Hydrophilicity as Compensation of Hydrogen-Bond Defects at Graphene Oxide Interfaces

Hydrophilicity is well-defined macroscopically, yet its molecular origin remains elusive. Here, molecular dynamics simulations combined with the V_4S structural index are used to resolve hydrophilicity at the molecular-site level and quantify the spatial extent of hydration perturbations induced by oxygen functional groups at graphene oxide interfaces. We show that epoxy and hydroxyl groups induce distinct hydration regimes: hydroxyl groups form strong hydrogen bonds and stabilize the interfacial network over larger spatial ranges, whereas epoxy groups promote weaker, distorted interactions that disrupt local connectivity. This contrast demonstrates that chemically similar functionalities can generate qualitatively different interfacial responses, highlighting the central role of nanoscale heterogeneity. Within this framework, hydrophobicity emerges as the inability of a surface to compensate for hydrogen-bond defects in its hydration layer, providing a microscopic basis for wettability and guiding the design of carbon-based nanomaterials.