Molecular Dynamics Study of Water Properties under Cylindrical Superhydrophilic Confinement Using the mW Model

Studying the anomalous behavior of nanoscale confined water is important for understanding biological processes at the molecular level as well as advancing materials science. Here, we elucidate the effect of strongly hydrophilic cylindrical confinement (pore radius: 10-50 Å) on the thermodynamic and structural properties of water using molecular dynamics simulations with the monatomic water (mW) model. The present work extends earlier slit-confinement analyses to curved cylindrical geometry and complements recent free-energy-based studies of solid-liquid coexistence under cylindrical confinement by focusing on the cooling-heating pathways of confined water and the associated phase transition behavior. The properties are found to depend on both pore radius and wall-fluid interaction strength. At small pore radii (R ≤ 10 Å), the solid phase exhibits a higher density than the liquid, deviating from bulk behavior due to the combined effects of extreme geometric confinement and stronger wall-fluid hydrophilicity. Specific heat capacity shows abrupt changes during phase transitions. Confinement notably impacts solid-phase entropy, with a negligible impact on the liquid phase. Hysteresis loops in density, enthalpy, potential energy, and entropy are observed around the transition temperatures, indicating a first-order phase transition. Freezing temperature increases with both pore radius and wall-fluid interaction strength, whereas melting temperatures display oscillations at lower pore sizes (R ≤ 25 Å), stabilizing beyond this threshold. A sudden rise in bond-orientational and tetrahedral order parameters during quenching indicates diamond-like crystallization. Interestingly, the observed shift in freezing temperature shows a linear correlation with the inverse effective pore radius, following a Gibbs-Thomson-like relation for confined phase transitions, which differs from what was observed in slit confinement in previous work.