Study of the interplay between geometry and chemistry in nanoscale hydration

V4S, a novel structural indicator developed to characterize water in hydration and nanoconfined environments, was recently introduced and initially applied to water in contact with self-assembled monolayers (SAMs), graphene-like systems, and proteins. In the present work, we employ this metric to characterize SAMs featuring cavities of varying sizes. We investigate the effects of geometry and chemical composition on surface hydration by incorporating hydroxyl groups (-OH) as hydrophilic sites. When hydration implies the disruption of hydrogen bonds at the hydration layer, a defect interaction threshold (DIT) should be satisfied in order to give rise to hydrophilic behavior. This threshold is given by the amount of energy compensation a hydrogen bond defect receives in bulk water and is significantly lower than the hydrogen bond energy. Besides signaling the transition to hydrophobicity, we find that the DIT value is also relevant for nanoconfined environments. In this regard, our findings reveal that a water molecule cannot sustain more than one hydrogen-bonding site with an interaction weaker than the DIT; if a second site exceeds this threshold, the molecule desorbs. This finding quantifies the energy loss a water molecule can tolerate while maintaining wetting or hydration. Finally, by means of preliminary calculations on simplified model systems, we show that such knowledge may be instrumental in main contexts like protein binding, where removal of hydration water contributes as a relevant driving force.