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Date of Award
Physics and Astronomy
Peter M. Hoffmann
Being hydrophilic, graphene oxide (GO) confines water between its layers and allows differential diffusion. When water is confined on a nanometer scale, it exhibits several unusual behaviors, including ordering, viscoelasticity, and long relaxation times. The structure and dynamics of confined water depend not only on the structure of the confining substrate, but also on the metal ions confined together with water molecules. We utilized the momentum transfer (Q)-dependence of Quasi-Elastic Neutron Scattering (QENS) to reveal the dynamics of confined liquids (water and ethanol) in pristine and metal ions intercalated GO powder and membranes at different temperatures and different orientations. The dynamics were measured across different length and time scales using several spectrometers. We also used the Atomic Force Microscopy (AFM) technique to probe the viscosity of the water confined in GO and spherical, flat cantilever tip. We found reduced diffusivities (up to 30 % in the case of water) and a depression of the transition temperatures in pristine GO. At the same time, the viscosity of nanoconfined water reduced up to 50% near the GO wall compared to its bulk value. Water showed near Arrhenius behavior with an almost bulk-like activation barrier in a temperature range of 280-310, the diffusivity of ethanol showed little temperature dependence. We found evidence for immobile and mobile fractions of the confined liquids in both pristine and ions intercalated GO for both water and ethanol. The mobile fraction exhibited jump-diffusion, with a jump length consistent with the expected average spacing of hydroxide groups in the GO surfaces. We found enhanced diffusivities and mean square displacements of water in the presence of structure breaker ions, chaotropes, (Cs+ and K+) and reduced diffusivities in the presence of structure maker ion, kosmotrope, (Na+) from picoseconds to nanoseconds time interval. On the other hand, relaxation times of water are diminished in the presence of kosmotrope and enhanced in the presence of chaotropes in both picoseconds and nanoseconds time scales. We also found the depression of the transition temperature of nanoconfined water at GO in the presence of chaotropes and enhanced transition temperature in the presence of kosmotrope compared to the water at pristine GO. From anisotropy measurements, we found weak anisotropy in the diffusion of water and ethanol, with the surprising result that diffusion was faster perpendicular to the membrane than parallel to it.
Acharya, Gobin, "Dynamics Of Confined Liquids At Multiple Time Scales" (2022). Wayne State University Dissertations. 3613.