Wetting as an Emergent Property of Water: Reformulating Young's Equation on Molecular Grounds

Young's equation provides a remarkably successful macroscopic description of wetting, yet its molecular origin─particularly for water─has remained elusive for over two centuries. Here, we make this molecular basis explicit by establishing a thermodynamic bridge that connects interfacial free energies, density fluctuations, and hydrogen-bond energetics. This leads to a molecular wetting coefficient, ω_m, that quantifies how an interface compensates the intrinsic energetic cost of hydrogen-bond defects relative to bulk water. Specifically, ω_m measures the water-stabilization ability of the surface─the interaction energy experienced by hydration water at the interface─and compares this interaction with water's intrinsic energetic scale (the defect interaction threshold, DIT), thus defining when wetting is favorable or not. Across a broad and continuous spectrum of hydrophilicities, spanning chemically diverse experimental and model surfaces, we show that macroscopic contact angles collapse onto a single universal master curve when expressed through ω_m, reflecting a common underlying free-energy balance. We further show that this collapse reflects a unified thermodynamic framework connecting interfacial free energies, density fluctuations, and molecular energetics across scales. Thus, wetting arises from the interplay between surface chemistry and water: while the surface determines the interaction energy experienced by interfacial molecules, the liquid sets the criterion separating wetting from nonwetting behavior by imposing a threshold that emerges from the intrinsic energetic scales of its hydrogen-bond network. Furthermore, this molecular reformulation closes Young's and Young-Dupré relations on energetic grounds, establishing a unified and predictive physical link between wetting, adhesion, cavitation, and nanoconfined filling. By anchoring interfacial behavior to water's intrinsic hydrogen-bond energetic scales, this work provides a transferable molecular framework that recalibrates energetic intuition and guides the rational design of aqueous interfaces.