Fundamental aspects of electric double layer force-distance measurements at liquid-solid interfaces using atomic...

Area of Science:

• Surface Science
• Physical Chemistry
• Nanotechnology

Background:

• Ionic liquids (ILs) exhibit complex layered ion structures at interfaces.
• Understanding these structures is crucial for applications in nanotechnology and materials science.
• Atomic force microscopy (AFM) is a key technique for probing nanoscale phenomena.

Purpose of the Study:

• To investigate the layered ion structure of ILs at a mica surface using AFM force-distance measurements.
• To determine the influence of AFM tip properties on measured force profiles.
• To compare experimental AFM data with molecular dynamics (MD) simulations of interfacial ion density.

Main Methods:

• Utilized atomic force microscopy (AFM) to perform force-distance measurements on ILs adsorbed onto a mica surface.
• Systematically varied AFM tip properties (e.g., radius) to assess their impact on force profiles.
• Employed molecular dynamics (MD) simulations to predict interfacial ion density profiles for comparison.

Main Results:

• AFM-measured ion positions were independent of tip properties, but tip radius influenced breakthrough and adhesion forces.
• Force measurements showed sensitivity to the ion with larger volume and mass, indicating excluded volume effects dominate selectivity.
• Electroneutrality was maintained above 1 nm, while van der Waals forces dominated at shorter distances.

Conclusions:

• AFM force-distance measurements can effectively probe the layered ion structure of ILs.
• Ion selectivity in AFM measurements is primarily governed by excluded volume effects, not electrostatic or chemical interactions.
• The study provides insights into interfacial charge distribution and forces governing IL behavior at surfaces.