Similar structure but different thermodynamic, dielectric, and frictional properties of confined water in twisted 2D materials: MoS2 vs. graphene

Abstract

Water-based nanofluidic devices, where water is confined in Angstrom scale nanochannels, are widely encountered in nanotechnology. Although it is known that the material of confinement has a significant influence on the properties of confined water, much less is known of the relationship between the structure of nanoconfined water and its properties, impacting the design of nanofluidic devices. We explore the behavior of a confined water monolayer within a bilayer molybdenum disulfide (MoS₂) structure, comparing its behavior with that within bilayer graphene. We find that only ∼2% of the entire structure has nearly perfect square order and the rest is filled with rhombus ordering. Surprisingly, we find that although the structure of monolayer confined water remains the same in both the 2D materials, thermodynamic analysis shows that confined water has a more favorable potential environment in MoS₂ than graphene for all twists explored here. However, with increasing twist angle, the encapsulating effect of water diminishes slightly in the case of graphene than MoS₂. Interestingly, the dielectric constant is anomalously lower in MoS₂ by ∼22% compared to the confined water dielectric constant in a graphene nanochannel. Finally, we show that the static friction coefficient of confined water in bilayer MoS₂ does not change with twist. However, unlike graphene, it does not show an order of magnitude reduction due to this extreme confinement. Overall, we show, counter-intuitively, that although confined water structures are similar in different 2D materials considered here, there exist differences in other properties of this structured water.