NMR Study of Water Molecules Confined in Extended Nanospaces

摘要

The study of water in confined geometries has been receiving much attention in chemistry, biology, and geology.[1] A variety of spectroscopic and theoretical investigations have demonstrated that water molecules confined in 1-nm-scale materials such as porous silica show unique properties not seen on the bulk scale.[2]-[7] A 1-nm-scale space is available as an experimental space for characterizing the behavior of an individual single molecule, while this scale is too small to illuminate the collective behaviors of liquid-phase molecules as condensed-phase matter. To elucidate the complicated properties of liquid-phase water molecules, a 10-100-nm-scale space is appropriate but has been almost unavailable. The technologies involved in micro- and nanochemistry on a chip are expected to allow the production of a physicochemically well-defined 10-10-nm-scale space on glass substrates (called an extended nanospace). Previously, we developed extended nanospaces by means of well-controlled micro-anofabrication techniques and made capillary and time-resolved fluorescence measurements of water confined in the spaces.[8] The results showed that, compared with bulk water, the water confined in the extended nanospaces had a higher viscosity and a lower dielectric constant. Similar size-confinement phenomena have been shown in hydrodynamic flow, conductivity, and ionic transport results in extended nanospaces.[9]-[12] However, little molecular-level information is available concerning the mechanisms for the novel confinement-induced nanospatial properties of water molecules in extended nanospaces. Here we present size-confinement effects of the molecular structure, motions, protonic mobility, and localization of proton-charge distribution of water and water-surface proton exchange in 295-5000-nm extended nanospaces by NMR spectroscopy measurements.