When does the Young-Laplace equation fail at the nanoscale? A cylindrical-interface test separating curvature...

Apparent deviations from the Young-Laplace equation at nanometer scales are often attributed to curvature-dependent interfacial tension and higher-order curvature corrections. This interpretation, however, assumes that the system still possesses distinct bulk-like phases and a well-defined curved interface, which are required to define a physically meaningful Laplace pressure. Here, we use cylindrical liquid-liquid interfaces, which contain only one nonzero principal curvature, to separate curvature corrections from breakdown of the capillary state. Formulating the pressure relation in terms of the equimolar radius Rₑ yields a cylindrical pressure relation ΔP=γ_∞/R_e that remains accurate through second order in curvature without requiring an additional Tolman parameter to be fitted. Molecular dynamics simulations of complementary n-octane-in-water and water-in-n-octane cylindrical nanodroplets show that, although the interfacial tension varies markedly with curvature, the independently measured pressure difference remains consistent with the prediction based on the planar interfacial tension γ_∞ and Rₑ throughout the physically admissible regime. Deviations arise only after the physical basis of the capillary description is lost: molecular ordering eliminates the bulk-like dispersed phase in oil-in-water systems, whereas water-in-oil cylinders lose metastability before a comparable ordered core develops. These results demonstrate that nanoscale curvature alone does not invalidate the Young-Laplace equation. For the cylindrical interfaces examined here, its practical applicability limit is governed by physical admissibility rather than curvature magnitude, providing an intrinsic fluid-fluid benchmark for identifying additional confinement- and boundary-induced effects in more complex nanoscale systems.