expansion of NH4-Mt is similar to that of K-Mt at >40% RH with slightly different d-values. However, NH4-Mt expansion is kept at ∼20% RH, while K-Mt tends to dehydrate. MD simulations indicate that hydrogen bonding with NH4+ ions causes the differences in hydration behaviors of NH4- and K-Mt, increasing basal spacing in dehydrated states and promoting hydration. This “hydration gap” may be attributed to differences in alteration to non-expandable minerals by dehydration, with NH4+ having less tendency for alternation and less of an effect on the barrier performance of bentonite than K+.

Successful Conversion of Pb-Contaminated Soils to Low-Bioaccessibility Plumbojarosite Using Potassium-Jarosite at Ambient Temperature.

Methods promoting lead (Pb) phase transformation in soils are essential for decreasing Pb bioaccessibility/bioavailability and may offer an in situ, cost-efficient process for mitigating contaminant exposure. Recent plumbojarosite (PLJ) conversion methods have shown the greatest potential to reduce soil Pb bioaccessibility, an in vitro bioaccessibility assay measurement of the proportion of Pb solubilized under gastric chemical conditions. Soils tested utilizing the recent PLJ method were found to have a Pb bioaccessibility of <1%, compared to original soils possessing bioaccessibility of >70%. However, this technique requires heat (95-100 °C) to promote mineral transformation. Jarosite-group minerals may incorporate multiple interlayer cations; therefore, we probed the potential for jarosite to remediate Pb via intercalation by reacting presynthesized potassium (K)-jarosite with aqueous Pb and/or Pb-contaminated soil at room temperature. Both K-jarosite and heated PLJ-treated samples were investigated by pairing bioaccessibility analyses with advanced bulk and spatially resolved X-ray absorption spectroscopy analyses. Samples treated with K-jarosite promoted Pb transformation to low-bioaccessibility (<10%) PLJ, with soil being converted to 100% PLJ using both heated and nonheated techniques. μ-X-ray fluorescence (μ-XRF) and μ-X-ray absorption near-edge structure (μ-XANES) showcase significant differences between elemental interactions for heated and nonheated PLJ-treated samples with anglesite impurities being found on the microscale. Although further development is necessary to accommodate for suitable field conditions, results indicate, for the first time, that K-jarosite may successfully convert soil Pb to PLJ without high-temperature conditions. The newfound utility of K-jarosite is expected to be key to future jarosite-based soil Pb remediation method development.

Hydrate phase equilibria in natural sediments: Inhibition mechanism and NMR-based prediction method

The hydrate phase equilibrium condition in natural sediments is an important prerequisite for gas hydrate exploitation and subsea sequestration of CO2. However, its rapid and accurate prediction remains a challenge. In this work, we proposed a novel method to predict the hydrate phase equilibria in various natural sediments using low-field NMR technology. Also, its potential inhibition mechanism was clarified from the perspective of different types of pore water. The results indicated the peaks of T2 spectra for all types of pore water shifted to lower transverse relaxation times during the hydrate formation, meaning that the formed hydrates act as solid matrices in sediments and thus reduce pore size. Furthermore, it was observed that hydrate formation from bound water was much more difficult than capillary water, even at the same pore size. More importantly, not all bound water participated in hydrate formation. During the hydrate dissociation, montmorillonite exhibited the greatest dissociation temperature depression, followed by SA sediments, while illite and kaolin showed a relatively low inhibition on hydrate phase equilibria. This difference in dissociation temperature depression is attributed to water adsorption on the mineral surface and interlayer cation hydration, in addition to the capillary effect. Among them, the capillary-induced phase equilibrium inhibition is the lowest, followed by interlayer cation hydration, and mineral surface adsorption shows the most strong inhibition. Finally, the feasibility and validity of the NMR-based method in predicting the hydrate phase equilibrium condition in natural sediments were verified. These findings not only add further insights into the hydrate phase behavior in clayey-silty sediments but also provide an effective tool for rapid and accurate predictions of hydrate phase equilibria.

Sodium‐Intercalated Manganese Oxides for Achieving Ultra‐Stable and Fast Charge Storage Kinetics in Wide‐Voltage Aqueous Supercapacitors

AbstractNanostructured birnessite with tunneled structures and nearly ideal capacitive behaviors are attractive as electrode material for aqueous asymmetric supercapacitors (ASCs). However, their practical application is hindered by inadequate structural stability, sluggish reaction kinetics, and the lack of deeper understanding of electrochemical mechanisms. Herein, oxygen defect modulated sodium‐intercalated manganese oxides (Na0.55Mn2O4‐x·1.5H2O, abbreviated as NMOx) with intercalating sodium ions and water crystals are massively fabricated, which can enable fast diffusion of cations and good structural stability. Systematical in situ and ex situ characterizations verify that the preeminent capacitive charge storage of NMOx is governed by interlayer cation intercalation and deintercalation, accompanied by interlayer spacing expansion/contraction. Subsequently, a horizontally oriented carbon nanotube microfilm with outstanding electrical conductivity and electrolyte wettability is reported for aqueous ASCs, which exhibits a wide operating voltage (2.4 V), a high energy density of 88.9 W h kg−1, and a superb cycle performance (92.7% retention after 50 000 cycles). Furthermore, a flexible planar ASC is prepared with landmark volumetric energy/power densities (60.2 mW h cm−3, 23.7 W cm−3), and excellent mechanical flexibility. This study provides not only an effective approach to fabricate tailoring structure and remarkable electrochemical properties of birnessite material, but also a deeper understanding of the charge storage mechanisms.

Strong Oxophilicity of Zr Species in Zr4+-Exchanged Montmorillonite Boosted Meerwein–Ponndorf–Verley Reduction of Renewable Carbonyl Compounds

Meerwein–Ponndorf–Verley (MPV) reduction of biomass-derived carbonyl compounds is a greatly promising route for biomass valorization. Robust, facile-prepared, and cost-effective inorganic Zr-based catalysts for this transformation remain highly desirable. Herein, Zr-containing montmorillonite (denoted as Zr-MMT) was constructed by the ion exchange of Zr4+ with the interlayer cations of montmorillonite (MMT). Very interestingly, as an inorganic catalyst, the prepared Zr-MMT showed high catalytic activity for MPV reduction of various biomass-derived carbonyl compounds. Systematic investigations revealed that the excellent performance of Zr-MMT predominantly originated from more positively charged Zr species ([Zrx(OH)y(H2O)n](4x−y)+), which could boost the activation of the carbonyl group and simultaneously promote the hydrogen transfer process assisted by suitable basic sites on MMT. Notably, the preparation of Zr-MMT avoided the utilization of surfactants or expensive organic ligands, and the prepared Zr-MMT showed better or comparable catalytic performance than most reported Zr-containing catalysts, significantly enabling it to be more practical.

Capturing surface interlayer cation migration in Na0.6[Li0.2Mn0.8]O2 layered cathode materials for sodium battery

Anionic redox of charge compensation is an effective strategy to release the excess capacity of the cathode material for sodium ion batteries. Nonetheless, the development of cathode materials with this electrochemical reaction mechanism faces great challenges in terms of voltage attenuation and irreversible loss of capacity. As a result of the complexity of its structure, the failure mechanism is not well understood. Here we conduct STEM coupled with EELS on a typical anionic redox material of sodium ion batteries (Na 0.6 [Li 0.2 Mn 0.8 ]O 2 ) after the electrochemical cycle. Surprisingly, it was found that the transition metal ions on the surface of the particles migrated into the alkali metal layer, forming a spinel structure, and resulting in a decrease in the valence of the transition metal on the surface. The possibility of this kind of migration is confirmed, and the most likely migration path is calculated by DFT. Combining the electrochemical curve and XPS spectroscopy, we found the evolution of the charge transfer mechanism, which gradually changes from anion to cation with the progress of the electrochemical cycle. The structural reconstruction of the particle surface and the evolution of the charge transfer mechanism lead to the voltage hysteresis and capacity loss of the cathode. The new failure mechanism provides an important basis for solving the problems of this type of material for sodium cathode.

Synthesis and ionic conductivity of novel high charged tetrasilisic type micas

The objectives of this study were the synthesis of novel high charged tetrasilisic type micas with the chemical compositions of Na4(MgxLi2(4-x))Si8O20F4 (x = 2, 3 and 4) and the estimation of ionic conductivity of the novel micas. The mica with x = 2 has no vacancies in the octahedral sheets and the ratio Mg2+/Li+ in the octahedral sheets is 2/4 in the chemical formula. The micas with x = 3 and 4 have vacancies in the octahedral sheets and the ratio Mg2+/Li+/vacancy is 3/2/1 and 4/0/2, respectively. The synthesis was tried by firing the mixtures of chemical reagents in sealed container. The obtained materials were characterized using X-ray diffraction analyzer, scanning electron microscopy, Fourier transform infrared spectrometer and 23Na magic angle spinning nuclear magnetic resonance equipment. The novel micas with x = 2, 3 and 4 were formed during cooling after melting the mixtures of the reagents. However, a part of Na+ ions which should be interlayer cations originally entered the vacant sites in the octahedral sheets of the obtained mica with x = 4. The mica with x = 2 showed the highest ionic conductivity and the conductivity was 2.4 × 10−2 S/cm at 600 °C. This resulted from the facts that the mica with x = 2 was actually high charged tetrasilisic type mica of which Na+ ions in the interlayers did not transfer to the octahedral sheets and had many mobile Na+ ions in the interlayers.

Microscopic molecular insights into methane hydrate growth on the surfaces of clay minerals: Experiments and molecular dynamics simulations

Clay is widely present in hydrate reservoirs and drilling and completion fluids. A quantitative understanding of hydrate growth characteristics on clay solid phase surfaces is vital for predicting hydrate formation and preventing secondary hydrate generation in solid-phase-containing drilling fluid systems. The experiments and nonequilibrium molecular dynamics simulations are performed to investigate the characteristics of methane hydrate growth on the surfaces of kaolinite and montmorillonite. The experiments and dynamics simulated results demonstrate that a large number of bound waters is absorbed on the clay surface. However, the bound water does not participate in hydrate formation. The mechanisms by which clay minerals influence the formation of hydrates are different. Kaolinite reduces the induction time of hydrate formation, but the massive amount of bound water on the kaolinite surface reduces the total amount of free water molecules, which hinders the formation of hydrates, and the inhibition effect on hydrates is more significant than that with montmorillonite. Significantly, the hydrolysis of interlayer cations on the montmorillonite surface inhibits the formation of hydrates. This study will help us to understand the mechanism for gas hydrates formation on reservoir clay minerals, and to predict the amount of hydrate reservoir reserves, which will further aid hydrate resource drilling and exploration.

Determination of layer charge density in expandable phyllosilicates with alkylammonium ions: A combined experimental and theoretical assessment of the method

Layer charge deficit is a key crystal-chemical parameter for the classification and nomenclature of 2:1 phyllosilicates that also controls some of their fundamental properties such as cation exchange capacity, expandability, water content, or rheology. The clay community has thus devoted significant efforts to determine this parameter either from the smectite/vermiculite chemical composition (the structural formula method) or experimentally using the alkylammonium method, that theoretically allows the determination of the mean layer charge density and of its distribution. In the present study density functional theory (DFT) and molecular dynamics (MD) simulations are compared to experimental X-ray diffraction data for a series of synthetic saponites with layer charge ranging from medium-charge smectite to medium-charge vermiculite. Consistency of computed and measured layer-to-layer distances confirms the ability of computational approaches to accurately predict the organization of alkylammonium cations in smectite interlayers. Thermalization of DFT-optimized structure models through MD simulations allows probing possible alternative interlayer configurations such as the electrostatically-favored location of ammonium heads above/below Al-substituted tetrahedra. X-ray diffraction results showed that layer-to-layer distances intermediate between those corresponding to the stable h1, h2, etc. interlayer configurations described in the literature result from the interstratification of different stable configurations. The overall consistency of computational and experimental results confirms also the ability of the alkylammonium method to accurately determine a mean value of layer charge density consistent with smectite structural formula when using the revised equations proposed by Laird et al. (1989). The validity of the method appears however limited to smectite-group minerals [layer charge density ranging from ~0.5 to ≤1.2 per O20(OH)4] most likely owing to the coexistence of both stable “layered” and paraffin-like interlayer configurations for higher values of layer charge. In addition, this consistency challenges the ability of the method to quantitatively describe distributions of layer charge in expandable phyllosilicates even when using the complete series of alkylammonium cations.

Mineralization mechanism of carbon dioxide with illite interlayer cations using molecular dynamics simulation and experiments

Clay minerals can be identified as a prospective target for long-term CO2 sequestration due to their accessible interlayer cations and periodic sheet structure. Understanding the reactive motion of mineral and fluids has dual advantages of resources and environment. To clarify the storage mechanism, an alternative strategy for CO2 mineralization was investigated through molecular dynamics (MD) simulation and scCO2H2Oillite experiments. The MD simulation predicts the protonation of non-bridging oxygen (NBO) at the illite surface in the first picoseconds, resulting in HCO3- ion formations via the bonding between CO2 molecules and hydroxyl group dissociated from H2O molecules. Surface protonation leads to interlayer K+ cations hopping to the illite/fluids interface since the middle stage, mainly after 1 ns of the reaction. The leached K+ cations bond with the HCO3- ions and later interact with the hydroxyl groups, forming K2CO3 molecules at the interface. In accordance with the experimental results, the K+ cations’ concentration in the filtrates progressively increases throughout the reaction. Results of SEM-EDS, Raman and XPS measurements find that free CO2 clusters in contact with the leached interlayer cations can be converted into carbonate species through the mineralization reaction, precipitating at the surface and thus inducing interlayer swelling. These observations reveal that the clay-related mineralization is estimated to undergo an accumulated process, accessibly enhancing the amount of captured CO2. This is a new report that demonstrates the mechanism of CO2 mineralization in clay minerals, presenting a potential solution for CO2 sequestration enhancement. Insights into K+ cations leaching and mineralization kinetics and their underlying mechanisms during the scCO2H2Oillite reaction is a matter of generalization in clay minerals for CO2 storage.

Delamination by Repulsive Osmotic Swelling of Synthetic Na-Hectorite with Variable Charge in Binary Dimethyl Sulfoxide-Water Mixtures.

Swelling of clays is hampered by increasing layer charge. With vermiculite-type layer charge densities, crystalline swelling is limited to the two-layer hydrate, while osmotic swelling requires ion exchange with bulky and hydrophilic organic molecules or with Li+ cations to trigger repulsive osmotic swelling. Here, we report on surprising and counterintuitive osmotic swelling behavior of a vermiculite-type synthetic clay [Na0.7]inter[Mg2.3Li0.7]oct[Si4]tetO10F2 in mixtures of water and dimethyl sulfoxide (DMSO). Although swelling in pure water is restricted to crystalline swelling, with the addition of DMSO, osmotic swelling sets in at some threshold composition. Finally, when the DMSO concentration is increased further to 75 vol %, swelling is restricted again to crystalline swelling as expected. Repulsive osmotic swelling thus is observed in a narrow composition range of the binary water-DMSO mixture, where a freezing point suppression is observed. This suppression is related to DMSO and water molecules exhibiting strong interactions leading to stable molecular clusters. Based on this phenomenological observation, we hypothesize that the unexpected swelling behavior might be related to the formation of different complexes of interlayer cations being formed at different compositions. Powder X-ray diffraction and 23Na magic angle spinning-NMR evidence is presented that supports this hypothesis. We propose that the synergistic solvation of the interlayer sodium at favorable compositions exerts a steric pressure by the complexes formed in the interlayer. Concomitantly, the basal spacing is increased to a level, where entropic contributions of interlayer species lead to a spontaneous thermodynamically allowed one-dimensional dissolution of the clay stack.