Molecular Dynamics Simulations Of Adsorption Research Articles

Molecular dynamics simulations of adsorption of hydrophobic 1,2,4-trichlorobenzene (TCB) on hydrophilic TiO 2 in surfactant emulsions and experimental process efficiencies of photo-degradation and -dechlorination

Molecular dynamics simulations have been carried out to assess event details in the adsorption of the hydrophobic substrate 1,2,4-trichlorobenzene (TCB) on the hydrophilic TiO 2 particle surface in the presence of the biosurfactant 4-O-(4′,6′-di-O-acetyl-2′,3′-di-O-alkanoyl-β- d-mannopyranosyl)- d-erythritol (MEL) and the SDS (sodium dodecylsulfate) surfactant in aqueous media at ambient temperature with interaction forces between organic molecules and TiO 2 calculated from Lennard–Jones and Coulomb potential models. The surfactant and the TCB substrate were arranged in the vicinity of TiO 2 using 768 water molecules in the simulations. Results indicate that the MEL molecules adsorb on the TiO 2 surface in ca. 5 ps ( k = 2 × 10 11 s −1) through the oxygen atoms of the hydrophilic functions. Contact distance between the MEL and the TiO 2 surface shortened further after 10 ps with adsorption controlled mostly by van der Waals’ forces. The TCB molecules are trapped within the alkyl chains of the MEL system, which subsumes TCB to a greater extent by the repulsion of water, thereby facilitating the TCB molecules to approach the highly hydrophilic and positively charged TiO 2 particle surface. Along with the simulations, also examined was the photodegradation of this hydrophobic TCB substrate that takes place at the TiO 2/water interface in the presence of the MEL biosurfactant. For comparison, the advantages of the MEL in this task and of the commonly used SDS surfactant were determined under otherwise identical experimental conditions (200 mg L −1 of surfactants) by examining the dynamics of the photo-induced degradation and dechlorination of TCB. The photodegradation of TCB was nearly quantitative in MEL/TiO 2 in contrast to only ca. 22% complete in SDS/TiO 2 dispersions. It is deduced that wastewater treatments with the highly hydrophilic TiO 2 are seriously limited in their photodegradation of hydrophobic pollutants, an issue easily resolvable by the presence of biodegradable surfactants.

Molecular dynamics simulations of adsorption and diffusion of gases in silicon-carbide nanotubes

Silicon carbide nanotubes (SiCNTs) are new materials with excellent properties, such as high thermal stability and mechanical strength, which are much improved over those of their carboneous counterparts, namely, carbon nanotubes (CNTs). Gas separation processes at high temperatures and pressures may be improved by developing mixed-matrix membranes that contain SiCNTs. Such nanotubes are also of interest in other important processes, such as hydrogen production and its storage, as well as separation by supercritical adsorption. The structural parameters of the nanotubes, i.e., their diameter, curvature, and chirality, as well as the interaction strength between the gases and the nanotubes' walls, play a fundamental role in efficient use of the SiCNTs in such processes. We employ molecular dynamics simulations in order to examine the adsorption and diffusion of N(2), H(2), CO(2), CH(4), and n-C(4)H(10) in the SiCNTs, as a function of the pressure and the type of the nanotubes, namely, the zigzag, armchair, and chiral tubes. The simulations indicate the strong effect of the nanotubes' chirality and curvature on the pressure dependence of the adsorption isotherms and the self-diffusivities. Detailed comparison is made between the results and those for the CNTs. In particular, we find that the adsorption capacity of the SiCNTs for hydrogen is higher than the CNTs' under the conditions that we have studied.

Molecular dynamics simulation of adsorption of an oil-water-surfactant mixture on calcite surface

An interface super molecular structure model for oil-water-surfactant mixture and calcite was established. By using a molecular dynamics method, the effects of rhamnolipid, sodium dodecyl benzene sulfonate and sodium hexadecyl sulfonate on the interface adsorption behavior of oil molecules were investigated. It was found that these three surfactants could reduce oil-calcite interface binding energy, and play a role of oil-displacing agent.

HIGH-SURFACE-AREA BIOCARBONS FOR REVERSIBLE ON-BOARD STORAGE OF NATURAL GAS AND HYDROGEN

AbstractAn overview is given of the development of advanced nanoporous carbons as storage ma-terials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean automobiles. The carbons are produced in a multi-step process from corncob, have surface areas of up to 3500 m2/g, porosities of up to 0.8, and reversibly store, by physisorp-tion, record amounts of methane and hydrogen. Current best gravimetric and volumetric storage capacities are: 250 g CH4/kg carbon and 130 g CH4/liter carbon (199 V/V) at 35 bar and 293 K; and 80 g H2/kg carbon and 47 g H2/liter carbon at 47 bar and 77 K. This is the first time the DOE methane storage target of 180 V/V at 35 bar and ambient temperature has been reached and exceeded. The hydrogen values compare favorably with the 2010 DOE gravimetric and volu-metric targets for hydrogen. A prototype adsorbed natural gas (ANG) tank, loaded with carbon monoliths produced accordingly and currently undergoing a road test in Kansas City, is de-scribed. A preliminary analysis of the surface and pore structure is given that may shed light on the mechanisms leading to the extraordinary storage capacities of these materials. The analysis includes pore-size distributions from nitrogen adsorption isotherms; spatial organization of pores across the entire solid from small-angle x-ray scattering (SAXS); pore entrances from scanning electron microscopy (SEM) and transmission electron microscopy (TEM); H2 binding energies from temperature-programmed desorption (TPD); and analysis of surface defects from Raman spectra. For future materials, expected to have higher H2 binding energies via appropriate sur-face functionalization, preliminary projections of H2 storage capacities based on molecular dy-namics simulations of adsorption of H2 on graphite, are reported.

Molecular Dynamics Simulations of Adsorption of Organic Compounds at the Clay Mineral/Aqueous Solution Interface

Computational studies of the sorption of organic compounds at clay mineral surfaces are described. Molecular dynamics simulations were performed with a recently developed empirical force field for dioctahedral clays. The studies allow the identification of three general mechanisms of adsorption. In the absence of water, organic compounds adsorb to mineral surfaces in such a way that contact area is maximized. In the presence of a sufficient amount of water, some molecules can adsorb via a single functional group, while the bulk of the molecular structure is immersed in the aqueous phase. When many water molecules are present, they form a structured layer, excluding organic adsorbates from the mineral basal plane. A detailed description is given of the characteristic structure found for water layers in the interlayer space of clays. The calculated trends are reasonable, but we also expect that current dynamics simulations may overestimate the extent of the structuring of water because of the absence of polarization terms in the available empirical force fields.

Modeling of Adsorption and Desorption in Pores of Simple Geometry Using Molecular Dynamics

We present results from molecular dynamics simulations of adsorption and desorption by diffusive mass transfer into model pores of well-defined geometry. The pore geometries considered were chosen to investigate issues related to classical theories of hysteresis in adsorption. We consider the slit pore with or without a closed end, a wedge or triangular pore, and finally a model with an ink bottle geometry. In each case we show that the molecular dynamics simulations yield the same results for adsorption and desorption as grand canonical Monte Carlo simulations of the same systems. Using computer graphics visualization, we show how hysteresis for the slit and wedge geometries is related to the evolution of vapor−liquid interfaces in a manner largely consistent with the classical picture of these systems. We find that for the ink bottle geometry the large pore can empty during desorption even while the small pore remains filled with fluid. This behavior is different from the classical picture of adsorption and desorption in this geometry based on the concept of pore blocking.

Statistical mechanics and molecular simulation of adsorption of ternary gas mixtures in nanoporous materials

We report the results of extensive equilibrium molecular dynamics simulation of adsorption of ternary gas mixtures in a class of model nanoporous materials, namely, carbon molecular sieve membranes (CMSMs), which are being utilized for separation of gas mixtures. We use a novel three-dimensional molecular pore network model for CMSMs based on Voronoi tessellation of the simulation cell. The simulations with such a model allow us to investigate the effect of the morphology of the pore space, i.e., its pore size distribution and pore connectivity, on the adsorption isotherms. We also extend a statistical mechanical theory of adsorption of single gases and their binary mixtures, recently developed by us, to ternary mixtures and use the results of the molecular simulations to test its accuracy. The theory is shown to provide very accurate predictions for the simulation results over wide ranges of the temperature and porosity of the membrane. The performance of the theory is also compared with that of another theory of adsorption based on a virial expansion.

Hysteresis in Monte Carlo and Molecular Dynamics Simulations of Adsorption in Porous Materials

We present a molecular dynamics simulation method for adsorption/desorption in a sample of a model porous material from a fluid reservoir at a fixed chemical potential and temperature. The method was developed to investigate the stability of the states associated with hysteresis in the adsorption/desorption isotherms obtained in grand canonical Monte Carlo (GCMC) simulations. For a model of a simple fluid adsorbed in a silica gel, we find that the hysteresis loops obtained in the molecular dynamics calculations are essentially identical to those obtained in GCMC simulations of the same system. The significance of this result is discussed.

Statistical Mechanics and Molecular Simulation of Adsorption in Microporous Materials: Pillared Clays and Carbon Molecular Sieve Membranes

We report the results of extensive equilibrium molecular dynamics simulation of adsorption of single-component and binary gas mixtures in models of two classes of microporous materials, namely, pillared clays and carbon molecular sieve membranes (CMSMs), both of which have been utilized for separation of gas mixtures. We develop a novel three-dimensional molecular pore network model for CMSMs based on Voronoi tessellation. The simulations allow us to investigate the effect of the morphology of the pore space, i.e., its pore size distribution and pore connectivity on the adsorption isotherms. The simulation results are also used to test the accuracy of a recently developed statistical mechanical theory of adsorption. The theory is shown to provide very accurate predictions for the simulation results for both classes of the microporous materials over a wide range of the porosity of the porous materials.

Molecular dynamics study of adsorption and spreading of a polymer chain onto a flat surface

We present results from a series of molecular dynamics simulations for adsorption and spreading of polymer chains onto a flat surface. We consider both homopolymer chains and “protein-like” copolymer chains in the simulations. For homopolymer chains, we have considered both good and poor solvent conditions, and for copolymer chains, we have considered several conformations of the non-adsorbing monomers. Our results indicate that when the adsorption strength is strong enough, a scaling description of the adsorption kinetics works well for homopolymers in both good and poor solvent conditions. When the adsorption strength is not strong enough, the chains adsorb partially, and one needs to consider effects of this partial adsorption in the scaling description. For each of the three primary structures of the copolymers considered in this study, the polymer chain does get adsorbed to the surface but the kinetics of the adsorption process depends on the specific structures of the copolymer chains.

Molecular dynamics simulation of adsorption of Ag particles on a graphite substrate

We have performed for the first time a molecular dynamics simulation of the adsorption of gas-phase Ag particles on a graphite substrate to provide an insight into the results of a comprehensive STM-based experiment on this system. Both pair-wise and many-body interatomic potentials have been employed, and a Morse-type Ag–C potential was specifically constructed to describe the interactions at the interface. Our simulation has successfully reproduced a significant portion of the experimental findings. We have also observed the intercalation of silver in graphite.

Molecular dynamics simulation of adsorption in electric double layers

Classical molecular dynamics is used to model the structure and dynamics of electric double layers that form when a metal electrode is in contact with an aqueous electrolyte solution containing simple ions and neutral organics. First attention is focused on the distribution of ions and neutrals next to an uncharged electrode. The electric field and potential across the system are calculated. The iodide ion adsorbs from neutral solution and causes a negative shift in the potential of zero charge (PZC) relative to fluoride. Adsorbed benzene causes a small positive shift of the PZC. Benzene desorbs when the metal electrode is charged and the surface electric field causes a layer of localized and oriented water to form next to the electrode. Non contact adsorbed hydrated sodium ions are more effective than contact adsorbed chloride in desorbing benzene.

Molecular dynamics simulations of adsorption and diffusion of Xe on bare and Xe covered Pt(111)

Molecular dynamics simulations are used to calculate adsorption probabilities and diffusion coefficients of Xe on Pt(111) and Xe/Pt(111). Experimental trends are reproduced. The adsorption mechanism is found to be the modified Kisliuk model, with the addition of direct and indirect insertion in the Xe islands. Up to almost saturation coverage, adsorption via migration to island borders is the most important way of adsorbate assisted adsorption. Stimulated desorption is observed at high incidence energies.

Molecular dynamics simulation of adsorption of ions from aqueous media onto charged electrodes

Molecular dynamics simulation of 216 water molecules (ST2 model) between charged flat electrodes 2.362 nm apart showed layering with a few molecules at each surface that broke H bonds with the bulk and oriented their charges towards the electrode. Compared to uncharged electrodes, the atomic and molecular distributions were unsymmetric. When a lithium and an iodide ion were substituted at random for two water molecules, the iodide ion contact adsorbed on the anode with no water molecules between it and the electrode. The iodide ion appeared weakly solvated on the solution side to water molecules that preferred to engage in hydrogen bonding with the network of the bulk solvent. In contrast, the lithium ion adsorbed without losing its primary solvation shell of six water molecules and was never observed further than two water molecules removed from the electrode. Its average position corresponded to an ion supported on a tripod of three waters. The average solvation number was not changed upon adsorption in this configuration. These qualitative observations and some quantitative results afford striking confirmation on the one hand and new insight on the other of some aspects of the standard model of the adsorption of ions on electrode surfaces. Time durations for simulations were generally between 200 and 800 ps with a basic integration time step of 2 fs.