Wettability of graphite under 2D confinement

TL;DR

This study uses molecular dynamics to explore how water's interfacial tension and wettability on graphite change under nanoconfinement, revealing oscillations and a critical pore size affecting diffusion and structure.

Contribution

It provides new insights into the thermodynamics of water-graphite interfaces under nanoconfinement, highlighting the impact of pore size on wettability and molecular structure.

Findings

Graphite becomes more hydrophobic at nanoscale confinement.

Surface tension oscillates with slit size before stabilizing.

Critical pore size of 0.9nm maximizes surface tension and minimizes diffusion.

Abstract

The thermodynamics of solid/liquid interfaces under nanoconfinement has tremendous implications for liquid transport properties. Here using molecular dynamics, we investigate graphite nanoslits and study how the water/graphite interfacial tension changes with the degree of confinement. We found that, for nanochannel heights between 0.7nm and 2.6nm, graphite becomes more hydrophobic than in bulk, and that the value of the surface tension oscillates before eventually converging towards a constant value for larger slits. The value of the surface tension is correlated with the slip length of the fluid and explained in terms of the effective and interfacial density, hydration pressure and friction coefficient. The study clearly indicates that there is a critical channel height of 0.9nm (achievable experimentally1) at which the surface tension reaches its highest value, but the water diffusion across the channel is at its minimum. The structural analysis shows that for this pore size a transition between a 2D and 3D hydrogen bond network is accompanied by an abrupt increase in conformational entropy. Our results show that the wettability of solid surfaces can change under nanoconfinement and the data can be used to interpret the experimental permeability data.