How Flexibility of the Nanoscale Solvophobic Confining Material Promotes Capillary Evaporation of Ionic Liquids

Capillary evaporation of ionic liquids in nanoscale confinement is an important process relevant to self-assembly and energy storage applications. Most studies focus on understanding ionic liquids confined by rigid materials, but the flexibility of the materials can also impact the thermodynamics of confinement. In this work, we investigate the effects of flexibility of confining surfaces on the structure and free energy underlying capillary evaporation of the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) in nanoscale solvophobic confinements. We employ indirect umbrella sampling to estimate the free energy profiles for [EMIM][BF₄] between both rigid and flexible square confining surfaces at several intersheet separations. The analysis of these free energy profiles shows that capillary evaporation of [EMIM][BF₄] is enhanced by the introduction of flexibility to the confining sheets. We examine the number density profiles for the ionic species in the confinement, which show distinct structural layering for different separations. By additionally characterizing the orientational ordering of cations in confinement, we quantify significant structural changes that occur as the intersheet distance is reduced and as capillary evaporation occurs at fixed confinement. We find that flexibility of the confining material also impacts the structural changes that occur as the ionic liquid evaporates from the intersheet region.