(512b) Relaxation Processes and Dynamics of Ionic Liquids in Nanoconfined Geometries

The Ionic liquids (ILs) are composed solely of ions that are liquids under ambient conditions. While phenomenological theories as well as advanced experimental observations are made in the aspects of confined and molecularly layered ILs, capturing the nature of the transitions from continuum to molecular behaviors in ILs thin films at different degrees of confinement has not been much successful.

Here, we present normal force and rheological measurements of 1-octyl-3-methylimidazolium tetrafluoroborate, [C8mim⁺][BF4^-], confined in gaps comparable to their molecular dimensions. Forces between two negatively charged mica as well as positively charged alumina surfaces across ILs were successfully captured, displaying distinctive features at different degrees of nanoconfinement, i.e. monotonic repulsive force with characteristic decay length in the long-range distance regime and oscillatory structural force with alternating attractive energy minima and repulsive energy maxima in the short-range distance regime. Dynamic rheological properties such as viscosity and storage/loss moduli were characterized in the short-range as well as long-range distance regimes (for the first time to our knowledge) in comparison with bulk properties thereof. The breakdown of continuum behavior was further evidenced in aids of dynamic scanning calorimetry as well as nuclear magnetic resonance which allowed us to quantitatively identify changes in thermal, dynamic as well as relaxational properties of ILs confined in alumina porous membranes of which diameters varied from ~15 nm all the way up to ~ 140 nm. Our findings obtained from these unique combination of experimental approaches lead to new fundamental insights on the drastic changes of physicochemical properties of confined ILs, delivering crucial perspectives for the applications where the space restriction is present such as supercapacitor, lithium batteries, and active lubrication.