Optimized intermolecular potential for nitriles based on Anisotropic United Atoms model

An extension of the Anisotropic United Atoms intermolecular potential model is proposed for nitriles. The electrostatic part of the intermolecular potential is calculated using atomic charges obtained by a simple Mulliken population analysis. The repulsion-dispersion interaction parameters for methyl and methylene groups are taken from transferable AUA4 literature parameters [Ungerer et al., J. Chem. Phys., 2000, 112, 5499]. Non-bonding Lennard-Jones intermolecular potential parameters are regressed for the carbon and nitrogen atoms of the nitrile group (–C≡N) from experimental vapor-liquid equilibrium data of acetonitrile. Gibbs Ensemble Monte Carlo simulations and experimental data agreement is very good for acetonitrile, and better than previous molecular potential proposed by Hloucha et al. [J. Chem. Phys., 2000, 113, 5401]. The transferability of the resulting potential is then successfully tested, without any further readjustment, to predict vapor-liquid phase equilibrium of propionitrile and n-butyronitrile

Use of plan quality degradation to evaluate tradeoffs in delivery efficiency and clinical plan metrics arising from IMRT optimizer and sequencer compromises

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135043/1/mp8118.pd

[Retracted publication] Molecular simulation of Zn2+, Cu2+, Pb2+, and NH4+ ion-exchange in clinoptilolite

Heavy metal ions (Zn, Cu, and Pb) along with (Formula presented.) are the most abundant cations in the domestic and industrial wastewaters to be removed before the discharge into surface waters due to the environmental regulations. Clinoptilolite (CLI) is one of the commercially available natural zeolites with high cation-removal potential from wastewaters. Cation (Na, K, Ca) compositions of this natural zeolite may vary significantly based of its origin. In this study, quantum mechanical calculations and molecular dynamics simulations are used to investigate the relationship between the extra-framework cation types of this zeolite and its removal performance for Zn, Cu, Pb, and (Formula presented.) cations from model wastewater through ion-exchange process. For this purpose, model CLI structures were constructed with either of Na, K, and Ca ions, as well as with their mixtures, as extra-framework cations. Results revealed that Na-exchanged CLI showed the highest ion-exchange capacity among the zeolite models considered and the selectivity sequence was obtained as (Formula presented.) > Pb > Zn > Cu for all CLI structures in agreement with previous experimental studies

Adsorption of perfluorohexane in BAM-P109 type activated carbon via molecular simulation

Perfluorohexane adsorption on porous activated carbon was studied via Monte Carlo methods. In order to estimate the microporous adsorption capacity at relative pressures (P/P-0) 0.1, 0.3, and 0.6, microporous structures were generated by simply packing corannulene and oxygenated corannulene fragments together into a simulation box and the benchmark data were validated by calculating CO2 and Ar adsorption isotherms prior to estimating the perfluorohexane adsorption capacity of the candidate material BAM-P109. Since similar to 50% of the total pore volume of this material is mesoporous, the mesoporous adsorption capacity was also estimated using a slit-pore model with varying pore widths at P/P-0=0.3 and 0.6. Prior to the adsorption simulations, the saturation pressure of perfluorohexane at 273 K was estimated as 8.13 kPa through a series of Monte Carlo simulations, which is in good agreement with the experimental data of 8.55 kPa

The Diffusion Process of Methane Through a Silicalite Single Crystal Membrane

The diffusion process of methane in a silicalite single-crystal membrane has been investigated using the Dual Control Volume-Grand Canonical Molecular Dynamics method. Simulations of full-membrane transport and the three individual contributions that comprise the overall process (entrance to the pores, intra-crystalline diffusion, and exit from the pores) show that the contribution of surface resistance to the overall transport resistance in zeolite membranes is larger and longer range than one might expect. A model is proposed on the basis of the additivity of these contributions. From the individual simulations of exit and entrance zones, it is shown that the adsorption and desorption resistances approach an asymptote with increasing crystal thickness. However, the asymptotic trend has not been observed in full membrane simulations within the thickness limit of this work, possibly because of the coupling between the entrance and exit effects. Since the surface resistance is limited to less than 1 μm and the single-crystal membrane comprises 100 μm, the surface resistance still represents a relatively small contribution to the overall resistance. Therefore, the diffusion process through the single-crystal membrane is dominated by the internal transport of the sorbate molecules along the principal (z-) axis of the crystal

Surface Resistance to Permeation Through the Silicalite Single Crystal Membrane: Variation With Permeant

The variation of surface resistances to diffusion of molecules through the silicalite single-crystal membranes as a function of permeants has been investigated using the Dual Control Volume-Grand Canonical Molecular Dynamics method. For this purpose three spherical molecules, CH 4, Ar, and CF 4, have been selected. This selection enabled the study of a range of molecular diameters and interaction energies. Simulation results showed that the magnitude of surface resistance in zeolite membranes depends on the permeant-crystal interaction size and energy. Furthermore, the range of the surface resistance, defined as the distance from the surface beyond which the surface resistance becomes constant, is primarily a function of molecular size: For smaller molecules the range of surface resistances is shorter while its magnitude is lower. Variations in mass-transfer resistances and diffusivities were studied in further detail with a parametric sensitivity analysis by varying permeant-crystal interaction size and well depth, as well as molecular weight in the manner of a factorial design. This procedure allowed checking for the significance of these factors and their cross-interactions during adsorption from the gas phase into the silicalite. The parametric study showed that the Lennard-Jones gas-crystal size interaction dominates the surface resistance of molecules that penetrate silicalite crystals, but interaction energy is also significant. Although, different sets of parameters yield similar equilibrium concentration values in adsorption studies, the surface resistance varies drastically with variations in these parameters

Gas Permeation Through Zeolite Single Crystal Membranes

Diffusion of methane and argon mixtures through single crystal membranes is studied using the Dual-Control Volume-Grand Canonical Molecular Dynamics method. This study focuses on understanding the impact of crystal structure on surface resistance and membrane performance by comparing diffusion through silicalite, mordenite, AlPO4-5 and ZSM-12. Results showed that the contribution of surface resistance on membrane selectivity varies with the structure of the zeolite framework. Surface resistance is larger and longer range in silicalite, with an overall trend of silicalite \u3e ZSM-12 \u3e mordenite \u3e AlPO4-5. This difference is attributed primarily to the smaller diameter of the silicalite pores, but the one-dimensional pore systems also seem to focus the translational momentum such that the surface resistance is smaller and shorter range. © 2005 Springer Science + Business Media, Inc

Effect of Surface Resistances on the Diffusion of Binary Mixtures in the Silicalite Single Crystal Membrane

Diffusion of methane and argon mixtures through the silicalite single-crystal membrane is studied using the dual-control volume-grand canonical molecular dynamics method to understand how surface resistances alter selectivity and permeance. Comparison of results from intracrystalline transport and entrance simulations for binary mixtures of CH 4 and Ar shows that the selectivity of silicalite membranes toward Ar is enhanced in the presence of the surface resistances. In both cases, however, diffusion of faster Ar molecules was inhibited by slower diffusing CH 4 molecules, whereas diffusion of the latter remained unaffected. This behavior was explained by the difference between the magnitudes of surface resistances for two molecules, which is much smaller for Ar because of its smaller permeant-crystal interaction size. We find that selectivity of the membrane at the surface depends strongly on total feed pressure and temperature, whereas this dependence is weak for intracrystalline diffusion. Furthermore, we show that the selectivity at the surface diminishes with crystal thickness until a certain thickness is reached, whereas the intracrystalline selectivity remains constant with increasing thickness. Finally, a study of diffusion of C 2H 6 and CF 4 mixtures shows that the diatomic ethane molecules diffuse faster inside the zeolite channels, but their desorption is hindered to a larger extent than that of a spherical molecule with larger diameter and lower heat of adsorption. This observation indicates that the difference in molecular geometry is also a significant factor to explain the exit effect