Molecular Structure and Dynamics in Thin Water Films at the Silica and Graphite Surfaces

Abstract

The structure and dynamic properties of interfacial water at the graphite and silica solid surfaces were investigated using molecular dynamics simulations. The effect of surface properties on the characteristics of interfacial water was quantified by computing density profiles, radial distribution functions, surface density distributions, orientation order parameters, and residence and reorientation correlation functions. In brief, our results show that the surface roughness, chemical heterogeneity, and surface heterogeneous charge distribution affect the structural and dynamic properties of the interfacial water molecules, as well as their rate of exchange with bulk water. Most importantly, our results indicate the formation of two distinct water layers at the SiO2 surface covered by a large density of hydroxyl groups. Further analysis of the data suggests a highly confined first layer where the water molecules assume preferential hydrogen-down orientation and a second layer whose behavior and characteristics are highly dependent on those of the first layer through a well-organized hydrogen bond network. The results suggest that water−water interactions, in particular hydrogen bonds, may be largely responsible for macroscopic interfacial properties such as adsorption and contact angle.

Monte Carlo Study on the Water Meniscus Condensation and Capillary Force in Atomic Force Microscopy

“…Therefore, a molecule in the first layer is believed to prefer the orientation where hydrogen atoms are closer to the surface. This preference for the hydrogen-down orientation of water molecules has been found in MD simulations of a carbon nanotube tip immersed in water, 55 of a water film on a silica surface, 57 and of water confined in nanoporous silica. 58 This was attributed to the hydrogen-down configuration being energetically more stable than any other orientation of molecule.…”

N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

“…As a proxy for materials often found in the sub-surface, the solid substrate considered here was modelled as β-cristobalite (1 1 1), whose surface non-bridging oxygen atoms were fully protonated, as described elsewhere. 12 Because quartz (made up by SiO4 tetrahedral structure) is an abundant mineral in earth, the cristobalite crystal with fully protonated non-bridging oxygen atoms is considered a reasonable proxy for hydrophilic rock pore surfaces. 13 Approximately, 1 n-octane molecule is 12.8 Å in length and 1 carbon dioxide is 5.4 Å.…”

Rapid Transport of Water via a Carbon Nanotube Syringe

“…Similar result were found in molecular simulations of liquid water near graphite and HS surfaces. 31 To determine if molecules can be more rapidly transferred across the CNM with the hydrophilic surface, we test the dependence of J on V w (Figure 5a). Curiously, the maximum V w we can achieve using the HS wall without breaking the water film in the chamber is identical to the maximum V w for the hydrophobic graphene wall.…”