From Individual Structure to Collective Topology: Cation Effects on the Hierarchical Organization of Interfacial Hydrogen Bond Networks

How electrolyte cations reshape the collective organization of interfacial water remains difficult to establish from local molecular descriptors. Here, we develop a graph-based hierarchical framework spanning molecular structure, network connectivity, and higher-order ring architecture, and apply it to molecular dynamics simulations of charged water/Pt(111) interfaces containing K + , Li + , and Ba 2+ . Molecular orientations respond primarily to surface charge, while molecular hydrogen bond statistics provide limited discrimination between Li + and Ba 2+ . Collective descriptors reveal additional cation effects: hydrogen bond networks containing Ba 2+ are more vulnerable than those containing Li + to fragmentation upon removal of interfacial water molecules. Despite reduced connectivity with increasing surface charge magnitude and ion adsorption, more than 85% of interfacial water molecules remain in the largest connected component on average under all conditions studied. Four-and five-member rings dominate the interfacial networks, whereas increasing surface charge magnitude reduces the population of large rings and inter-ring connectivity. Connections involving large rings become less frequent from K + to Li + to Ba 2+ , revealing ion-dependent differences in ring organization. Larger rings preferentially involve water molecules with fewer hydrogen bonds, connecting higher-order architecture to local molecular environments.