Surface tension enhancement in nanoconfined water: The role of confinement and excluded volume effects

In this work, the surface tension (γ) of nanoconfined water between planar interfaces was evaluated using atomistic simulations combined with the test-area method. To elucidate the role of spatial restriction, two types of confinement were considered: solid and liquid. In the case of solid surfaces, a strong increase in surface tension was observed compared to the unconfined case-i.e., water in contact with a single interface. In contrast, no significant change in surface tension was found under hydrophobic liquid confinement, indicating that spatial restriction alone does not fully account for the increase in γ. Our results demonstrate that the enhancement of surface tension arises from a synergistic interplay between confinement and excluded volume effects, amplified by a lack of miscibility between the two phases. These findings highlight the importance of interfacial structure and phase compatibility in determining the interfacial properties of confined fluids.