Deconstructing the Confinement Effect upon the Organization and Dynamics of Water in Hydrophobic Nanoporous Materials: Lessons Learned from Zeolites

The properties of confined water are relevant to many chemical, geological, and biological phenomena, where they underpin essential changes to molecular scale reactivity-perturbing both the energetic and configurational landscape. Though much prior literature has focused on hydrophilic confinement, the hydrophobic confinement of water is less well understood. Here, we use molecular dynamics simulations to investigate the structures and dynamics of water in hydrophobic all-silica zeolites that have sequentially smaller pore dimensions. Of special interest is the role that pure geometric restriction imparts, relative to the rugged potential energy landscape for water interacting with the atomistic pore surface. These two effects were studied via the hydrogen bond dynamics, specifically the rates and mechanisms of hydrogen bond breakage and formation. Measuring the dynamic features as a function of scaling the water:zeolite interaction energy revealed that geometric restriction is responsible for 67%-86% of the total perturbations to water upon confinement in MFI (depending on the property) while the water:surface interactions are responsible for 14%-33%. The relative magnitude of the interaction of water with the pore surface was confirmed by second order Møller-Plesset perturbation theory. Thus, in a highly confined environment, the weak water-surface interaction should not be neglected - even in hydrophobic adsorbents to which zeolites and other materials like carbon nanotubes belong.