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Notes: We present results from nonequilibrium molecular dynamics (NEMD) simulations of simple fluids undergoing planar Poiseuille flow in a slit pore only a few molecular diameters in width. The calculations reported in this publication build on previous results by including the effects of attractive forces and studying the flow at narrower pore widths. Our aims are: (1) to examine the role of attractive forces in determining hydrodynamic properties, (2) to provide clearer evidence for the existence of a non-Markovian generalization of Newtons law, (3) to examine the slip-stick boundary conditions in more detail by using a high spatial resolution of the streaming velocity profiles, (4) to investigate the significance of the recently proposed cross-coupling coefficient on the temperature profiles. The presence of attractive interactions gives rise to interesting packing effects, but otherwise, does not significantly alter the spatial dependence of hydrodynamic quantities. We find the strongest evidence to date that Newton's Law breaks down for very narrow pores; the shear viscosity exhibits singularities. We suggest a method to test the validity of the non-Markovian generalization of Newton's Law. No-slip boundary conditions are found to apply, even at these microscopic length scales, provided one takes into account the finite size of the wall atoms. The effects of any strain rate induced coupling to the heat flow are found to be insignificant. © 2000 American Institute of Physics.

Notes: We study the freezing of CCl4 in microporous activated carbon fibers (ACF), using Monte Carlo simulation and differential scanning calorimetry (DSC). Microporous activated carbon fibers are well characterized porous materials, having slit-shaped pores due to the voids formed between graphitic basal planes. They serve as highly attractive adsorbents for simple nonpolar molecules, the adsorbent–adsorbate interaction being mostly dispersive (of the van der Waals-type). Recent molecular simulation studies have predicted an upward shift in the freezing temperature (ΔTf=Tf,pore−Tf,bulk〉0) for simple fluids confined in such highly attractive carbon slit pores. Our DSC experiments verify these predictions about the increase in Tf. The results also indicate significant deviation from the prediction of ΔTf based on the Gibbs–Thomson equation (simple capillary theory). We employ a recently developed free energy method to calculate the exact freezing temperature in these confined systems using molecular simulation, in order to address the failure of the simple capillary theory. © 1999 American Institute of Physics.

Notes: We report grand canonical Monte Carlo simulations for a Lennard-Jones (LJ) fluid modeled on methane in slit-shaped pores of several materials and pore widths. Three types of pore wall were considered: graphitic carbon (strongly attractive walls), "methane'' walls (wall attractions equal to those in the adsorbate phase), and hard walls. For each system the change from a fluidlike to a solidlike adsorbed phase was observed, and the shift in freezing or melting temperature from that of the bulk adsorbate material was determined. As well as changes in the overall properties of the adsorbate phase, corresponding changes in the individual adsorbate layers in the pore were studied. In addition hysteresis on heating and cooling was examined. For the graphitic carbon walls the freezing temperature was raised relative to that of the bulk material, the elevation being greater for smaller pore widths; however, no freezing transition was observed for pore widths below about 5.3σ. In addition, the contact layer of adsorbate froze at a temperature higher than the inner layers. For pores with methane walls (walls of LJ molecules having the same density and intermolecular interactions as the adsorbate phase) no shift in freezing temperature occurred, while pores with hard walls showed a decrease in freezing temperature relative to the bulk; in the case of hard walls, the contact layer of adsorbate froze at a lower temperature than the inner layers. Considerable hysteresis was observed in some cases, and the width of the hysteresis loop was sensitive to pore size, being wider for pores in which the adsorbed layers are tightly packed. The results indicate that the direction and magnitude of the shift in freezing temperature in the pore is strongly dependent on the strength of the attractive forces between the adsorbate molecules and the wall, and particularly on the magnitude of this relative to such forces between the adsorbate and a wall composed of the same adsorbate molecules. A simple thermodynamic model based on this idea is proposed, and showed to give a good account of the simulation results for methane in carbons. In the simple systems studied here the confinement causes little change in the solid lattice structure of the bulk material. This is unlikely to be the case for more complex pore geometries, and the analysis of such cases is likely to involve additional structural effects.© 1997 American Institute of Physics.

Notes: We report molecular simulation studies of the freezing behavior of fluids in nano-porous media. The effect of confinement is to induce spatial constraints as well as energetic heterogeneity on the confined fluid, thereby altering the bulk phase behavior drastically. We consider the effect of the fluid-wall interaction energy on the shift of the freezing temperature and on the fluid structure, using a novel approach to calculate the free energy surface based on Landau theory and order parameter formulation. Corresponding states theory is then used to map out the global freezing behavior of a Lennard-Jones (LJ) fluid in model slit-shaped pores of varying fluid-wall interaction strengths. Using LJ parameters fitted to thermophysical property behavior, we predict the qualitative freezing behavior for a variety of fluids and nano-porous materials, based on a global freezing diagram. We have attempted to verify these predictions by comparing with experimental data for several systems, and show that in these cases, the experimental observations and the predictions are in agreement. © 2000 American Institute of Physics.

Notes: We report the phase coexistence properties of polarizable Stockmayer fluids of reduced permanent dipoles |m0*|= 1.0 and 2.0 and reduced polarizabilities α*= 0.00, 0.03, and 0.06, calculated by a series of grand canonical Monte Carlo simulations with the histogram reweighting method. In the histogram reweighting method, the distributions of density and energy calculated in Grand Canonical Monte Carlo simulations are stored in histograms and analyzed to construct the grand canonical partition function of the system. All thermodynamic properties are calculated from the grand partition function. The results are compared with Wertheim's renormalization perturbation theory. Deviations between theory and simulation results for the coexistence envelope are near 2% for the lower dipole moment and 10% for the higher dipole moment we studied. © 1997 American Institute of Physics.

Notes: The adsorption of methane–carbon dioxide mixtures in micropores having either slit or cylindrical geometries, and different adsorption energies, has been studied using molecular simulation. The pore models were constructed from close packed layers of simple atoms whose Lennard-Jones interaction parameters could be varied. For realistic choices of the adsorbent parameters, adsorption selectivity outside the molecular sieving regime was found to vary over a range between about 4 and about 1/4, denoting a preference for either CO2 or for methane, depending on geometry and choice of parameters. The variations in selectivity are interpreted in terms of a balance between energetic factors, including adsorbate–adsorbent interactions and cooperative adsorbate interactions, and entropic factors associated with the constraints on rotational freedom of the CO2 molecule imposed mainly by the geometry of the pore spaces. © 1996 American Institute of Physics.

Notes: The results of recently developed Monte Carlo methods for simulation of strongly associating fluids show an apparent disagreement with Wertheim's theory of associating fluids. In this comment we show that the disagreement observed arises largely from approximations used for the reference-fluid pair distribution function required by the theory. © 1995 American Institute of Physics.