Macroscopic Contact Angle Masks Molecular Complexity at Solid/Liquid Interfaces

Water at interfaces governs essential phenomena spanning protein folding, heterogeneous catalysis, nanofluidics, and materials synthesis. Yet even after extensive research, separating the pure influence of interfacial interaction strength from complicating factors like surface roughness, chemical heterogeneity, and specific bonding geometries continues to pose significant difficulties. This fundamental limitation impedes our capacity to connect molecular organization with macroscopic wetting behavior, constraining rational design approaches for technological surfaces. We tackle this problem using molecular dynamics simulations on ideally flat surfaces, which allows systematic exploration of surface hydrophilicity spanning a continuous range free from geometric or chemical complexity. Our findings identify a fundamental transition in interfacial water behavior that emerges through thermodynamic properties rather than structural features. Crucially, cavity free energy calculations at molecular dimensions reveal pronounced differences between surfaces that are indistinguishable at macroscopic scales, showing that hydrophilicity is scale dependent and that contact angle measurements alone fail to capture the full interfacial picture.