Morphology and stability of water droplets under 2D cross-sectional confinement in amorphous silica nanopores: Roles of size, cross-sectional shape, and surface chemistry

Molecular dynamics simulations are performed to investigate the coupled effects of pore size (1–6 nm), cross-sectional geometry (circular, hexagonal, square, triangular), and surface chemistry (hydroxyl vs. methyl functionalization) on water droplets confined in amorphous silica nanopores. Droplet stability, density distributions, and pressure profiles are evaluated across stable and metastable states. Density analyses indicate that hydrophilic pores promote continuous wetting films extending beyond the droplets, while hydrophobic pores restrict water to the primary droplet volume. Under extreme 1 nm confinement, distinct liquid-vapor interfaces vanish, and water forms a uniform dense phase regardless of surface wettability. Additionally, water molecules penetrate the silica interstices in highly confined (<2 nm) pores across both surface chemistries. Pressure profiles reveal a non-monotonic size dependence in hydrophilic pores, where maximum negative pressure—and thus peak liquid retention—occurs at approximately 2 nm rather than 1 nm. Conversely, 1 nm hydrophobic pores undergo a thermodynamic inversion, exhibiting highly positive internal pressures that promote spontaneous liquid expulsion. These mechanical responses are further amplified by cross-sectional geometry. The triangular pores yield the most negative pressures at 2 nm (enhancing retention) and the highest positive pressures at 1 nm (driving expulsion). Finally, cross-sectional geometry dictates the stability of metastable states under 2D confinement. The large surface area of triangular hydrophilic pores disrupts continuous film formation, facilitating droplet coalescence. In contrast, this identical geometry in hydrophobic pores stabilizes separated, metastable liquid droplets due to favorable water-solid interactions.