Reverse micelles (RMs) provide unique systems of study for water in a confined environment (Figure 1A). As the water loading (w0 or water to surfactant molar ratio) increases, the internal water pool develops more bulk-like character and grows in size. The water pool is often described by a two-component core/shell model, with water in the core behaving like bulk water and water in the shell behaving like interfacial water. This picture however can be difficult to verify experimentally and its details contested across different techniques. To obtain an enriched understanding of the water pool inside RMs of different sizes, we invoke a two-pronged NMR approach: Deuterium relaxometry for probing the rotational mobility and average hydrogen bonding of water, and Overhauser Dynamic Nuclear Polarization (ODNP) for probing the translational motion. Expanding upon important existing work toward characterizing these systems by Deuterium NMR, we study RMs up to an approximate w0 of 20 (Figure 1B). We monitor changes in the water environment upon changing surfactant or dispersant and investigate the impact of including osmolytes such as glucose and polyethylene glycol. We also report the first comprehensive liquid-state ODNP analysis on the translational motion of water inside these RMs at different w0 (Figure 1C). Through careful selection of spin probes which preferentially reside in the core or in the shell, we anticipate the ability to observe core and shell signal separately and develop a more complete picture of the nanoscale water dynamics.

Inside perspective: Characterizing water pools within reverse micelles using magnetic resonance relaxometry techniques

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