Energetic and Structural Insights into Water Confined in Hydrophobic Nanopores

Water
confined within hydrophobic nanopores exhibits unusual thermodynamic
and structural behavior that governs a wide range of nanofluidic and
energy-conversion phenomena. Here, we combine high-pressure scanning
calorimetry with molecular dynamics simulations in the temperature
range of 298–380 K to elucidate the energetics and mechanisms
of water intrusion into pure-silica LTA zeolites featuring cage-like
pores. A new data-processing approach separates compression and intrusion
contributions in pressure–volume curves, enabling direct quantification
of temperature-dependent heat effects. Intrusion is exothermic at
ambient temperature (≈−5 J g–1) and
becomes nearly thermoneutral above 338 K. Simulations reveal slow
intrusion kinetics, collective cage filling through hydrogen-bond-mediated
chains, and pronounced stabilization at occupancies of 17–22
H2O molecules per supercell, balancing enthalpic and entropic
factors. The results demonstrate that intrusion pressures correlate
with accessible surface area, free pore volume, pore geometry, and
connectivity rather than aperture size, thereby invalidating classical
Laplace–Washburn scaling for nanopores. These findings establish
microscopic design principles for tailoring wetting thermodynamics
in hydrophobic nanoporous frameworks for energy storage, mechanical
actuation, nanofluidic systems, and nanodevices.