Temperature-Driven Behavior of Surface Tension in Liquids Under Closed Nano-Confinement

The manipulation of liquids in nanoscale confinement, which is pivotal for advancing nanofluidics technologies, sheds light on predominant surface effects, with surface tension being the most important property, yet still understudied. This study investigates the surface tension of liquids narrowly nanoconfined between rigid graphene sheets under isochoric and closed conditions. I reveal a significant increase in the solid–liquid surface tension compared to the bulk liquid–vapor phase, with pronounced effects in narrower confinements. Temperature-dependent analyses highlight an unusual increase in surface tension, opposite to what is typically observed at the liquid–vapor interface. I show that the repulsive van der Waals interactions between graphene and ethanol molecules dominate the surface tension contributions. Further, the structured layering of ethanol molecules near the graphene surfaces associated with specific molecular arrangement is identified as a key factor of negative liquid–liquid surface tension contribution. These findings provide fundamental insights into interfacial interactions in nanoscale systems and underscore the critical role of confinement in modulating physical properties.