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Past research has conclusively shown that confined pockets of water exhibit
properties that differ from those of unconfined ("bulk") water. The differences
between confined water and bulk, as well as between different types of confined
water environments impact a far-reaching range of target applications. However,
the measurements that discriminate between different variants of confined water
tend to rely on sophisticated techniques that frequently involve specialized
instrumentation or facilities. Here, we demonstrate a straightforward and
automated technique compatible with most NMR spectrometers that can analyze a
wide range of nanoporous or mesoporous systems. It generates a 2D plot that
correlates the approximate rotational correlation time (from deuterium
relaxation measurements) against the approximate average hydrogen bond strength
(from the diamagnetic shielding, i.e., chemical shift).

The water pools inside reverse micelles (RMs), chosen here as a demonstration
system, exhibit a range of properties as the water loading ($w_0$, or water to
surfactant molar ratio) changes. Small $w_0$ correspond to severe confinement
(isolation of tens to hundreds of water molecules), and as the $w_0$ increases,
the RMs grow in size. As a result, measurements of RMs with differently sized
water pools ($w_0$) sweep out a characteristic shape in the 2D correlation
spectrum.

This simple, automated measurement demonstrates striking differences in how
the properties of differently confined waters change as the lengthscale of the
confinement (controlled in RMs by $w_0$) changes. The results here report on a
total of 45 different RM samples prepared with a range of $w_0$, surfactants,
dispersants, and guest molecules. This technique should be widely applicable
both in terms of facilities where it can be implemented as well as chemical
systems to which it applies.