Phase transitions in nanoconfined fluids have been contentious for two decades. In the 1980s and early 1990s a large number of surface force apparatus (SFA) experiments on a variety of ultrathin nonpolar liquid films (e.g., such as dodecane, cyclohexane and octamethylcyclotetrasiloxane (OMCTS)), reached a common conclusion: When their confinement between molecularly smooth mica sheets reached the order of several molecular diameters (approximately 3 or less, depending on the fluid being studied) they exhibited behavior typical of the stick-slip response of a crystalline solid structure.

In contrast to the solid-like behavior under extreme nanoconfinement, when the mica surface separation is sufficiently large, the confined fluid exhibits bulk-like liquid behavior. Thus, a phase transition as a function of separation must exist. In this talk, we review the two-decade-old debate on the nature of this phase transition (first order vs continuous), and its effective resolution using very high fidelity molecular dynamics simulations. In particular, the origin of the phase transition from fluid to solid-like behavior is, unexpectedly, driven by electrostatic interactions between ions in mica and partial charges on the atoms in the nonpolar organic molecules.

More recently, our interest in nanoconfined fluids has focused on novel energy storage devices: electrical double layer (EDL) capacitors, also called supercapacitors. Supercapacitors have attracted considerable attention, owing to their desirable properties, such as high power density, high capacitance, and excellent durability. As emerging electrolytes for these supercapacitors, room-temperature ionic liquids (RTILs) have attracted considerable attention due to their wide electrochemical windows, excellent thermal stability, non-volatility, relatively inert nature, and high ionic conductivity. With high specific surface area and electrical conductivity, nanoporous carbon-based materials are the most widely used electrodes for supercapacitors, including activated carbons, templated and carbide-derived carbons (CDC). Using molecular simulations, model porous carbon electrodes (e.g., CDC), supercapacitors composed of slit-shaped micropores ranging in size from 0.67 nm to 1.8 nm in an IL were studied to investigate the dependence of capacitance on pore size. The capacitance was found to show an oscillatory behavior with pore size. In good agreement with experiment, we find that, as the pore shrinks from 1.0 nm to 0.7 nm, the capacitance of the micropore increases anomalously. The persistence of oscillations in capacitance beyond 1.0 nm is a new theoretical prediction currently being probed experimentally.