Short-range electrostatic screening in ionic liquids as inferred by direct force measurements

Area of Science:

• Physical Chemistry
• Materials Science
• Surface Science

Background:

• Experimental reports on ionic liquid (IL) interactions show long-range forces, contradicting theoretical predictions.
• Discrepancies exist regarding the experimental range of electrostatic screening in ILs.

Purpose of the Study:

• Investigate discrepancies in literature concerning experimental ranges of electrostatic screening in ILs.
• Clarify the nature of interactions between surfaces confining ILs.

Main Methods:

• Utilized two advanced Surface Force Apparatuses (SFAs) to study mica and borosilicate surfaces confining ILs.
• Employed complementary measurement techniques (stepwise and continuous approach) and varied confinement geometries.
• Performed measurements at ultra-low surface velocities (down to 9 pm/s) with extended equilibration times (up to 90 s).

Main Results:

• Identified two distinct force regimes: oscillatory forces at small separations (IL structuration) and monotonic repulsion at larger separations.
• Demonstrated that force magnitude and spatial extent critically depend on motion conditions and equilibration times.
• Observed that slow surface displacements lead to short-range screening, consistent with Poisson-Boltzmann theory, while fast displacements create an illusion of long-range interactions.

Conclusions:

• Resolved a decade-old controversy regarding force measurements in confined ionic liquids.
• Revealed slow relaxation dynamics and out-of-equilibrium behavior in ILs, akin to aging phenomena.
• Highlighted the critical importance of approaching thermodynamic equilibrium in measurements to accurately determine screening lengths.