Molecular dynamics study of swelling patterns of Na/Cs-montmorillonites and hydration of interlayer cations

We report on a molecular dynamics study of swelling patterns of an Na-rich/Cs-poor montomorillonite and a Cs-montomorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduce the swelling patterns of the Na and Cs-montomorillonite, suggesting a mechanism of its hydration different from that of the montomorillonite. In the meanwhile, we find that the differences in size and hydration energy of Na and Cs ions have strong implications for the structure and the internal energy of interlayer water. In particular, our results indicate that the hydrate difference in the presence of coexistent Na and Cs has a larger influence on the behavior of clay-water system. For Na-rich/Cs-poor montomorillonite, the hydration energy values of Na ions and water molecules each have a dramatic increase compared with those in Na-montomorillonite on the interlayer spacing, and the hydration energy values of Cs ions and water molecules decrease somewhat compared with those in Cs-montomorillonite.     

Monte Carlo molecular simulation of the Na-, Mg-, and mixtures of Na/Mg-montmorillonites systems,in function of the pressure

In this paper, Monte Carlo (MC) simulation has been used to study the swelling pattern of Na-montmorillonite (Na-Mnt), Mg-montmorillonite (Mg-Mnt), and Na/Mg-mixture montmorillonite (4NaMg-Mnt; 2Na2Mg-Mnt). The molecular simulation was performed in the NVT (number of molecules, volume and temperature are constant) ensemble at normal temperature (300 K) and 225, 300, and 340 bar over an H₂O content 147, 196, and 294 mg g^?1 of clay. The simulations reproduce the swelling pattern of Na-Mnt and Mg-Mnt. The predicted spacing of the Na/Mg-Mnt mixtures is closely related to that of Mg-Mnt and confirms the results reported in the literature for Na-rich/Mg-poor Mnt. The results of the water adsorption and the swelling properties on the system Na-Mnt, Mg-Mnt, and the Na/Mg-Mnt mixtures are reflected with a transformation to two-hydrate stages. The probability of the coordination number of Na⁺, Mg²⁺, and mixtures tends to increase with an increasing amount of H₂O molecules, but decreases with increasing pressure. The cation–oxygen distances (Na–O or Mg–O) show two signals, corresponding to the first and second coordination shells, which indicates that the ions behave as in bulk water.     

Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K+ 相似文献   

Temperature effects on the diffusion of water and monovalent counterions in the hydrated montmorillonite

We investigate here the effect of temperature on the diffusion of water and cations in the Wyoming-type montmorillonite clay. The considered cations are monovalent compensating ions, such as Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ in one-, two- and three-hydration states. For this purpose, molecular dynamics simulations have been performed to obtain the dynamic behaviour regarding the interlayer ions and water molecules under a temperature range between 260 and 400 K. The diffusion coefficient of water and cations in different hydrated clays increases with temperature. The influence of temperature on the diffusion of water is much greater than that of cations in one-, two- or three-hydrated clay. The degree of hydration plays an important role on the diffusion behaviour of water and counterions. We found that the effect of temperature is negligible in weakly hydrated clay, whereas it became significant in highly hydrated one. Besides, the size and mass of cations’ hydrate also affect the diffusion behaviour of water and cations in the interlayer space of hydrated clay.     

Properties of Hydrated Alkali Metals Aimed at the Ion Channel Selectivity

The hydration structure properties of different alkali metal ions with eight water molecules and potassium ions with different numbers of water molecules are studied using the mixed density functional theory, B3LYP, with 6-3110 basis set. The hydration structures are obtained from structure optimization and the optimum numbers of water molecules in the innermost hydration shell for the alkali metal ions are found. Some useful information about the ion channel selectivity is presented.     

应用分子动力学模拟和拉曼光谱对钙、镁、氯离子 水化现象的研究

应用分子动力学方法,模拟了298 K下,密度为1.0 g/cm~3的水溶液中Ca²⁺,Mg²⁺,Cl~-的水化现象,得到了相应离子周围水分子的微观分布情况.发现在钙离子周围,水分子以其氧离子去靠近中心离子;而在氯离子周围,水分子则以其中的一个氢原子去靠近中心离子.通过分析三种离子的径向分布函数、配位数曲线、水化数、水化半径,发现Ca²⁺的水化数和水化半径均大于Mg²⁺,即Ca²⁺的水合能力比Mg²⁺强.与以往研究结果相比,本文计算所得的自扩散系数更接近实验所得结果.为了使模型更好的代表真实水溶液体系,本文还应用分子动力学和拉曼光谱法研究了不同浓度的CaCl₂水溶液.分子动力学研究发现随着浓度的升高,CaCl₂溶液中Ca²⁺,Cl~-的配位数分别呈降低趋势.同时,随着浓度的升高,Ca²⁺,Cl~-的自扩散系数也呈现降低的趋势.作者推断这是由于浓度的升高,加剧了离子的微观反...     

Revised superconducting phase diagram of hole-doped Na(x)(H3O)(z)CoO2 x yH(2)O

We have studied the superconducting phase diagram of NaxCoO2.yH(2)O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O+ ions into NaxCoO2, together with water, acts as an additional dopant, indicating that Na substoichiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal T(C) is achieved through a Co valence range of 3.24-3.35, while T(C) decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped nonsuperconducting anhydrous samples, differing from the superconducting hydrate only in interlayer spacing.     

Salt permeation and exclusion in hydroxylated and functionalized silica pores

We use combined ab initio molecular dynamics (AIMD), grand canonical Monte Carlo, and molecular dynamics techniques to study the effect of pore surface chemistry and confinement on the permeation of salt into silica nanopore arrays filled with water. AIMD shows that 11.6 A diameter hydroxylated silica pores are relatively stable in water, whereas amine groups on functionalized pore surfaces abstract silanol protons, turning into NH3+. Free energy calculations using an ab initio parametrized force field show that the hydroxylated pores strongly attract Na+ and repel Cl- ions. Pores lined with NH3+ have the reverse surface charge polarity. Finally, studies of ions in carbon nanotubes suggest that hydration of Cl- is more strongly frustrated by pure confinement effects than Na+.     

Temperature effect in a montmorillonite clay at low hydration—microscopic simulation

The effect of temperature in the range 0–150°C was studied for homo-ionic montmorillonite clays with Na⁺ and Cs⁺ compensating ions in low hydration states. Monte Carlo and molecular dynamics simulations were employed to provide both static and dynamic information concerning the interlayer ions and water molecules, and emphasis was laid on the temperature activation of the diffusion coefficients. Principal structural changes were limited to the interlayer water phase. In the monohydrated systems, neither of the cations was seen to enter into the hexagonal cavities of the clay. Cs⁺ exhibited clear site-to-site diffusion between sites allowing coordination to six oxygen atoms of the clay sheets, this behaviour persisting to high temperatures. Preferential sites for the Na⁺ counterion were much less well-defined, even at low temperatures. The behaviour of the water phase in the monohydrated states was similar for the two ions. A rapid approach to bulk dynamics was seen in the transition from monohydrated to bihydrated Na-montmorillonite. A detailed quantitative comparison of the temperature activation of diffusion for a two-dimensional water phase and three-dimensional bulk water is presented for the first time.     

On the compressibility of alkali metals

An analysis of experimental P-V-T data for Na and Cs is given in terms of pseudopotential theory. It is shown that for these metals the contribution to compressibility due to the higher-order terms in electron energy practically does not depend on the position of the ions.     

Molecular dynamics computer simulation of the hydration of two simple organic solutes

The hydration of two simple organic solutes has been studied using the molecular dynamics (MD) computer simulation method. Results of the simulations of a single 1,4-dioxane or 1,3-dioxane molecule dissolved in 122 water molecules are compared with those of a MD simulation of an empty cavity of corresponding size in 216 water molecules. This yields the opportunity to trace the specific effects of the polar and dispersion solute-solvent interactions on the properties of the water molecules in the hydration shell of the solute.

The hydration shell properties of 1,4-dioxane (μ) = 0·14 D) are very similar to those of the corresponding cavity, whereas those of 1,3-dioxane (μ) = 1·91 D) show significant deviations. Earlier conclusions that water structure-making and water structure-breaking properties of 1,4-dioxane are about equally balanced, while 1,3-dioxane is definitely structure-breaking, are confirmed. Moreover, it is shown that a slower self-diffusion and reorientation of water molecules upon addition of a cosolvent does not necessarily point at structure-making properties, additional to those that are already induced by the cavity formation. The introduction of an empty cavity also slows down self-diffusion and molecular reorientation in the hydration shell.     

Water characteristics depend on the ionic environment. Thermodynamics and modelisation of the aquo ions

We have characterized the aquo ion in solution by five parameters: the charge q and the corresponding crystallographic radius R_c of the ion with the coordination number N, the radius R_w of the water molecule in the aquo complex and the number H of water molecules in the second hydration shell. A consistent set of these parameters has been derived from literature data. From an analysis of experimental determinations of the cation-oxygen distance and evaluation of electrostriction phenomena, we have concluded that the radius R_w of the water molecule depends on the charge of the central ion and on its crystallographic radius.

The different ion-water interactions and previous models are discussed, and we propose an ionic model and a general relation which gives the free hydration energy, ΔG(hyd), of a monatomic ion with ionic character. It depends on the five fundamental characteristics of the aquo ion and reproduces the experimental data with standard deviation less than 0.3%. In the case of lanthanides, it is shown that the model including average values of the oxygen-cation distance, or different distances, taking into account the local structure of the aquo ion, gives similar thermodynamic data. The proposed model gives also the possibility to deduce properties of the ions such as consistent values of the coordination numbers.     

Dielectric constant of solvents at nano to bulk regimes: linear response theory and statistical mechanics-based approaches

Statistical mechanics and generalised linear response theory based approaches are employed to derive the analytical expressions for size-dependent dielectric constant and normalised orientation polarisation of solvents. As an illustrative example, water is considered and the dielectric constants for the same are calculated over the entire range of water clusters. Our results reveal that the dielectric constant and normalised orientation polarisation are monotonically increasing with the increase in the number of solvent molecules and converge to the respective bulk values in the thermodynamic limit. More importantly, the dielectric constant of water is found to be independent of the nature, geometry and microscopic charges of the non-spherical ions. This finding offers a new platform for calculating the hydration energy and orientation polarisation based on linear response theory for different kinds of ions in the solvent medium.     

Lattice relaxation at na(100) and na(110) surfaces

The surface relaxation of the first interlayer spacing for both the (110) and (100) faces of Na has been calculated selfconsistently using the density functional formalism and a new density matrix method which obviates the use of wavefunctions. For the Na(110) surface we find no relaxation of the first interlayer spacing in agreement with LEED studies, while for the Na(100) surface we predict a 2% reduction. Force constants for the movement of the last atomic layer are also determined.     

普适性双原子分子解析势能函数的研究

提出了一种构造解析势能函数的新方法,由此得到了一种既适用于中性双原子分子又适用于带电双原子分子离子的解析势能函数。本文用八种基本类型的双原子分子——同核中性基态双原子分子Na₂-X1Σ+_g,同核中性激发态双原子分子C₂-A1Π_u,同核带电基态双原子分子离子He+₂-X2Σ+_u,同核带电激发态双原子分子离子N+₂-B2Σ+_u,异核中性基态双原子分子NaLi-X1Σ+_g,异核中性激发态双原子分子BH-B1Σ+,异核带电基态双原子分子离子(BC)--X3Π,异核带电激发态双原子分子离子(CS)+-A2Π等共21个算例对势能函数进行了验证并与RKR (Rydberg-Klein-Rees)实验数据进行了比较,计算结果与RKR数据符合很好。     

Solvation of halogen ions in aqueous solutions at 500 K–600 K under 100 atm

Structural properties of the pure water and halogen solutions at high temperatures and pressures are studied by using the molecular dynamics simulations and quantum molecular simulations. The related characters are calculated as functions of temperature and pressure. The results show that the hydrogen bonded networks become looser as temperature increases,with the collapse of the traditional tetrahedral structure. It is similar to the concentration-dependent collapse in the Na Cl solutions. However, adding other halogen elements has no further effects on the already weakly bonded water molecules.At the phase changing points, the process of hydration is evident for the bigger ions, so that the bigger the ion is, the smaller a cluster is formed.     

Theoretical assessment on mixing properties of liquid Tl–Na alloys

Thermodynamic and structural properties of mixing of molten Tl–Na alloys at 673 K have been investigated using quasi-chemical model. To understand the mixing behaviour in more detail, emphasis is placed on the role of interaction energy term, and viscosity and surface tension of the alloys have also been analysed under statistical considerations. Our study shows negative deviation from the Raoultian behaviour in the properties of Tl–Na alloy thereby indicating hetero-coordination in the Tl–Na melt at 673 K in the full range of concentration. Theoretically, computed thermodynamic data at 673 K agree very well with the corresponding experimental data. The viscosities of the alloys computed from Kaptay equation show small negative deviation and those computed from Singh and Sommer’s formulation show small positive deviation from ideal values while the Budai-Benko-Kaptay equation predicts noticeable negative deviation in Na-rich end and positive deviation in Tl-rich end of the composition. The calculations of surface tension reveal that results obtained from layered structure approach and compound formation model are in good agreement in the Na-rich side and in reasonable agreement in Tl-rich side of the composition, while those computed from Butler equation show noticeable deviations in the intermediate compositions. Both the viscosity and surface tension of liquid Tl–Na alloys increase with addition of Tl-component, viscosity having approximately linear variation with concentration. The study shows that there is non-linear variation in surface composition with bulk concentration and for most of the compositions the surface of the alloy is enriched with Na-atoms which segregate to the surface.     

Structure of water adsorbed on a mica surface

Monte Carlo simulations of hydration water on the mica (001) surface under ambient conditions revealed water molecules bound closely to the ditrigonal cavities in the surface, with a lateral distribution of approximately one per cavity, and water molecules (interposed between K+ counter ions) in a layer situated about 2.5 A from a surface O along a direction normal to the (001) plane. The calculated water O density profile was in quantitative agreement with recent x-ray reflectivity measurements indicating strong lateral ordering of the hydration water but liquidlike disorder otherwise.     

Effect of FeCl3 and acetone on the structure of Na–montmorillonite studied by Mössbauer and XRD measurements

⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and infrared spectroscopy were used to study the effect of FeCl₃ and acetone on the structure of a Na–bentonite. XRD indicated the incorporation of Fe³⁺ ions into the interlayer space since the basal lattice spacing of montmorillonite increased to 1.6 from 1.24 nm after treatment with FeCl₃ dissolved in acetone. Interlayer Na⁺ ions could be exchanged to Fe³⁺. Magnetically split Mössbauer subspectra with internal magnetic fields 41 and 46 T at 74 K, were associated with two main Fe³⁺ microenvironments within the interlayer regions. The resultant Fe–montmorillonite was successfully applied as a catalyst in the preparation of 1,1-diacetates from aromatic aldehydes and acetic acid anhydride.     

Layered double hydroxide: Inorganic organic conjugate nanocarrier for methotrexate

Layered double hydroxide (LDH)-methotrexate (MTX) nanohybrids were successfully synthesized using ex situ and in situ processes. X-ray diffraction patterns of the synthesized nanopowders revealed that intercalated MTX molecules were stabilized in tilted longitudinal conformation into the hydroxide interlayer space. Two separate hydroxyl peaks were found in the FTIR spectra of LDH-MTX nanopowders suggesting successful intercalation of the MTX molecule into LDH matrix. The synthesized powders were further characterized using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern. HRTEM images showed an increase in interlayer spacing in hydrothermally crystallized LDH-MTX nanohybrids as compared to pristine LDH. The study showed that depending on the synthesis route used to synthesize LDH-MTX nanohybrid, its particle size as well as morphology can be varied at nano scale.