Diffusion of water in clays – microscopic simulation and neutron scattering

Effect of Temperature on the Diffusion and Sorption of Cations in Clay Vermiculite

The MD method for modeling vermiculite containing Na⁺, Rb⁺, Cs⁺, Mg²⁺, and Ba²⁺ cations shows the following: With a weak swelling of clay, the temperature has no significant effect on the diffusion of water and cations through vermiculite. With a high content of water in vermiculite, the effect of temperature on the diffusion coefficient of water is greater than that of cations. We studied the structure of RDF ions in Na⁺-vermiculite, in which some of the cations are replaced by Rb⁺, Cs⁺, Mg²⁺, and Ba²⁺. Cations of alkali and alkaline earth metals compete with Na⁺ ions for adsorption sites on the surface of the clay layer. The alkaline earth metal cations are in the middle between the clay layers due to their higher charge and stronger hydration. In this case, Na⁺ is localized at the surface of the clay layer. Thus, cations of alkaline earth metals have little effect on the temperature dependence of the diffusion coefficient Na⁺.

Water and Ion Dynamics in Confined Media: A Multi-Scale Study of the Clay/Water Interface

This review details a large panel of experimental studies (Inelastic Neutron Scattering, Quasi-Elastic Neutron Scattering, Nuclear Magnetic Resonance relaxometry, Pulsed-Gradient Spin-Echo attenuation, Nuclear Magnetic Resonance Imaging, macroscopic diffusion experiments) used recently to probe, over a large distribution of characteristic times (from pico-second up to days), the dynamical properties of water molecules and neutralizing cations diffusing within clay/water interfacial media. The purpose of this review is not to describe these various experimental methods in detail but, rather, to investigate the specific dynamical information obtained by each of them concerning these clay/water interfacial media. In addition, this review also illustrates the various numerical methods (quantum Density Functional Theory, classical Molecular Dynamics, Brownian Dynamics, macroscopic differential equations) used to interpret these various experimental data by analyzing the corresponding multi-scale dynamical processes. The purpose of this multi-scale study is to perform a bottom-up analysis of the dynamical properties of confined ions and water molecules, by using complementary experimental and numerical studies covering a broad range of diffusion times (between pico-seconds up to days) and corresponding diffusion lengths (between Angstroms and centimeters). In the context of such a bottom-up approach, the numerical modeling of the dynamical properties of the diffusing probes is based on experimental or numerical investigations performed on a smaller scale, thus avoiding the use of empirical or fitted parameters.

Diffusion of confined fluids in microporous zeolites and clay materials

Fluids exhibit remarkable variation in their structural and dynamic properties when they are confined at the nanoscopic scale. Various factors, including geometric restriction, the size and shape of the guest molecules, the topology of the host, and guest-host interactions, are responsible for the alterations in these properties. Due to their porous structures, aluminosilicates provide a suitable host system for studying the diffusion of sorbates in confinement. Zeolites and clays are two classes of the aluminosilicate family, comprising very ordered porous or layered structures. Zeolitic materials are important due to their high catalytic activity and molecular sieving properties. Guest molecules adsorbed by zeolites display many interesting features including unidimensional diffusion, non-isotropic rotation, preferred orientation and levitation effects, depending on the guest and host characteristics. These are useful for the separation of hydrocarbons which commonly exist as mixtures in nature. Similarly, clay materials have found application in catalysis, desalination, enhanced oil recovery, and isolation barriers used in radioactive waste disposal. It has been shown that the bonding interactions, level of hydration, interlayer spacing, and number of charge-balancing cations are the important factors that determine the nature of diffusion of water molecules in clays. Here, we present a review of the current status of the diffusion mechanisms of various adsorbed species in different microporous zeolites and clays, as investigated using quasielastic neutron scattering and classical molecular dynamics simulation techniques. It is impossible to write an exhaustive review of the subject matter, as it has been explored over several decades and involves many research topics. However, an effort is made to cover the relevant issues specific to the dynamics of different molecules in microporous zeolites and clay materials and to highlight a variety of interesting features that are important for both practical applications and fundamental aspects.

Green earth pigments dispersions: Water dynamics at the interfaces

HYPOTHESIS

The objective is to elucidate the multiscale dynamics of water within natural mixtures of minerals, green earth pigments that are mainly composed of phyllosilicates containing large amount of iron. In particular, the interaction of water with the different kinds of surfaces has to be probed. One issue is to examine the influence of surface type, basal or edge, on the dispersion quality.

EXPERIMENT

The study was carried out using ¹H variable field NMR relaxometry on various green earth pigment dispersions and concentrations. To analyse the data, a new analytical model was developed for natural phyllosilicates containing large amount of paramagnetic centres.

FINDING

The proposed theoretical framework is able to fit the experimental data for various samples using few parameters. It allows to determining water diffusion and residence times in complex phyllosilicate dispersions. Furthermore, it makes it possible to differentiate the contribution of the basal and edge surfaces and their respective surface area in interaction with water. Moreover, NMR relaxation profile reveals to be highly sensitive to the structural aspect of the phyllosilicates and to the accessibility of water to iron, hence allowing to discriminate clearly between two very similar phyllosilicates (glauconite and celadonite) that are difficult to distinguish by standard structural methods.

Classical Polarizable Force Field to Study Hydrated Hectorite: Optimization on DFT Calculations and Validation against XRD Data

Following our previous works on dioctahedral clays, we extend the classical Polarizable Ion Model (PIM) to trioctahedral clays, by considering dry Na-, Cs-, Ca- and Sr-hectorites as well as hydrated Na-hectorite. The parameters of the force field are determined by optimizing the atomic forces and dipoles on density functional theory calculations. The simulation results are validated by comparison with experimental X-ray diffraction (XRD) data. The XRD patterns calculated from classical molecular dynamics simulations performed with the PIM force field are in very good agreement with experimental results. In the bihydrated state, the less structured electronic density profile obtained with PIM compared to the one from the state-of-the-art non-polarizable force field clayFF explains the slightly better agreement between the PIM results and experiments.

Modeling the arrangement of particles in natural swelling-clay porous media using three-dimensional packing of elliptic disks

Swelling clay minerals play a key role in the control of water and pollutant migration in natural media such as soils. Moreover, swelling clay particles' orientational properties in porous media have significant implications for the directional dependence of fluid transfer. Herein we investigate the ability to mimic the organization of particles in natural swelling-clay porous media using a three-dimensional sequential particle deposition procedure [D. Coelho, J.-F. Thovert, and P. M. Adler, Phys. Rev. E 55, 1959 (1997)]. The algorithm considered is first used to simulate disk packings. Porosities of disk packings fall onto a single master curve when plotted against the orientational scalar order parameter value. This relation is used to validate the algorithm used in comparison with existing ones. The ellipticity degree of the particles is shown to have a negligible effect on the packing porosity for ratios ℓ(a)/ℓ(b) less than 1.5, whereas a significant increase in porosity is obtained for higher values. The effect of the distribution of the geometrical parameters (size, aspect ratio, and ellipticity degree) of particles on the final packing properties is also investigated. Finally, the algorithm is used to simulate particle packings for three size fractions of natural swelling-clay mineral powders. Calculated data regarding the distribution of the geometrical parameters and orientation of particles in porous media are successfully compared with experimental data obtained for the same samples. The results indicate that the obtained virtual porous media can be considered representative of natural samples and can be used to extract properties difficult to obtain experimentally, such as the anisotropic features of pore and solid phases in a system.

Anisotropy on the collective dynamics of water confined in swelling clay minerals

Collective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si(7.3)Al(0.7)Mg(6)O(20)(OH)(4)Na(0.7) in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental results.

A coarse-grained model of DNA with explicit solvation by water and ions

Solvation by water and ions has been shown to be vitally important for biological molecules, yet fully atomistic simulations of large biomolecules remain a challenge due to their high computational cost. The effect of solvation is the most pronounced in polyelectrolytes, of which DNA is a paradigmatic example. Coarse-grained (CG) representations have been developed to model the essential physics of the DNA molecule, yet almost without exception, these models replace the water and ions by implicit solvation in order to significantly reduce the computational expense. This work introduces the first coarse-grained model of DNA solvated explicitly with water and ions. To this end, we combined two established CG models; the recently developed mW-ion model [DeMille, R. C.; Molinero, V. J. Chem. Phys. 2009, 131, 034107], which reproduces the structure of aqueous ionic solutions without electrostatic interactions, was coupled to the three-sites-per-nucleotide (3SPN) CG model of DNA [Knotts, T. A., IV; et al. J. Chem. Phys. 2007, 126, 084901]. Using atomistic simulations of d(CGCGAATTCGCG)(2) as a reference, we optimized the coarse-grained interactions between DNA and solvent to reproduce the solvation structure of water and ions around CG DNA. The resulting coarse-grained model of DNA explicitly solvated by ions and water (mW/3SPN-DNA) exhibits base-pair specificity and ion-condensation effects and it is 2 orders of magnitude computationally more efficient than atomistic models. We describe the parametrization strategy and offer insight into how other CG models may be combined with a coarse-grained solvent model such as mW-ion.

Molecular simulation of aqueous solutions at clay surfaces

We report a molecular simulation study of aqueous solutions at montmorillonite clay surfaces. Unlike most previous studies, ours does not focus on the interlayer nanopores, but looks at both kinds of external surfaces of clay particles: basal surfaces along the clay layers, and lateral surfaces through which interlayer and larger interparticle pores are linked. We present results on structural, dynamic and thermodynamic properties and phenomena, including hydration complexes of ions, H bonding networks, modification of the water dynamics with respect to the bulk, and the role of water in the cation exchange between interlayer and interparticle pores.

Dynamics in Clays - Combining Neutron Scattering and Microscopic Simulation

Mobility of ions and water in clays is at the heart of their remarkable properties of water retention and ion-exchange. It has been addressed here using two microscopic techniques: neutron scattering and molecular dynamics simulations. Neutron scattering gives access exclusively to water dynamics in clays, due to the exceptional sensitivity of neutrons to H atoms. The data interpretation can be challenging, especially for natural clays such as montmorillonite, with inhomogeneous swelling characteristics. A great improvement is achieved with the use of synthetic materials, as demonstrated here on the case of synthetic (fluoro)hectorite. The standard analytical models for long-range diffusive motion, isotropic translation and its derivative, powder averaged two dimensional translation, have been used to interpret the neutron scattering data. They both agree on the order of magnitude for the diffusion coefficient of water in monohydrated and bihydrated clays, 10−10 m2s−1 and 10−9 m2s−1 respectively. While the two-dimensional nature of water diffusion in clays is seen clearly from molecular dynamics simulations, its signature in neutron scattering data is obscured by the powder-averaging of the signal. A novel method, based on a multi-resolution analysis of scattering functions from powder samples, allows never-the-less a clear determination of the dimensionality of water motion in the system. Extracting information on local water motion is difficult on the basis of neutron scattering data only. Various models for localised motion, rotation on a sphere or jump diffusion, have been proposed and used to interpret the observed neutron data, however their applicability is questionable in light of information from molecular dynamics simulations. Aside from aiding the interpretation of neutron scattering data, MD simulations are most valuable in providing information on the behaviour of ions in clays. MD estimates the interlayer ion coefficients as of the some order of magnitude as water, even if the details of ionic motion are strikingly different between the two ions considered here, Na+ and Cs+. Further, MD has also allowed to address the topic of ion exchange between clay interlayers and bulk aqueous solution. The microscopic picture of water and ion motion in clays, emerging from neutron scattering and MD simulations, should be treated as a building block of the overall modelling of macroscopic transport in clays, the ultimate property of interest for many clay applications.

Molecular hydraulic properties of montmorillonite: a polarized fourier transform infrared spectroscopic study

Understanding the rates at which fluid flows into clay interlayers at the molecular level is fundamental to designing an effective clay barrier system. In this work, molecular interactions at the Na-montmorillonite (MMT)-water interface, emphasizing the flow properties of the clay interlayer, have been studied at the molecular and nanoscale level using polarized Fourier transform infrared (FT-IR) spectroscopic and X-ray diffraction (XRD) techniques. Clay-water slurries were smeared on inert gold-coated metal substrates for FT-IR experiments and slurries were smeared on quartz plates for XRD experiments. By analyzing the O-H stretching and H-O-H bending vibrations in clay slurries, it was concluded that the molecular behavior of interlayer water is significantly different from the molecular behavior of bulk water. With increasing clay-water interaction time, it was also seen that the Si-O stretching bands of clay are being significantly altered by the water molecules in the interlayer. Using these spectroscopic techniques we have estimated the time required for water to flow into the clay interlayer. Further, by analyzing the particle size of the clay using atomic force microscopy (AFM) imaging, we were able to estimate the flow velocity of the water in the clay interlayer. This velocity is found to be 3.23 x 10(-9) cm/s. This flow velocity was found to be of the same order of magnitude as the hydraulic conductivity of smectite-type clay reported elsewhere. Also described in this work is the correct positioning of the Si-O out-of-plane vibration band of MMT at the two-layer saturation level in the interlayer. This band was only observed in p-polarized spectra at 1211 cm(-1). Thus, we attribute this band to the Si-O out-of-plane vibration band.

Modelling studies of water in crystalline nanoporous aluminosilicates

The paper presents a review of molecular modelling studies of hydrated nanoporous aluminosilicates (zeolites and clays) performed during the last decade. A special emphasis is set on the calculation of the dynamical quantities and collective properties of the confined water. Some new results concerning the behaviour of water molecules in the siliceous silicalite and zeolite beta structures are presented.

Ion dynamics in compacted clays: Derivation of a two-state diffusion-reaction scheme from the lattice Fokker-Planck equation

We show how a two-state diffusion-reaction description of the mobility of ions confined within compacted clays can be constructed from the microscopic dynamics of ions in an external field. The diffusion-reaction picture provides the usual interpretation of the reduced ionic mobility in clays, but the required partitioning coefficient K(d) between trapped and mobile ions is generally an empirical parameter. We demonstrate that it is possible to obtain K(d) from the microscopic dynamics of ions interacting with the clay surfaces by evaluating the ionic mobility using a novel lattice implementation of the Fokker-Planck equation. The resulting K(d) allows a clear-cut characterization of the trapping sites on the clay surfaces and determines the adsorption/desorption rates. The results highlight the limitations of standard approximation schemes and pinpoint the crossover from jump to Brownian diffusion regimes.

Diffusion of Water in Clays on the Microscopic Scale: Modeling and Experiment

Diffusion of water in montmorillonite clays at low hydration has been studied on the microscopic scale by two quasi-elastic neutron scattering techniques, neutron spin-echo (NSE) and time-of-flight (TOF), and by classical microscopic simulation. Experiment and simulation are compared both directly on the level of intermediate scattering functions, I(Q, t), and indirectly on the level of relaxation times after a model of atomic motion is applied. Regarding the dynamics of water in Na- and Cs-monohydrated montmorillonite samples, the simulation and NSE results show a very good agreement, both indicating diffusion coefficients of the order of (1-3) x 10(-10) m(2) s(-1). The TOF technique significantly underestimates water relaxation times (therefore overestimates water dynamics), by a factor of up to 3 and 7 in the two systems, respectively, primarily due to insufficiently long correlation times being probed. In the case of the Na-bihydrated system, the TOF results are in closer agreement with the other two techniques (the techniques differ by a factor of 2-3 at most), giving diffusion coefficients of (5-10) x 10(-10) m(2) s(-1). Attention has been also paid to the elastic incoherent structure factor, EISF(Q). Simulation has played a key role in understanding the various contributions to EISF(Q) in clay systems and in clearly distinguishing the signatures of "apparent" and true confinement. Indirectly, simulation highlights the difficulty in interpreting the EISF(Q) signal from powder clay samples used in experiments.