  Here, Overhauser Dynamic Nuclear Polarization (ODNP) analyzes the
translational dynamics of water confined inside reverse micelles (RMs). It
offers a unique insight into the translational diffusion of water inside RMs,
in particular measuring the diffusion in a "RM-fixed" frame, rather than
relative to the laboratory frame. This study seeks to confirm whether or not
the act of tuning the size of the RMs, which can be achieved simply by
adjusting the "water loading" ($w_0$, i.e., the mole ratio of surfactant to
water) offer a near-continuous tuning of the translational diffusion of water.
The results here confirm this supposition, while offering two additional
interesting observations (1) surprisingly, translational diffusion slows to a
near-stop for RMs that are small, but that still contain hundreds of water
molecules in their core and (2) even relatively large RMs exhibit translational
dynamics in their core (center) that is only as fast as translational dynamics
on the surface of a lipid bilayer; extrapolation to larger sized water pools
implies the need for tens of thousands of water molecules present in a confined
pool in order to recover bulk-like translational dynamics. The parts of this
study that take place at the lowest water loadings represent the first known
instance of an ODNP measurement where the probing spin label interacts
continuously with a full complement of water molecules (as opposed to sitting
buried inside a lipid membrane or protein core), and yet clearly observes only
slow translational diffusion of water. The particular system here, with an
NMR-silent CCl$_4$ dispersant, shoud prove easy to reproduce. Thus, this study
also opens up the pathway to future measurements where RMs serve as a ruler for
dynamics, in particular for smaller resonance frequencies that can ostensibly
detect the dynamics of the slower water inside severely confined (i.e., small)
RMs.
