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

Abstract

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. H_R 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 H_R = 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 H_R = 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.