Ab initio molecular dynamics study of the interlayer and micropore structure of aqueous montmorillonite clays

Ab initio molecular dynamics simulations have been performed to gain an understanding of the interfacial microscopic structure and reactivity of fully hydrated clay edges. The models studied include both micropore and interlayer water. We identify acidic sites through dissociation mechanisms; the resulting ions can be stabilized by both micropore and interlayer water. We find clay edges possess a complex amphoteric behavior, which depends on the face under consideration and the location of isomorphic substitution. For the neutral (110) surface, we do not observe any dissociation on the timescale accessible. The edge terminating hydroxyl groups participate in a hydrogen bonded network of water molecules that spans the interlayer between periodic images of the clay framework. With isomorphic substitutions in the tetrahedral layer of the (110) clay edge, we find the adjacent exposed apical oxygen behaves as a Br?nsted base and abstracts a proton from a nearby water molecule, which in turn removes a proton from an AlOH2 group. With isomorphic substitutions in the octahedral layer of the (110) clay edge the adjacent exposed apical oxygen atom does not abstract a proton from the water molecules, but increases the number of hydrogen bonded water molecules (from one to two). Acid treated clays are likely to have both sites protonated. The (010) surface does not have the same interfacial hydrogen bonding structure; it is much less stable and we observe dissociation of half the terminal SiOH groups (?Si-O-H??SiO-+H+) in our models. The resulting anions are stabilized by solvation from both micropore and interlayer water molecules. This suggests that, when fully hydrated, the (010) surface can act as a Br?nsted acid, even at neutral pH.Qatar National Research Fund (QNRF) of Qatar Foundation, National Priorities Research Program (grant number 09-26-01-048). Our work made use of the facilities of HECToR, the UK's national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRC's High End Computing Programme. Access to HECToR was through grants EP/F00521/1, EP/E045111/1, EP/I017763/1 and the UK Consortium on Mesoscopic Engineering Sciences (EP/L00030X/1). We also made use of High Performance Computing facilities at University College London.Scopu

Molecular and all solid DFT studies of the magnetic and chemical bonding properties within KM[Cr(CN)6_6] (M = V, Ni) complexes

A study at both the molecular and extended solid level in the framework DFT is carried out for KM[Cr(CN)$_6$] (M = V, Ni). From molecular calculations, the exchange parameters J are obtained, pointing to the expected magnetic ground states, i.e., antiferromagnetic for M = V with J = -296.5 cm$^{-1}$ and ferromagnetic for M = Ni with J = +40.5 cm$^{-1}$. From solid state computations the same ground states and J magnitudes are confirmed from energy differences. Furthermore an analysis of the site projected density of states and of the chemical bonding is developed in which the cyanide ion linkage is analyzed addressing some isomerism aspects.Comment: new results, 5 tables, 7 fig

Modeling molecular crystals formed by spin-active metal complexes by atom-atom potentials

We apply the atom-atom potentials to molecular crystals of iron (II) complexes with bulky organic ligands. The crystals under study are formed by low-spin or high-spin molecules of Fe(phen)$_{2}$(NCS)$_{2}$ (phen = 1,10-phenanthroline), Fe(btz)$_{2}$(NCS)$_{2}$ (btz = 5,5$^{\prime }$,6,6$^{\prime}$-tetrahydro-4\textit{H},4$^{\prime}$\textit{H}-2,2$^{\prime }$-bi-1,3-thiazine), and Fe(bpz)$_{2}$(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2$^{\prime}$-bipyridine). All molecular geometries are taken from the X-ray experimental data and assumed to be frozen. The unit cell dimensions and angles, positions of the centers of masses of molecules, and the orientations of molecules corresponding to the minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal structures are in a good agreement with the experimental data. Sources of the residual discrepancies between the calculated and experimental structures are discussed. The intermolecular contributions to the enthalpy of the spin transitions are found to be comparable with its total experimental values. It demonstrates that the method of atom-atom potentials is very useful for modeling organometalic crystals undergoing the spin transitions

Experimental and theoretical studies on the mechanical and structural changes imposed by the variation of clay loading on poly(vinyl alcohol)/cloisite 93A nanocomposites

Polymer nanocomposites (PNC) structures are promising materials due to their novel properties. However, many of their characteristic physical, mechanical, and chemical behavior have not been quantified. Depending on the interface interactions between polymeric chains and clay sheets, various classes of polymer/clay nanocomposite exist: Intercalated, flocculated and exfoliated nanocomposites. In this work, we present an experimental study of poly(vinyl alcohol) (PVA)-cloisite 93A followed by a classical molecular dynamic (MD) simulations. The structural properties of the system were studied using X-ray diffraction (XRD), nanoscanning electron microscopy (NSEM). Both revealed intercalation between PVA chains and cloisite 93A nanoclay. Another evidence of the intercalation between PVA and cloisite 93A nanoclay was realized from the differential scanning calorimetry (DSC) which confirmed as surge in crystallinity upon intercalation. A main focus for the intercalated structure was to investigate the impact of nanofillers content on the mechanical performance. Intercalation significantly influenced both Young's modulus and the % of elongation of PVA/cloisite 93A blends. Young's modulus and tensile stress were raised with the content of the filler content up to 3 wt%. For higher content, opposite observations are addressed owing to the formation of aggregates of nanofillers and as consequence construction of microvoids. From the MD simulations, the intercalation has been shown by the increase of the d-spacing with the clay loading. By calculating the density profile, it has been demonstrated that in case of low clay loading, the cloisite 93A has its nitrogen groups on the clay surface and the long tails form layers. For the high loading of clay, nitrogen and carbon groups are together on the surface. Young modulus calculated theoretically follows the same experimental trend where an increase of the values has been observed with a clay loading up to 3 wt%, followed by a decrease of the values for higher clay loading. J. VINYL ADDIT. TECHNOL., 25:172181, 2019.This paper was made possible by an NPRP 09-260-1-048 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. We are also grateful to University College London for using their high performance computing facilities.Scopu

Quantum mechanical calculations on FeOH nanoparticles

Density functional theory calculations were performed on Fesingle bondOsingle bondH nanoparticles based on the goethite and a proposed ferrihydrite structures. Both molecular cluster and a periodic boundary condition approaches were applied and the resulting structures compared to the bulk values and to the other computational method. Reaction energies for Al3 +-substitution into each nanoparticle were calculated. The implications of these results for ferrihydrite and goethite nucleation and the ferrihydrite to goethite phase transition are discussed

Impact of magneto-dielectric materials on the miniaturization of UWB monopole antennas

International audienceThis article presents the miniaturization of an UWB planar monopole antenna covering the VHF band through the use of low loss Magneto-Dielectric Materials (MDM). A new structure of the planar monopole antenna using slots have been proposed, which allows to reach high miniaturization rates by covering partially the antenna with MDM. This new monopole structure shows a miniaturization rate of 60% by covering about only 12 % of the antenna surface by the MDM

Characterization of ferrite tunability at microwave frequencies Application to VHF tunable antennas

International audienceA microwave characterization method that allows extracting permeability variations of ferrite as a function of applied static magnetic field is presented. This method is based on a coaxial line method for which a calibration technique is proposed to take into account the variations of the bias system. Measured permeability spectra are used to design a tunable antenna in VHF frequency band. © 2019 IEEE