Simulations of adsorptions and phase transitions

The objective of this thesis is to develop simulation tools that will allow us to study many phenomena from a molecular level. The topics covered in this thesis include bulk phase transitions, phase transitions in adsorbed fluids, and the application of single-walled carbon nanotubes as a gas storage media.Multiple histogram reweighting and mixed-field finite-size scaling techniques have been developed to calculate the phase diagram for classical and quantum fluids in bulk and adsorbed phases. We show, for the first time, that capillary condensation shows a crossover of the effective exponent for the width of the coexistence curve from 2-D Ising-like (1/8) farther away from the critical point to mean-field (1/2) near the critical point. The first prewetting transitions clearly observed from simulation of quantum fluids are presented. The experimental wetting temperature of 19.1 K is reproduced from the simulation with a modified potential. Hydrogen adsorbing on a 15 A thick film of Rb on Au gives a wetting temperature of about 1 K less than H2 on pure Rb. This prediction should be observable from experiments.Hydrogen adsorption onto single walled carbon nanotube bundles has been performed from computer simulations and compared with the experiments. We study the effect of CO2 oxidation of the nanotubes on adsorption. Isotherms computed with a standard graphitic potential give remarkably good agreement with the experimentally measured isotherms before activation with CO2. The effect of activation is modeled by independently increasing the nanotube spacing and the solid-fluid interaction potential. It is found that only a combination of increased nanotube spacing and increased solid-fluid potential gives rough agreement with experiments.Gases such as CH4, Xe, and Ar have been studied on both the homogeneous (same tube diameter) and heterogeneous (different tube diameters) closed single-walled carbon nanotube bundles constructed from the basin-hopping method. Experimental gas adsorption data on SWNT bundles have previously been analyzed in terms of an over-simplified model of homogeneous nanotubes packed into perfect arrays. This analysis has led to the general conclusion that gases do not adsorb inside interstitial channels of homogeneous nanotube bundles. Our analysis overturns the current paradigm of gas adsorption on SWNTs by showing that adsorption inside interstices of heterogeneous SWNT bundles is vitally important to accurately describing these materials.