Molecular-driven extreme surface tension of water in carbon nanotubes

At the nanoscale when the system shrinks down, interfacial interactions govern fluid behavior, often defying bulk expectations. Using classical molecular simulations, we directly calculate the surface tension of water confined in carbon nanotubes (CNTs). Unlike planar interfaces, where confinement enhances surface tension monotonically, cylindrical confinement reveals a striking non-monotonic trend. In narrow CNTs, strong water–carbon (WC) repulsion yields giant surface tensions exceeding twice the bulk value. At a critical pore radius, water organizes into ordered hexagonal and pentagonal networks, driving an unexpected collapse—and even negative values—of the surface tension. These behaviors were also recovered for confined methane and for a different water model, further supporting the confinement effect on surface tension. Comparison with thermodynamic models, including Gibbs and Tolman representations as well as the recently introduced concepts of differential and integral surface tension, highlights their limitations in capturing the pore-radius dependence and the anomalous values observed. We establish water–carbon repulsion, curvature, and molecular structuring as the key determinants of confined-water surface tension, with broad implications for nanofluidics and interfacial thermodynamics.