(151j) Interfacial Properties of Nanoconfined Ionic Liquids

Ionic liquids (ILs) are a relatively new class of liquids that consist only of cations and anions. Negligibly low vapor pressure, fire resistance, excellent chemical and thermal stability, wide liquid temperature ranges, and wide electrochemical windows are examples of the useful properties of typical ILs. Because of these excellent properties, ILs have been used or considered for use in organic synthesis, catalysis, chemical separation, fuel and solar cells, and their applications continue to expand. In spite of a plethora of emerging processes involving ionic liquids, the understanding of interfacial properties of ILs under confinement such as rheological properties and glass transition temperatures, Tg is still very limited compared to that of other confined material classes such as polymers, electrolyte solutions, and liquid crystals. In this work, we studied the rheological and thermal properties of ILs under nanoconfiment using the surface forces apparatus (SFA) and template-assisted difference scanning calorimetry (DSC). It was found that Newtonian behavior of IL breaks down when the separation between shearing surfaces fall below 15 nm. The ILs with larger molecular size of cation (more number of carbons in cation) showed a higher Tg in bulk. The Tg of the confined ILs was found to be strongly dependent on the pore sizes regardless of types of ILs studied and such a dependence appeared to be relatively more pronounced (e.g. sharper increase in Tg as decreasing a pore size) for the ILs with smaller cation size. These results can be interpreted based on the surface-enhanced molecular organization/ordering under nanoconfinement that requires more thermal energy to induce a liquid state, thereby increasing Tg.