(474h) Assembly and Dynamics of Nanoconfined Ionic Liquids – Effect of Solid Surface Chemistry

Ionic liquids (ILs) have demonstrated significant potentials in fields such as energy storage, lubrication, catalysis, etc., many of which involve ILs in confined geometries such as nanopores and slits. ILs under solid confinement can behave differently due to the additional IL-surface interactions and the limitations in space, which lead to deviated assembly and dynamic behaviors from bulk. While IL bulk behaviors are relatively well-known, IL behaviors under confinements are less so.

Here we present a series of force profiles and rheological measurements of two types of ILs [C₄mim⁺][BF₄^-] and [C₈mim⁺][BF₄^-], confined between opposing two solid surfaces. Three types of solid surfaces were selected as confining boundaries: pristine mica surfaces (hydrophilic, negatively charged), PDMS surfaces (hydrophobic, negatively charged) and octadecyltriethoxysilane (OTE) covered mica surfaces (hydrophobic, non-charging). Long range, monotonic, exponentially decaying repulsion, followed by short range oscillatory features were observed in the force profiles, of which magnitude and range varied with surface chemistry. From rheological measurement, slipping planes were identified along with effective viscosity and shear moduli under different degrees of confinement. The slipping planes were found to extend from solid surfaces by odd number of layers, among which mica surfaces demonstrated slipping plane extending the farthest while hydrophobic surfaces shorter due to weaker adsorption. Effective viscosities were found to deviate from bulk measurements at separation distances roughly correlating with decay length of long-range repulsion. A transition from liquid to solid-like behavior was observed in oscillatory region, marked by the substantially higher storage modulus comparing with loss modulus.

We anticipate these findings ultimately serve as new fundamental insights for rational design of ILs with desired interfacial properties in use of fabricating superior functional materials and devices.