Statistical Mechanical and Quantum Mechanical Modeling of Condensed Phase Systems

Understanding adsorption in nanoporous media such as carbon nanotubesare vital to improving fluid storage and separations processes. Onemajor objective of this research is to shed light on an on-goingcontroversy in literature over where gases adsorb on single walledcarbon nanotube bundles. Grand-canonical Monte Carlo simulations havebeen performed using models of carbon nanotube bundles composed oftubes of all the same diameter (homogeneous) and tubes of differentdiameters (heterogeneous). We use three metrics with which we compareour simulation results to those found in experiments on HiPconanotubes: the specific surface area, the isosteric heat ofadsorption, and adsorption capacity. Simulations of classicallybehaved fluids Ar, CH$_4$, and Xe indicate that nanotubes prepared bythe HiPco process are best described by a model consisting ofheterogeneous bundles with $sim11\%$ of the nanotubes opened. Nerequires additional considerations to describe the quantum effects atthe temperatures of interest. Simulation results from Ne simulationsare consistent with those from classical fluids. However, Nesimulations strongly indicate that the small interstitial channelsformed by exactly three nanotubes are closed. Combined with previousstudies on classically behaved fluids Ar, CH$_4$, and Xe, experimentaldata including Ne adsorption are best matched by hetergeneous bundleswith $sim11\%$ open nanotubes.The development of a heterogeneous Co/C/O reactive force field(ReaxFF) potential has also been a major objective of this research.ReaxFF provides a method to describe bond-breaking and bond-formingevents that can be applied to large-scale molecular dynamicssimulations. The many-bodied semi-empirical potential has been trainedfrom emph{ab initio} density functional theory calculations. Thetraining set originally included description of bulk and surfacecondensed phase cobalt systems, but was later expanded to includebinary (Co/C, Co/O) and tertiary (Co/C/O) heterogeneous interactions.We have tested these parameters against additional DFT calculationsnot included in the fitting data set. The parameter optimization hasproduced a force field capable of describing new configurations withsame accuracy as those used in the fitting procedure. The optimizedparameters have been used to predict the melting point and diffusioncoefficients of bulk fcc cobalt. Large-scale simulations of a Co$_2$Cphase nanoparticle show segregation on short time scales (less than300 ps), with all C atoms forming graphene precursors on the surfaceof a Co nanoparticle core. ReaxFF has also been used to showdiffusion of Co is more energetically favorable than oxygen through acobalt oxide crystal. This is consistent with experimentalobservations that oxidized cobalt nanoparticle form hollow cobaltoxide nanospheres. These two binary applications demonstrate thatReaxFF is transferable to heterogeneous systems and a computationallyinexpensive means by which transition metal surface reactions can beexplored

Surfactant Assisted Dispersion of Single-Walled Carbon Nanotubes in Polyvinylpyrrolidone Solutions

Obtaining stable aqueous dispersions is one of the main challenges hindering a widespread and effective use of single-walled carbon nanotubes (SWNT) in many applications. Although it has been recognized that their versatility makes them an extremely attractive material, the unique molecular structure that gives SWNTs their unmatched electronic, mechanical, and thermal properties is also responsible for strong van der Waals interactions. This, combined with extremely high aspect ratios and flexibility, causes SWNTs to adhere strongly into tightly bundled ropes. In these bundles, SWNTs are not as useful as their linearized unbundled equivalents. Thus, in order to take advantage of their properties effectively, SWNTs must be debundled. In this contribution we will report the characterization of a novel non-covalent system using the surfactant, cetyltrimethylammonium bromide (CTAB) and the polymer, polyvinylpyrrolidone (PVP) at different molecular weights. Initial tests using Vis-NIR spectroscopy showed that although individually these molecules are poor dispersers of SWNTs, they show a synergic effect when combined for all cases. We have probed for a mechanism using a battery of characterization techniques including Vis-NIR, atomic force microscopy (AFM), viscosity, dynamic light scattering (DLS), surface tension, and pH. Our data suggests that CTAB binds normally to nanotubes while PVP is augmenting dispersion through a physical mechanism specifically linked to its hydrodynamic radius. We propose our approach as a facile way of augmenting current nanotube dispersion techniques, potentially allowing for increased usage in the world today

Simulations of Adsorptions and Phase Transitions

The objective of this thesis is to developsimulation tools that will allow us to study many phenomena from amolecular level. The topics covered inthis thesis include bulk phasetransitions, phase transitions in adsorbed fluids,and the application of single-walled carbon nanotubes as a gasstorage media.Multiple histogram reweighting and mixed-field finite-size scaling techniqueshave been developed to calculate the phase diagram for classical andquantum fluids in bulk and adsorbed phases.We show, for the firsttime, that capillary condensation showsa crossover of the effectiveexponent for the width of the coexistence curve from 2-DIsing-like (1/8) farther away from the critical point tomean-field (1/2) near the critical point.The first prewetting transitions clearlyobserved from simulation of quantum fluids are presented.The experimentalwetting temperature of 19.1 K is reproduced from the simulationwith a modified potential. Hydrogen adsorbing on a 15 AA thickfilm of Rb on Au gives a wetting temperatureof about 1 K less than H$_2$ on pure Rb.This prediction should be observablefrom experiments.Hydrogen adsorption onto single walled carbonnanotube bundles has been performed from computer simulationsand compared with the experiments. We studythe effect of CO$_2$ oxidation of the nanotubes on adsorption. Isothermscomputed with a standard graphitic potential giveremarkably good agreement with the experimentally measured isotherms beforeactivation with CO$_{2}$. The effect of activation is modeled byindependently increasing the nanotube spacing and the solid-fluidinteraction potential. It is found that onlya combination of increased nanotube spacing and increased solid-fluidpotential gives rough agreement with experiments.Gases such as CH$_4$, Xe, and Arhave been studied on both the homogeneous (same tubediameter) and heterogeneous (different tube diameters)closed single-walled carbon nanotube bundles constructed fromthe basin-hopping method.Experimental gas adsorption data on SWNT bundles have previously been analyzedin terms of an over-simplified model of homogeneous nanotubespacked into perfect arrays. This analysis has led to the generalconclusion that gases do not adsorb inside interstitial channelsof homogeneous nanotube bundles.Our analysis overturns the current paradigm of gas adsorption onSWNTs by showing that adsorption inside interstices ofheterogeneous SWNT bundles isvitally important to accurately describing these materials