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  • 1
    Electronic Resource
    Electronic Resource
    Diffusion, adsorption, and reaction in pillared clays. I. Rod-like molecules in a regular pore space (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5107-5118 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the results of the first computer simulation of transport, adsorption, and reaction processes in pillared clays, which are a class of catalytic materials with high catalytic activities. These materials have a very restricted pore structure which gives rise to the phenomenon of hindered diffusion in their pore space. We develop a dynamic Monte Carlo method and study diffusion, adsorption, and reaction phenomena in such systems. The pore space of the pillared clays is represented by parallel silicate layers connected to one another by pillars of various sizes, and the molecules are in the form of long, needlelike objects. Diffusion is represented by a random walk process, the adsorption of the molecules on the surface of the pillars takes place with a probability proportional to a Boltzmann factor, and the efficiency of the reaction properties of the pillared clays is investigated by measuring the average distance that a molecule has to travel in the pore space in order to reach a reactive site. Our results indicate that the structure of the pore space of the clays, the intermolecular interaction, the size of the molecules, and their adsorption on the surface of the pillars strongly affect their effective diffusivity and, even if the pore space of the clays is very regular and homogeneous, they can give rise to anomalous diffusion in which the effective diffusivity of the molecules varies slowly with time. Moreover, if the size of the molecules is comparable to the effective size of the pores, the irreversible adsorption of the molecules gives rise to a percolationlike phenomenon, in which the effective diffusivity decreases as the number of adsorbed molecules increases. The effective diffusivity would ultimately vanish if enough molecules are adsorbed so that a sample-spanning path of open pores would no longer exist.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458544
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  • 2
    Electronic Resource
    Electronic Resource
    Nonequilibrium molecular dynamics simulation of transport and separation of gases in carbon nanopores. II. Binary and ternary mixtures and comparison with the experimental data (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 910-922 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results of extensive nonequilibrium molecular dynamics simulations of transport and separation characteristics of binary and ternary gas mixtures consisting of CO2, CH4, and H2 through a carbon nanopore, in the presence of an external chemical potential gradient. The gas molecules are represented as Lennard-Jones (LJ) hard spheres. The effect of the various factors, such as the temperature, feed composition, and the pore size, on the transport, adsorption, and separation characteristics is investigated in detail. The simulations' predictions are compared with experimental data obtained with a carbon molecular-sieve membrane. In some cases, there is good agreement between the predictions and the experimental data, while in other cases the simulations' results and the data do not agree. Possible causes for the (dis)agreement are discussed, including the crucial interplay between two main factors in gas separation in a pore space, namely, adsorption on the pores' walls versus the morphology (the pores' interconnectivity and size distribution) of the porous material. Improved models are thus suggested. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480618
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  • 3
    Electronic Resource
    Electronic Resource
    Statistical mechanics and molecular simulation of adsorption of ternary gas mixtures in nanoporous materials (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 7196-7210 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357802
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  • 4
    Electronic Resource
    Electronic Resource
    A combined experimental and theoretical study of the neutral, cationic, and anionic Si3N cluster molecule (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2871-2879 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collision experiments and high-level ab initio calculations are used for the first characterization of the neutral, cationic, and anionic Si3N cluster molecules. The experimentally observed fragmentation processes are well described by calculated reaction energies of all dissociation channels. In addition, the geometries and harmonic frequencies of SinN (n=1–3) have been calculated for the cationic, anionic, and neutral species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467601
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  • 5
    Electronic Resource
    Electronic Resource
    Nonequilibrium molecular dynamics simulation of transport and separation of gases in carbon nanopores. I. Basic results (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3252-3264 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results of extensive nonequilibrium Molecular Dynamics simulations of transport of a binary gas mixture of CO2 and CH4 through a carbon nanopore, in the presence of an external chemical potential or concentration gradient. The CH4 molecules are represented as Lennard-Jones (LJ) hard spheres, while the CO2 molecules are modeled both as LJ hard spheres, as well as three-site linear structures to account for their quadrupole moments. The effect of the various factors, including the structure of the pore walls, temperature, feed composition, and the molecular models and their parameters, on the transport, adsorption, and separation of the gases is investigated in detail. While the structure of the pore walls has virtually no effect on the separation factor, the temperature of the system, the pore size, and the molecular models and their parameters strongly influence it. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479663
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  • 6
    Electronic Resource
    Electronic Resource
    Solitons in polyacetylene: Magnetic hyperfine constants from ab initio calculations (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7698-7716 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spin distribution of the neutral soliton in trans-polyacetylene is controversial, as evidenced by the markedly different interpretations given by the various active experimental groups to their ENDOR measurements of proton hyperfine coupling (hfc) constants. To help resolve the discrepancies, ab initio electronic structure calculations were carried out for proton and carbon-13 hfc on the series of model polyene radicals C5H7,C9H11,...,C21H23 and the most important characteristics of the spin distribution were extrapolated to the long chain limit. Here σ spin polarization was treated through single excitation configuration interaction, both with respect to Hartree–Fock and to multiconfiguration reference spaces that include the most significant effects of π electron correlation. Particular attention was paid to basis set requirements necessary for proper treatment of hfc. It turns out that the ab initio hfc does not agree with that obtained by any of the major experimental groups. In particular, the calculated width of the bell-shaped distribution for the central and other even proton sites is much narrower than all experimental reports, and the anisotropic coupling calculated for the central proton along the fibril axis is much larger. Several assumptions commonly made in analyzing the ENDOR spectra are shown not to be quantitatively justifiable, but their improvement is unlikely to resolve the large discrepancies we have found between theory and current experimental interpretations. Thus, it appears likely that the experimental spectra are significantly influenced by environmental effects and cannot be adequately modeled by an isolated long chain polyene radical as heretofore assumed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461344
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  • 7
    Electronic Resource
    Electronic Resource
    Surface tension of binary liquid–vapor mixtures: A comparison of mean-field and scaling theories (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6749-6761 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use two different methods to estimate surface tension of binary liquid–vapor mixtures of CO2 and a hydrocarbon near a critical point. The first method is based on the gradient theory, which is essentially a mean-field approximation to the problem that reduces the determination of the interface's structure and the surface tension to a boundary value problem. The theory's input is an equation of state of homogeneous fluid and the influence parameters of inhomogeneous fluid. The Peng–Robinson equation and a modification of it are used as the equation of state of homogeneous fluid. The second method is based on the concept of two-scale-factor universality which can predict the surface tension from the singularity in the thermodynamic properties of the bulk fluid. The inputs of the method are an equation of state and certain universal amplitude ratios near the critical point. As the equation of state, we use a modification of a model first proposed by Leung and Griffiths, and further developed by Moldover, Rainwater, and co-workers. We use the two models to examine in detail CO2+n -butane and CO2+n -decane mixtures. While both models provide accurate estimates of surface tension of CO2+n -butane mixtures, only the gradient theory can predict accurately surface tension of CO2+n -decane mixtures. Moreover, while the gradient theory and the Peng–Robinson equation of state use very few adjustable parameters (at most three parameters), calculation of surface tension based on two-scale-factor universality and the corresponding equation of state requires many adjustable parameters whose number has to be increased dramatically as the fluid mixture becomes more complex. We then use the gradient theory to predict surface tension of binary liquid–vapor mixtures of CO2 and benzene, cyclohexane, and n-hexadecane. In all cases, the predictions of the gradient theory are in good agreement with the available experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461514
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  • 8
    Electronic Resource
    Electronic Resource
    Monte Carlo simulation of the three-dimensional dilute Widom model of microemulsions (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1385-1387 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Widom suggested a spin-1/2 Ising model for microemulsions. As a simple model of microemulsions in a porous medium such as sandstone, we simulate it on a randomly dilute simple-cubic lattice with sizes up to 252.3 Our results for the critical demixing temperature are somewhat similar to those of Velgakis in the square lattice.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459995
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  • 9
    Electronic Resource
    Electronic Resource
    Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Hartree–Fock static longitudinal hyperpolarizability of polyacetylene (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 5350-5356 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio restricted Hartree–Fock 6-31G calculations are reported for the static longitudinal hyperpolarizability of the linear polyenes C4H6 through C44H46. Using a new extrapolation technique the infinite chain value in polyacetylene is determined with an accuracy similar to that achieved for small molecules. This is the first in a series of articles leading to a comprehensive ab initio treatment for the nonlinear optical properties of conjugated polymers. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469262
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of gas mixtures in porous media. I. Adsorption (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2178-2188 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper and its sequel we report the results of Molecular Dynamics simulation of single component and binary gas mixtures in a porous medium with interconnected pores. The porous medium used in the study is a model pillared clay. In the present paper we study adsorption of binary gas mixtures, and investigate in detail the effect of various factors, such as the morphology of the pore space and the adsorbent-adsorbate interactions. A new mean-field statistical mechanical theory of adsorption is developed, and shown to provide very accurate predictions for the simulation results over wide ranges of the pressure, temperature and porosity of the system. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475598
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