Influence of the extent of hydrophobicity on water organization and dynamics on 2D graphene oxide surfaces.

Graphene oxide (GO) nanomaterials are being extensively explored for a wide spectrum of applications, ranging from water desalination to fuel cell applications, due to their tunable mechanical, thermal, and electrical properties. In this paper, we have investigated the influence of the hydrophobic extent on the adsorption of water on 2D GO surfaces by performing a series of grand canonical Monte Carlo simulations at various relative pressures, P/P0, at 298 K and discuss the implications of our findings on proton transport characteristics. HR is defined as the ratio of the hydrophobic to hydrophilic areas on the GO surface. The structure of adsorbed water is studied by analyzing density distributions and hydrogen bonds. At moderate relative pressures of P/P0 < 0.6, a monolayer of adsorbed water, spanning the hydrophilic and hydrophobic regions of the GO surface, is observed for HR = 0, 0.5 and 1, and at higher pressures, a percolating hydrogen-bonded network is formed, which results in the formation of a thick water film. At intermediate water pressures, bridging water networks form across the hydrophobic regions. The GO surface of HR = 1 is seen to have a strong signature of a Janus surface, displaying increased fluctuations in adsorbed water molecules and hydrogen bonds. Our results suggest that if there is sufficient hydrophilicity on the GO surface, a relative humidity between 70 and 80% results in the formation of a fully formed contact water layer hydrogen-bonded with the surface functional groups along with a second layer of adsorbed water molecules. This coincides with hydration levels at which a maximum in the proton conductivity has been reported on 2D GO surfaces. Molecular dynamics simulations reveal a higher reorientational relaxation time at lower water hydration and the rotational entropy of interfacial water at lower hydration is higher than that of bulk water, indicating broader rotational phase space sampling.

Influence of surface hydrophilicity and hydration on the rotational relaxation of supercooled water on graphene oxide surfaces.

Hydration or interfacial water present in biomolecules and inorganic solids has been shown to exhibit a dynamical transition upon supercooling. However, understanding the extent of the underlying surface hydrophilicity as well as the local distribution of hydrophilic/hydrophobic patches on the dynamical transition is unexplored. Here, we use molecular dynamics simulations with a TIP4P/2005 water model to study the translational and rotational relaxation dynamics of interfacial water on graphene surfaces. The purpose of this study is to investigate the influence of both the surface chemistry and the extent of hydration on the rotational transitions of interfacial water on graphene oxide (GO) surfaces in the deeply supercooled region. We have considered three graphene-based surfaces: a GO surface with equal proportions of oxidized and pristine graphene regions in a striped topology, a fully oxidized surface and a pristine graphene surface. The dipole relaxation time of interfacial water (high hydration) shows a strong-to-strong transition, strong nature, and strong-to-strong transition on these surfaces, respectively, in the temperature range of 210-298 K. In contrast, bulk water shows a fragile-to-strong rotational transition upon supercooling. In all these cases at high hydration, interfacial water co-exists with a thick water film with bulk-like properties. To investigate the influence of bulk water on dynamical transitions, we simulated a low hydration regime where only bound water (surface water) is present on the GO surfaces and found that the rotational relaxation of surface water on both the GO and fully oxidized surfaces shows a single Arrhenius temperature dependence. Bulk water is found to have a greater influence on the rotational relaxation in the presence of a hydrophobic surface and the dipole angular distributions show distinct differences on the surfaces upon supercooling. Our results indicate that not only does the local extent of surface hydrophilicity play a role in determining the energy landscape explored by the water molecules upon supercooling, but the presence of bulk water also modulates the dynamic transition.

Dynamical Transitions of Supercooled Water in Graphene Oxide Nanopores: Influence of Surface Hydrophilicity.

Molecular dynamics simulations are carried out to explore the dynamical crossover phenomenon in strongly confined and mildly supercooled water in graphene oxide nanopores. Since the extent of hydrophilicity can be varied on graphene oxide surfaces, they offer energetically heterogeneous environments that can potentially modulate the rotational and translational relaxation dynamics of confined water. The influence of the physicochemical nature of the graphene oxide surface on the dynamical transitions is investigated by varying the extent of hydrophobicity on the confining surfaces placed at an intersurface separation of 10 Å. Water forms two distinct layers in contact with the graphene oxide surface at this separation. All dynamical quantities show a typical slowing down as the temperature is lowered from 298 to 200 K; however, the nature of the transition is a distinct function of the surface type. Water confined between surfaces consisting of alternating hydrophilic and hydrophobic regions exhibit a strong-to-strong dynamical transition in the diffusion coefficients and rotational relaxation times at a crossover temperature of 237 K and show a fragile-to-strong transition in the α-relaxation time at 238 K. The observed crossover temperature is higher than the freezing point of the SPC/E water model used in this study, indicating that these dynamical transitions can occur with mild supercooling under strong confinement in the absence of bulk-like water. In contrast, water confined in a hydrophilic nanopore shows a single Arrhenius energy barrier over the entire temperature range. Our results indicate that in addition to confinement, the nature of the surface can play a critical role in determining the dynamical transitions for water upon supercooling.

Enhancing the Dynamics of Water Confined between Graphene Oxide Surfaces with Janus Interfaces: A Molecular Dynamics Study.

Graphene oxide membranes have been widely studied for their potential applications in water desalination applications. To understand the influence of surface oxidation and the inherent heterogeneity imposed by opposing surfaces formed in macroscopic membranes, molecular dynamics simulations of water confined in nanopores (8-15 Å) made up of different surface types are carried out. The greatest differences are observed at 8 Å, 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 2 order decrease in the dipole-dipole relaxation time observed for the Janus confinement consisting of an oxidized surface adjacent to a graphene surface. The translational and rotational density of states show distinct blue shifts and red shifts, respectively, at the smaller separations, with the extent of the shifts dependent on the surface type. Self-intermediate scattering functions show a pronounced plateau region for the OO surfaces at 8 Å, suggestive of glasslike dynamics, and extended α-relaxations were observed for the other surfaces. Although 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. The free energy to transfer a water molecule from bulk water was found to be the smallest (∼4 kJ/mol) for the Janus surfaces, which have the lowest number of hydrophilic groups among the different systems studied. Thus, the Janus interface appears to provide the optimal environment for water transport, providing a design strategy while assembling graphene oxide-based membrane stacks for water purification.