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

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• Interfacial hydration structures are critical for applications like batteries, colloids, and lubrication.
• Multivalent ions (e.g., Mg2+, La3+) are hypothesized to have unique interfacial roles due to their hydration structures, but experimental evidence is lacking.

Purpose of the Study:

• To experimentally observe and characterize the interfacial hydration structures of multivalent ions at the molecular level.
• To elucidate the influence of ion concentration and other factors on these hydration structures.
• To provide a mechanistic understanding for optimizing interfacial properties in various applications.

Main Methods:

• Utilized atomic force microscopy (AFM) for molecular-resolution imaging of interfacial hydration structures.
• Employed theoretical calculations and molecular simulations to interpret experimental observations.
• Conducted experiments across diverse solid-liquid interfaces and compared with existing literature.

Main Results:

• First direct observation of layered hydration structures at La(NO3)3 solution-mica interfaces.
• Observed concentration-dependent changes in layer number, interlayer thickness, and hydration force.
• Demonstrated that multivalent cations form inner-sphere complexes, leading to ion condensation and structural evolution.
• Confirmed the universality of the observed mechanism across various interfaces and ion types.

Conclusions:

• Multivalence, concentration, and solvent dielectric constant are key factors governing interfacial hydration structures.
• The findings provide a mechanistic basis for designing and controlling solid-liquid interphases.
• This research offers guidance for enhancing battery performance, colloid stability, and lubricant efficiency.