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.