Quantum isotope mixture studies using molecular simulations

The objective of this thesis is to understand thermodynamic properties and fundamental behavior of quantum isotope mixtures using computer simulations. Path integral simulations have been carried out to determine non-idealities and phase equilibrium of quantum liquid mixtures, and adsorption and separation of quantum isotope mixtures in carbon nanotubes. New algorithms using path integral technique are presented for applications to quantum systems.; We have simulated volumetric properties of liquid mixtures of neon and hydrogen by combining techniques of classical simulations with the path integral formalism for quantum fluids. We obtain good agreement between simulations and experiments by using realistic interatomic potentials. Calculation of excess molar volumes in these mixtures shows large positive deviations from ideal mixing. However, contrary to the existing understanding that large positive deviations from ideal mixtures are caused due to quantum effects in Ne-H ₂ mixtures, both classical as well as quantum simulations predict similar large positive deviations from ideal mixtures. Several potentials for Ne-H ₂ cross interaction have been evaluated. The non-idealities in volumetric properties of Ne-H₂ mixtures are concluded to be due to the cross interaction potential. Similar calculations have been done for mixtures of H₂-D₂, H₂-T₂, and H₂-HD. Calculations show very small deviations from ideal mixing. Mixture vapor-liquid phase diagrams of H₂-D₂ have been computed and compared with experiments. Calculation of activity coefficients indicates small positive deviations from ideal phase behavior.; We have presented the first detailed predictions of quantum sieving based on realistic models of microporous materials. Carbon nanotubes, and the interstices of nanotube bundles, can act as highly effective quantum sieves for hydrogen isotopes. We have evaluated several carbon nanotubes and interstices of nanotubes to separate mixtures of light isotope species. Good agreement is found between analytical predictions and detailed path integral calculations. We have also simulated adsorption of hydrogen isotopes in carbon nanotubes and interstices. Pure fluid isotherms, mixture isotherms, selectivities, and density profiles have been evaluated. Good agreement is found between path integral simulations and theories based on adsorption at low to moderate coverages. High selectivities are obtained at high coverages for mixtures of H₂-T₂ and H₂-D₂.