In Situ Characterization of the Hydration Structure at the Silica-Water Interface

The structure of interfacial water at silica-water interfaces plays a key role in surface forces, colloidal stability, and interfacial transport processes. However, a quantitative understanding of how interfacial forces govern hydration-layer formation remains incomplete, particularly under varying electrolyte conditions. In this work, colloidal-probe atomic force microscopy (AFM) and three-dimensional AFM are combined to simultaneously characterize interfacial forces and hydration structures on silica surfaces in pure water and NaCl solutions. In pure water, the interface exhibits a bilayer hydration structure with relatively large layer spacing, whereas in electrolyte solutions, a transition to a more compact trilayer structure is observed. Force measurements reveal that the effective interaction evolves nonmonotonically with increasing ionic strength, reflecting the combined contributions of electrostatic double-layer forces, van der Waals interactions, and short-range hydration forces. Beyond identifying structural transitions, the key novelty of this work lies in establishing a quantitative correlation between real-space hydration-structure evolution and the synchronous, nonmonotonic variation of net interfacial forces on realistic oxide surfaces. By correlating force-distance profiles with hydration-layer thickness, we show that repulsive interactions are associated with expanded and loosely structured hydration layers, while attractive interactions promote confinement and reduced layer thickness. The results further indicate that surface silanol groups play a dominant role in stabilizing compact hydration structures across a wide range of ionic strengths. These findings provide a quantitative link between interfacial forces and hydration-layer organization at silica-water interfaces and contribute to a better understanding of hydration forces in aqueous systems.