N. Quirke

A new analysis method for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements

Abstract A new analysis method has been developed for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements. The method is based on a molecular model for the adsorption of nitrogen in porous carbon. It allows, for the first time, the distribution of pore sizes to be determined over both the micropore and mesopore size ranges using a single analysis method. In addition to carbons, this method is also applicable to a range of adsorbents, such as silicas and aluminas.

Fluid flow in carbon nanotubes and nanopipes

Nanoscale carbon tubes and pipes can be readily fabricated using self-assembly techniques and they have useful electrical, optical and mechanical properties. The transport of liquids along their central pores is now of considerable interest both for testing classical theories of fluid flow at the nanoscale and for potential nanofluidic device applications. In this review we consider evidence for novel fluid flow in carbon nanotubes and pipes that approaches frictionless transport. Methods for controlling such flow and for creating functional device architectures are described and possible applications are discussed.

Molecular modeling of electron traps in polymer insulators: Chemical defects and impurities

The presence of space charge in the polymeric insulation of high-voltage cables is thought to be correlated with electric breakdown. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. It is clear that both physical (e.g., conformational disorder) and chemical defects (e.g., broken bonds and impurities) are present in insulating materials and that both may trap electrons. We have shown that by defining the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the material. In a recent paper [M. Meunier and N. Quirke, J. Chem. Phys. 113, 369 (2000)] we proposed methods that have made it possible to provide estimates of the energy, number and residence times of electrons in conformational traps in polyethylene. Typical physical trap energies are of the order of 0.15 eV and all are less than 0.3 eV. In the present paper we focus on the role o...

Fluid flow in nanopores: Accurate boundary conditions for carbon nanotubes

Steady-state Poiseuille flow of a simple fluid in carbon nanopores under a gravitylike force is simulated using a realistic empirical many-body potential model for carbon. Building on our previous study of slit carbon nanopores we show that fluid flow in a nanotube is also characterized by a large slip length. By analyzing temporal profiles of the velocity components of particles colliding with the wall we obtain values of the Maxwell coefficient defining the fraction of molecules thermalized by the wall and, for the first time, propose slip boundary conditions for smooth continuum surfaces such that they are equivalent in adsorption, diffusion, and fluid flow properties to fully dynamic atomistic models.

Enhanced fluid flow through nanoscale carbon pipes.

Recent experimental and theoretical studies demonstrate that pressure driven flow of fluids through nanoscale ( d < 10 nm) carbon pores occurs 4 to 5 orders of magnitude faster than predicted by extrapolation from conventional theory. Here, we report experimental results for flow of water, ethanol, and decane through carbon nanopipes with larger inner diameters (43 +/- 3 nm) than previously investigated. We find enhanced transport up to 45 times theoretical predictions. In contrast to previous work, in our systems, decane flows faster than water. These nanopipes were composed of amorphous carbon deposited from ethylene vapor in alumina templates using a single step fabrication process.

Molecular modeling of electron trapping in polymer insulators

The presence of space charge in the polymeric insulation of high-voltage cables is correlated with electric breakdown. There is a vast literature concerned with the experimental characterization of space charge and with phenomenological models of space charge formation and discharge. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. In this paper, we suggest a new scheme that constitutes a first step in linking microscopic defects to the formation of space charge. Although our goal is to understand the role of defects at the molecular level in electron trapping and the formation of space charge in polyethylene, we start by considering a “model” material; the wax tridecane (n-C13H28). It is clear that both physical (e.g., conformational defects) and chemical defects (e.g., broken bonds) may be present in insulating materials and may both trap electrons. In the present paper, we focus on the role of physical defects. Our analysis ...

Molecular simulation of methane and butane in silicalite

Molecular simulation studies of methane in silicalite at room temperature are reported. Adsorption isotherms and diffusion coefficients have been calculated for a range of pressures. At low pressures (<20 bar) adsorption is predominantly at specific potential-energy minima in the silicalite channels while at higher pressures adsorption is determined by the total accessible channel volume. Diffusion is found to be strongly anisotropic with the fastest diffusion along the straight channels. Good agreement is obtained with experimental results for orientationally averaged diffusion coefficients. At low pressure, diffusion is well described by a pseudo-Bosanquet formula which identifies two independent contributions to diffusional resistance (from collisions with walls and from intermolecular collisions). At short times significant molecular force correlations arise due to the diffuculty of methane molecules passing each other in the channel intersections of the silicalite lattice. Some preliminary results for butane diffusion coefficients in silicaliate are also reported.

Computer simulation of molecular liquid mixtures. I. A diatomic Lennard‐Jones model mixture for CO2/C2H6

Molecular dynamics simulations of model anisotropic molecules have been carried out which are intended to represent the molecular mixture CO2/C2H6. By paying careful attention to the methodology of the simulation it has been found possible to reduce the statistical and systematic errors to such an extent that the pressure‐composition curve can be predicted for the mixture. Note that for the experimental system the difference between the maximum pressure of the mixture and the saturated vapor pressure of the pure components differ by about 10 atm. The model mixture displays an azeotrope in qualitative agreement with experiment. We believe this is the first time azeotropy has been observed by molecular dynamics. Detailed results are presented for the saturated liquid structure as a function of composition.

Fluid flow in nanopores: An examination of hydrodynamic boundary conditions

Steady-state Poiseuille flow of a simple fluid in carbon slit pores under a gravity-like force is simulated using a realistic empirical many-body potential model for carbon. In this work we focus on the small Knudsen number regime, where the macroscopic equations are applicable, and simulate different wetting conditions by varying the strength of fluid–wall interactions. We show that fluid flow in a carbon pore is characterized by a large slip length even in the strongly wetting case, contrary to the predictions of Tolstoi’s theory. When the surface density of wall atoms is reduced to values typical of a van der Waals solid, the streaming velocity profile vanishes at the wall, in accordance with earlier findings. From the velocity profiles we have calculated the slip length and by analyzing temporal profiles of the velocity components of particles colliding with the wall we obtained values of the Maxwell coefficient defining the fraction of molecules thermalized by the wall.

Non-destructive molecular-dynamics simulation of the chemical potential of a fluid

We have evaluated the chemical potential by molecular-dynamics simulation for a Lennard-Jones (L-J) fluid and for a Lennard-Jones shifted-force (L-J, sf) fluid over a wide range of temperature and density by Widoms [6] particle-insertion-energy method. We have also investigated, in some detail, more recent methods [10, 11] using the energy of a real particle, and combinations of both methods using the energy-distribution functions. We find that these methods are accurate, convenient and economical but are no better than the straightforward Widom method. We confirm that the molecular-dynamics method is as good, if not better, than the corresponding Monte-Carlo techniques and the grand-canonical Monte-Carlo method. We present results for the properties of the L-J, sf3 fluid, in particular the liquid-vapour co-existence curve, including the critical point, and compare them with those for the L-J fluid and for liquid argon. The method is of general application and, in particular, may be directly used for mol...