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.