Evolution of Interfacial Hydration Structure Induced by Ion Condensation and Correlation Effects

Interfacial hydration structures are crucial in wide-ranging applications, including battery, colloid, lubrication. Multivalent ions like Mg²⁺ and La³⁺ show irreplaceable roles in these applications, which are hypothesized due to their unique interfacial hydration structures. However, this hypothesis lacks experimental supports. Here, we provide the first observation for their interfacial hydration structures with molecular resolution using atomic force microscopy. We observed the evolution of layered hydration structures at La(NO₃)₃ solution-mica interfaces. As concentration increases from 25 mM to 2 M, the layer number varies from 2 to 1 and back to 2, and the interlayer thickness rises from 0.25±0.05 to 0.34±0.03 nm, with hydration force increasing from 0.27±0.07 to 1.04±0.24 nN. Theory and molecular simulation reveal that the cations form inner-sphere complexes. Multivalence induces concentration-dependent ion condensation and correlation effects, resulting in compositional and structural evolution within interfacial hydration structures. Additional experiments at seven different solid-liquid interfaces together with literature comparison confirm the universality of this mechanism for both multivalent and monovalent ions. New factors affecting interfacial hydration structures are revealed, including concentration and solvent dielectric constant. This insight provides guidance for designing interfacial hydration structures to optimize solid-liquid-interphase for battery life extension, modulate colloid stability and develop efficient lubricants.