Free Energy Analysis of Solid–Liquid Coexistence in Water Under Superhydrophilic Cylindrical Confinement

Here, we explore the effect of strongly hydrophilic cylindrical confinement (R = 10-50 Å) on the solid-liquid coexistence and structural dynamics of water using molecular dynamics simulations in LAMMPS with the monatomic water (mW) model. Studying the melting behavior of water under nanoscale confinement is essential for elucidating biological processes and materials science at the atomic or molecular level, yet the underlying mechanisms remain poorly understood. This study extends our previous work on slit-pore confinement [Sinha, V. K.; Das, C. K. <i>J. Mol. Liq.</i> 2025, 435, 128163], while demonstrating that cylindrical nanopores exhibit distinct thermodynamic melting behavior and structural ordering owing to their surface curvature and larger interfacial area (circumference) exposed to water molecules. We analyzed the Gibbs free energy difference (<i>ΔG</i>) between the solid and liquid phases to rigorously determine the melting temperature (<i>T</i>_<i>m</i>) from the coexistence condition (<i>ΔG</i> = 0), which is also supported by structural analysis. Depression in <i>T</i>_<i>m</i> with smaller pore radii or higher surface hydrophilicity indicates greater stability of the liquid phase under confinement. Structural analysis shows that the fraction of 2D hexagonal motifs at the pore surface increases with stronger surface interactions, while cubic and hexagonal diamond structures dominate mainly the bulk region. The variation in ice polymorph distribution provides a structural rationale for the observed depression of <i>T</i>_<i>m</i> under confinement. Similar trends but significantly lower melting temperatures for the cylindrical nanopores were observed compared to our previous results on slit-pore confinement. This work signifies the importance of nanoscale confinement geometry and surface hydrophilicity when dealing with solid-liquid phase transformation of water.