In this work, the surface tension ($\gamma$) 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 $\gamma$. 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.

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• Confinement
• Excluded volume interaction
• Surface tension effects