Entropic-dielectric interplay governs ion adsorption in inner electric double layers.

Ion-specific effects at aqueous interfaces are fundamental to chemistry, biology, and environmental science, yet unraveling their thermodynamic origins remains challenging. Here, we quantitatively resolve interfacial thermodynamics at molecular resolution by combining concentration-dependent adsorption isotherms obtained from atomic force microscopy imaging, molecular dynamics simulations, and sum-frequency generation spectroscopy. We show that strongly hydrated ions disrupt and compact interfacial water structure, leading to entropic penalties and increased dielectric screening that weakens their interaction with the surface. Conversely, weakly hydrated ions incur lower entropy costs and substantially decrease screening via water depletion, promoting inner-sphere overadsorption on a charged surface. This balance between entropic penalties and screening governs inner double-layer formation. Our findings establish a thermodynamically grounded molecular framework for ion-specific adsorption, quantitatively linking interfacial entropy, water structure, and electrostatics. This approach generalizes the Hofmeister concept and provides a foundational approach for understanding interfacial structure in complex electrolytes.