| dc.description.abstract | Graphene oxide (GO) is a chemically functionalized graphene with various oxygen-containing functional groups, such as epoxy (-O-), hydroxyl (-OH), and carboxyl (-COOH) on the basal plane. As a consequence, GOs have hydrophilic and hydrophobic regions at the molecular level. GO membranes are found to be a promising potential material for water filtration and desalination, gas separations, and fuel cell applications. The GO structure is non-unique and complex in nature due to variations in the extent of the oxidized regions. In this thesis, we use molecular simulations to understand the structure and dynamics of water confined in GO nanopores and on GO surfaces, as well as the manner in which the extent of hydrophobicity modulates these properties.
In the first part of this thesis, we model the GO surface to represent various scenarios. Here we study the adsorption of water on a GO surface as a function of varying levels of hydrophobicity using grand canonical Monte Carlo (GCMC) simulations. GCMC simulations reveal interesting water film growth as the vapor pressure is increased. Water is found to adsorb at the hydrophilic - hydrophobic interface at low pressures. Subsequently, a water bridge spanning the hydrophilic region is observed, and large number fluctuations are observed due to the Janus nature of the interface. The extent of hydrophobicity determines water layering, organization, and the adsorption isotherms.
We investigate the influence of surface chemistry on the dynamical transitions of supercooled interfacial water on the GO surface. Using the TIP4P/2005 water model, molecular dynamics (MD) simulations reveal that the rotational relaxation of bound water can undergo either a strong-to-strong or a single Arrhenius behaviour as a function of the surface hydrophilicity. This is in contrast to bulk water, which exhibits a fragile-to-strong transition upon supercooling. Molecular dynamics simulations on surface water where the influence of bulk-like water is absent, revealed a single Arrhenius behaviour during supercooling. Our results provide novel insights into the strong role played by the presence of bulk water as well as the influence of surface chemistry on the dynamical transitions of interfacial water.
In order to understand the influence of surface oxidation and the inherent heterogeneity imposed by opposing surfaces formed in macroscopic membranes, MD simulations of water confined in nanopores (8 - 15 Angstrom) made up of different surface types are carried out . The greatest differences are observed at 8 Angstrom, which is the optimal separation distance for molecular sieving of ions. The dipole-dipole relaxation and HH rotational relaxation of confined water are the slowest between fully oxidized (OO) surfaces with a two-order decrease in the dipole-dipole relaxation time observed for the Janus confinement consisting of an oxidized surface adjacent to a graphene surface. Although the water diffusivity is an order of magnitude smaller than bulk diffusivities at the smaller surface separations, water between the Janus surfaces always had the highest diffusivities. Thus the Janus interface appears to provide the optimal environment for water transport, providing a design strategy while assembling GO-based membrane for water purification.
Molecular dynamics simulations have been carried out to explore the dynamical crossover phenomenon in strongly confined and layered water in GO nanopores. In contrast to studies where confinement is used to study the properties of bulk water, we are interested in the dynamical transitions for strongly confined water in the absence of any bulk-like water. Graphene oxide surfaces having different degrees of hydrophilicity are placed at interlayer separation of d = 10 Angstrom, to produce an in-registry pore (IR) and a fully hydrophilic (OO) nanopore. Water confined in the IR pores exhibits a strong-to-strong dynamical transition in the diffusion coefficient and rotational relaxation time at 237 K and shows a fragile-to-strong transition in the alpha-relaxation at 238 K. In contrast, water confined in the OO pores did not display a dynamical crossover in any of the dynamical quantities studied. Our results indicate that water under strong confinement can undergo a dynamic transition, which is a strong function of the physicochemical nature of the confining surface.
In the last part of this thesis, we have studied the adsorption of pure CH4, CO2, N2, and H2 gases in GO nanopores of d = 10 Angstrom using grand canonical Monte Carlo (GCMC) simulations and have also evaluated adsorption selectivity for equimolar gas mixtures of CH4/CO2, CO2/N2, CO2/H2, and CH4/H2 in GO nanopores of d = 10 Angstrom at 298 K. Adsorption isotherm and isosteric heats of adsorption reveal that CO2 adsorption is highest, and adsorption of H2 is lowest. Adsorption selectivity of CO2 in all gas mixtures for the GO pores is highest, specifically in the CO2/H2 mixture, and in the case of the CH4/H2 mixture, CH4 selectivity is higher. These findings suggest that GO nanopores have the potential to be used as adsorbents for the CO2 capture from flue gas and natural gas streams. | en_US |
