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Sahimi, Muhammad

Publications and source records attributed to Sahimi, Muhammad.

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Molecular Dynamics Simulation of Hydration and Swelling of Mixed-Layer Clays in the Presence of Carbon Dioxide

Swelling of clay minerals is a prime anomaly that is relevant to many problems in geoscience, oil and gas reservoirs, geophysics, and engineering. The phenomenon is even more complex when carbon dioxide is also present. Since sequestration of CO 2 in sedimentary rock has been under active consideration as a way of alleviating the increasing concerns over climate change for which CO 2 is a main culprit, studying swelling of clays minerals in the presence of CO 2 has taken on added urgency. The swelling changes the permeability and porosity of porous formations, which in turn may lead to a reactivation of dormant fractures, triggering seismic activities, and opening up new pathways for CO 2 to return to the porous formations’ surface. Although the large majority of sedimentary rocks contain various types of mixed-layer clays (MLCs) with intermixed stacking sequences of several types of distinct layers within a single crystal, the vast majority of the past experimental and computational studies of the swelling was focused on pure clays. We present the results of extensive molecular dynamics simulation of hydration and swelling of illite-montmorillonite (I-MMT) MLCs, the most common type of mixed clays, in the presence of both water and CO 2 and various combinations of interlayer cations, Na + and K + . To understand the differences with pure clays, we also report on the same phenomenon in the MMT only. At low CO 2 concentrations in Na-MMT, which has its layers’ charge concentrated in its octahedral sheet, weak ion–surface interactions result in fully hydrated ions and, therefore, more extensive swelling than in I-MMT. Without CO 2 , adsorption of the cations at the illite surface increases the hydrophobicity of its surface. Thus, in the asymmetric interlayer of I-MMT, illite with stronger surface–ion interactions causes accumulation of cations near its surface, which limits their hydration and, therefore, controls swelling of the MLCs. Further inhibition of swelling of Na-MLC can be achieved by increasing the concentration of K + in the interlayer. At higher CO 2 concentrations, however, intercalation of water and CO 2 results in a completely different behavior, since in the 2W and 3W hydration states, the hydrophobicity of the MMT surface is stronger than that of illite. Thus, the density distributions of the intercalated molecules are considerably different from that with water only, which together with disruption of the water network by CO 2 reduces the difference between the extent of swelling of the MLC and pure MMT.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Molecular Dynamics Simulation of Hydration and Swelling of Mixed-Layer Clays

Swelling of clay minerals is important to a broad class of problems in science and engineering. While the problem has been extensively studied experimentally, past molecular modeling of the phenomenon was focused on pure clays of one type or another. In practice, however, there is a diverse class of mixed-layer clays (MLCs) in sedimentary rock with intermixed stacking sequence of two or more types of distinct layers within a single crystal. In fact, more than 60% of sedimentary rocks in the U.S. contain various types of MLCs. In this work, we present the results, to our knowledge, of the first molecular dynamics simulation of hydration energetics and swelling of illite–montmorillonite (I–MMT) MLCs, the most common type of mixed clays. The swelling is studied as a function of the water concentration with three combinations of interlayer cations, namely, Na + and K + . The hydration energies, the radial distribution functions, and the density profiles in the interlayer region are computed. For regular Na–MMT with layer charge concentrated in the octahedral sheet, weak cation–surface interaction results in fully hydrated ions and significant swelling. In the asymmetric interlayer of the MLC, however, the illite sheet with stronger interaction of surface and ions causes adsorption of the cations deep in the ditrigonal cavities of the siloxane surface. Given that the hydration enthalpy of K + is smaller than that of Na + , its hydration shell is quite unstable compared with that of Na+. Therefore, swelling is inhibited as the ratio K + /Na + increases. Finally, the results demonstrate the significant differences between the hydration and swelling properties of pure clays and the mixed ones, which have important implications in practice, particularly for sequestration of CO 2 in sedimentary rock.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Theoretical Model and Numerical Simulation of Adsorption and Deformation in Flexible Metal–Organic Frameworks

We report that metal–organic frameworks (MOFs) have recently attracted considerable attention as new nanoporous materials with applications to separation of fluid mixtures, catalysis, gas capture and storage, and drug delivery. A subclass of MOFs, the MIL-53 (with Al or Cr) family, exhibits a complex structural phase transition, often referred to as the breathing transition, which occurs when gas molecules are adsorbed in its pore space. In this phenomenon, the material morphology oscillates between two distinct phases, usually referred to as the narrow-pore (np) and large-pore (lp) phases. We describe a statistical mechanical model based on the energetics of the system that couples the adsorbates with the host and the ensuing structural deformation of the materials. The Hamiltonian of the system consists of the elastic energy of the MOF, the fluid phase energy, and the interaction energy between the gas and MOFs. Minimizing the total energy with respect to both the gas density and the displacement field in the material yields the governing equations for both. The model is used to study the adsorption of CO 2 and CH 4 in MIL-53(Al). Here, the results demonstrate that all reported experimental features of the phenomenon are produced by the model. In particular, consistent with experimental data, the model predicts that the material experiences contraction in the np phase, whereas it undergoes swelling in the lp phase, and that the two phases coexist in the transition zone. The model is, however, completely general and is applicable to any type of guest-responsive material that undergoes deformation as a result of its interaction with the guest molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Pore-network model of evaporation-induced salt precipitation in porous media: The effect of correlations and heterogeneity

Salt transport and precipitation in porous media constitute a set of complex and fascinating phenomena that are of considerable interest to several important problems, ranging from storage of CO 2 in geological formations, to soil fertility, and protection of pavements and roads, as well as historical monuments. The phenomena occur at the pore scale and are greatly influenced by the heterogeneity of the pore space morphology. In this paper, we present a pore-network (PN) model to study the phenomena. Vapor diffusion, capillary effect at the brine-vapor interface, flow of brine, and transport of salt and its precipitation in the pores that plug the pores partially or completely are all accounted for. The drying process is modeled by the invasion percolation, while transport of salt in brine is accounted for by the convective-diffusion equation. We demonstrate that the drying patterns, the clustering and connectivity of the pore throats in which salt precipitation occurs, the saturation distribution, and the drying rate are all strongly dependent upon the pore-size distribution, the correlations among the pore sizes, and the anisotropy of the pore space caused by stratification that most natural porous media contain. In particular, if the strata are more or less parallel to the direction of injection of the gas that dries out the pore space (air, for example) and/or causes salt precipitation (CO 2 , for example), the drying rate increases significantly. Moreover, salt tends to precipitate in clusters of neighboring pores that are parallel to the open surface of the porous medium.

42 ENGINEERING↗
DOE OSTI2017

Flow, Transport, and Reaction in Porous Media: Percolation Scaling, Critical‐Path Analysis, and Effective Medium Approximation

Abstract We describe the most important developments in the application of three theoretical tools to modeling of the morphology of porous media and flow and transport processes in them. One tool is percolation theory. Although it was over 40 years ago that the possibility of using percolation theory to describe flow and transport processes in porous media was first raised, new models and concepts, as well as new variants of the original percolation model are still being developed for various applications to flow phenomena in porous media. The other two approaches, closely related to percolation theory, are the critical‐path analysis, which is applicable when porous media are highly heterogeneous, and the effective medium approximation— poor man's percolation —that provide a simple and, under certain conditions, quantitatively correct description of transport in porous media in which percolation‐type disorder is relevant. Applications to topics in geosciences include predictions of the hydraulic conductivity and air permeability, solute and gas diffusion that are particularly important in ecohydrological applications and land‐surface interactions, and multiphase flow in porous media, as well as non‐Gaussian solute transport, and flow morphologies associated with imbibition into unsaturated fractures. We describe new applications of percolation theory of solute transport to chemical weathering and soil formation, geomorphology, and elemental cycling through the terrestrial Earth surface. Wherever quantitatively accurate predictions of such quantities are relevant, so are the techniques presented here. Whenever possible, the theoretical predictions are compared with the relevant experimental data. In practically all the cases, the agreement between the theoretical predictions and the data is excellent. Also discussed are possible future directions in the application of such concepts to many other phenomena in geosciences.

Hunt, Allen G.↗
DOE OSTI2017

Nucleation of Salt Crystals in Clay Minerals: Molecular Dynamics Simulation

Nucleation of salt crystals in confined media occurs in many processes of high importance, such as injection of CO 2 in geological formations for its sequestration. Specifically, salt precipitation in clays, a main component of sedimentary rock, is an important phenomenon. The crystals precipitate on the pores’ surface, modify the pore space morphology, and reduce its flow and transport properties. Despite numerous efforts to understand the mechanisms of nucleation of salt crystals in confined media, the effect of the clay’s chemistry on the growth, distribution, and properties of the crystals is not well understood. We discuss the results of extensive molecular dynamics simulation of nucleation and growth of NaCl crystals in a clay pore using molecular models of two types of clay minerals, Na-montmorillonite and kaolinite. Clear evidence is introduced for the nucleation of the salt crystals that indicates that the molecular structure of clay minerals affects their spatial distribution, although the nucleation mechanism is the same in both types of clays.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Microstructural characterization of random packings of cubic particles

Understanding the properties of random packings of solid objects is of critical importance to a wide variety of fundamental scientific and practical problems. The great majority of the previous works focused, however, on packings of spherical and sphere-like particles. We report the first detailed simulation and characterization of packings of non-overlapping cubic particles. Such packings arise in a variety of problems, ranging from biological materials, to colloids and fabrication of porous scaffolds using salt powders. In addition, packing of cubic salt crystals arise in various problems involving preservation of pavements, paintings, and historical monuments, mineral-fluid interactions, CO 2 sequestration in rock, and intrusion of groundwater aquifers by saline water. Not much is known, however, about the structure and statistical descriptors of such packings. We have developed a version of the random sequential addition algorithm to generate such packings, and have computed a variety of microstructural descriptors, including the radial distribution function, two-point probability function, orientational correlation function, specific surface, and mean chord length, and have studied the effect of finite system size and porosity on such characteristics. Here, the results indicate the existence of both spatial and orientational long-range order in the packing, which is more distinctive for higher packing densities.

36 MATERIALS SCIENCE↗
DOE OSTI2015

First principles-based multiparadigm, multiscale strategy for simulating complex materials processes with applications to amorphous SiC films

Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2011

Hybrid adsorptive membrane reactor

A hybrid adsorbent-membrane reactor in which the chemical reaction, membrane separation, and product adsorption are coupled. Also disclosed are a dual-reactor apparatus and a process using the reactor or the apparatus.

Tsotsis, Theodore T.↗