Anisotropy and Hybrid Heterosurface-Modulated Two-Dimensional Hydrogen Bond Network of Water

We report the structure and dynamics of layered water structure near a bilayer heterosurface using classical molecular dynamics simulations and network graph theory. The heterosurfaces of our choice are technologically essential and well-studied hexagonal boron nitride (h-BN) and graphitic carbon nitride (g-C3N4). Starting with the water density distribution profile, we have studied different hydrogen bondings (HBs) associated with distance and angle distributions and correlated them with the HB count, followed by the investigations of dynamic properties. We realize the oddity while corresponding to the tetrahedral order parameter (TOP), HB donor/acceptor count, and HB lifetime. A quantitative gap is observed between the TOP and HB counts, and we perceive another structural factor playing a significant role. We observe a 2D sheetlike arrangement of hydrogen-bonded water molecules, namely, 2D-HB, in the interfacial layer instead of 3D tetrahedral arrangements. 2D-HB qualitatively acts as a key descriptor of the hydrophobicity of the interface. The probability of higher TOP among the interfacial layer water molecules is less for h-BN compared to g-C3N4. However, the per-water HBs for both the heterosurfaces remain indistinguishable, implying the existence of another significant physical factor related to HB. Hence, 2D-HB serves as a deciding factor. In the case of h-BN, water molecules in the first layer form extended 2D sheets compared to the second and bulk, which is not a scenario for g-C3N4. Following the network graph theory, we calculated the temporal evolution of the maximum number of water molecules connected in a single network. The layer 1 water molecules have slower orientational dynamics, higher HB lifetime, and consequently slower translational diffusion than layer 2 and bulk water molecules. The experimental studies suggested that the contact angle of water on h-BN can vary from 61.4 to 85.8°, higher than that of g-C3N4, depending on the surface stacking, support material, and exposure to air, which effectively makes h-BN more hydrophobic compared to g-C3N4. The interfacial water molecules have a smaller HB lifetime and a faster diffusion coefficient than g-C3N4. The HB lifetime of water molecules in bulk is less than that in the interface, irrespective of the surface. A slower reorientation lifetime, that is, 2.69 ps, for g-C3N4 interfacial water molecules is observed versus 2.20 ps for h-BN. Our results agree with the experimental observations and can help to understand the hydrophobicity of other relevant materials