(607g) Interfacial Dynamics of Ionic Liquids under Nanoconfinement

The Ionic liquids (ILs) are a relatively new class of liquids that have been used or considered for use in various important fields, such as energy storage, lubrication, catalysis, etc. While many of those applications often require the ILs near surfaces/interfaces under confined geometry at nano- and/or micro-length scales, the structure-property relationships of ILs and their interfacial responses under confinement are not well understood. This talk is primarily aimed at addressing this knowledge gap. Here, we present normal force and rheological measurements of two types of ionic liquids, [C4mim⁺][BF4^-] and [C8mim⁺][BF4^-], confined in gaps comparable to their molecular dimensions. Forces between two mica surfaces across ILs showed distinctive differences at different degrees of nanoconfinement. In the long range (~10 nm to 50 nm), monotonic repulsive force showed exponential decay with increasing gap distance. The fitting of Poisson-Boltzmann equation in this region resulted in a shorter Debye screening length for [C4mim⁺][BF4^-] compared to [C8mim⁺][BF4^-], implying the species with smaller cation has a higher degree of effective ion pair dissociation. In the short range (< 10 nm), oscillatory features were observed in the force profiles for both species, indicating strategized assembly. By considering and comparing the dimension of ion species and hard-wall distances in the force profiles, we propose a possible bilayer formation in the more amphiphilic pair, i.e. [C8mim⁺][BF4^-] , which is absent in [C4mim⁺][BF4^-]. Dynamic rheological properties such as viscosity and storage/loss moduli were characterized in the short range (for the first time to our knowledge) with different numbers of ion or ion-pair layers across the interface. Viscosity of both ionic liquid species increased by four orders of magnitude from bulk values as the degree of confinement progressed from the long range into the short range oscillatory region, implying the breakdown of continuum behavior. In the long range (bulk) region, both ionic liquid species demonstrated Newtonian liquid behavior. A transition from liquid to solid-like behavior was observed in the oscillatory region, marked by a substantially higher storage modulus in comparison to the loss modulus. We anticipate the findings obtained from this unique experimental approach ultimately serve as new fundamental insights for rational design of ILs with desired interfacial properties in use of fabricating superior functional materials and devices.