Direct Observation of the Translational Immobilization of Water Molecules Under Nanoscale Confinement

Abstract: ODNP (Overhauser Dynamic Nuclear Polarization) detects an experimental measurable associated directly with translational motion of water at the nanoscale, a quantity that few other methods can detect. This study offers a unique insight into the translational diffusion of water inside RMs (reverse micelles). It finds that simply adjusting the "water loading" ($w_0$, i.e. the mole ratio of surfactant to water) to tune the size of the RMs achieves a near-continuous tuning of the translational diffusion of water. Furthermore, (1) water molecules in the core of relatively large RMs ($w_0=10$, diameter of water nanopool $\approx 3.5$ nm) diffuse only as fast as those on the surface of a lipid bilayer and (2) surprisingly, translational diffusion slows to a near-stop for RMs that are small, but still contain hundreds of water molecules in their core. Extrapolation to larger sized water pools implies that in order to recover bulk-like translational dynamics, tens of thousands of water molecules are required. The data from the small RMs also represent a breakthrough as the first example where a spin probe that is completely exposed to water (as opposed to buried inside a macromolecule) observes dramatic slowing of the translational diffusion.