Date of Award

2004

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Jayendran C. Rasaiah

Second Committee Member

R. Dean Astumian

Third Committee Member

Raymond C. Fort

Abstract

Liquids in narrowly confined spaces have properties that are very different from those of the bulk. In this thesis three different systems with semi-confined water have been studies using computer simulations. The first is a molecular dynamics study of liquid water between parallel plates subjected to an external electric field. Unlike earlier simulations, this system is open to a heat and particle bath. A uniform electric field between the plates is seen to deplete the water density in this region, in contrast to predication of earlier studies. The second study is a Monte Carlo simulation of water inside a carbon nanotube. We study the electric field dependence of the equilibrium between the empty and filled states of the nanotube. We also investigate the electric field dependence of the equilibrium between the "up" and "down" states of the filled tube that correspond to two orientations of the total dipole moment of the water chain along the tube axis. A two-state statistical mechanics model shows very good agreement with simulations for the equilibrium between the filled and empty states and also for the relative probabilities for the "up" and "down" states as a function of the electric field. By studying the temperature dependence of the free energy of occupancy, we evaluate the energy and entropy of the water inside the tube relative to bulk water. The energy of transfer is found to be sensitive to the water-tube interaction potential; the entropy of one-dimensionally ordered water chains is comparable to that of bulk water. Finally we apply these Monte Carlo methods to water occupancy of idealized hydrophobic cavities and calculate the thermodynamics and structure of water in these cavities. It is found that the filling of non-polar cavities with water is thermo-dynamically favorable under conditions which are critically dependent on the cavity size and the interactions with the cavity walls. This interior water is in thermal and chemical equilibrium with bulk water, but the extensively hydrogen-bonded water molecules in the cavities have structures very similar to those observed and studied spectroscopically by Saykally and his group [Keutsch and Saykally, 2001] in the gas phase.

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