Phase Diagrams of Water Confined by Graphene and Graphene Oxide

Abstract

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and water filtration and desalination processes. Remarkably, nanoscale confinement can drastically alter the properties of water. Understanding these changes in the physical behavior of water can provide new insights into many scientific questions and technical challenges. This thesis focuses on phase diagrams of water confined by graphene and graphene oxide. First, by performing Molecular Dynamic (MD) simulations, we constructed phase diagrams of water confined by graphene, a hydrophobic smooth surface. We found that the phase behaviors of water confined by graphene are complicated. In the phase diagram, monolayer square ice, bilayer square ice, liquid and vapor phases were presented. The non-monotonic cavitation pressures as a function of walls separations was unexpected. The values of cavitation pressures significantly deviated from the classical prediction for bulk water. Next, I moved to water under hydrophilic confinements. The first model used was a hydrophilic graphene-based surface where graphene C-water O interactions were tuned to create a hydrophilic surface but maintaining the geometry of the graphene. The phase diagram of water confined by hydrophilic graphene is presented. The extremely high magnitude of cavitation pressures found in this analysis suggests that energy can be converted efficiently from changes in relative humidity. Furthermore, the oscillation of cavitation pressures as a function of walls separations is relevant to water transportation. By randomly distributing hydroxyl groups on graphene, we saw similar cavitation pressures in a graphene oxide (GO) model.