Influence of Surface Chemistry on <i>t</i>-Layer Dimension and Density of Nanoconfined Water by Fine-Tuning the Functional Groups of Periodic Mesoporous Organosilicas

This study investigates the behavior of water confined in cylindrical nanopores with hydrophilic, charged, and hydrophobic surfaces using water vapor sorption at 298.15 K. MCM-41 silica and periodic mesoporous organosilicas (PMOs) serve as host materials, with PMOs featuring a silica backbone linked by organic bridging units, allowing precise control over surface chemistry. Using a consistent divinylbenzene-based structure, the study assesses how functional groups influence confined water properties. A novel hydrophilicity index is introduced, incorporating contact angle data and adsorption layer thickness to quantify surface chemistry effects. Hydrophilic materials exhibit distinct adsorption profiles compared to hydrophobic PMOs, where surface chemistry and confinement influence water density, adsorption behavior, and t-layer proportions. In hydrophobic materials, such as divinylbenzene- and N,N-dimethyldivinylaniline-bridged PMOs, thicker water adsorption layers with lower average density are observed, which can be attributed to the localization of water near silanol groups and its absence near hydrophobic regions. Fluorinated PMOs fluctuate between hydrophobic and hydrophilic tendencies depending on water saturation. Introduction of surface charges enhances water adsorption, leading to a larger t-layer and higher adsorbed water density. These findings provide critical insights into how surface chemistry and pore size collectively govern water behavior, offering a framework for future studies on nanoconfined water.