Periodic Mesoporous Organosilicas as Host Materials for Studying Surface Chemistry Effects on the Freezing and Melting Behavior of Nanoconfined Water

The impact of surface hydrophilicity on the freezing, melting, and nonfreezable layer of nanoconfined water remains debated. Variability in material types, pore sizes, surface functionality organization, and experimental conditions complicate direct comparisons and conclusions. To address this, periodic mesoporous organosilicas (PMOs) with a uniform divinylbenzene bridging unit were used. By fine-tuning hydrophilicity and pore sizes, confined water’s phase behavior was studied via differential scanning calorimetry (DSC). Comparing gas-phase adsorption and incipient wetness as filling methods revealed an increased t-layer density with the latter. Surface hydrophilicity has little effect on melting point depression in larger pores but becomes increasingly influential as pore size decreases. The nonfreezable layer thickness was evaluated using the Gibbs–Thomson equation and geometric enthalpy-based calculations. In hydrophobic PMOs, water exhibited larger melting point depression, lower specific enthalpies, and thicker interfacial layers than in hydrophilic ones. In contrast, charged PMOs behaved differently: despite higher hydrophilicity, confined water exhibited a larger melting temperature depression, lower specific enthalpy, larger critical pore radius, and comparatively thicker t-layers, likely due to higher disorder of the hydrogen-bonding network close to the surface. Moreover, the t-layer density did not follow a simple trend based solely on hydrophilicity. These results highlight the complex interplay between pore size, surface chemistry, and interfacial water behavior, offering valuable insights into confined water properties and phase transitions.