Confinement quenches the ability of water molecules to engage in collective motion, and proves relevant to the structure and function of both synthetic materials and biological macromolecules. Reverse micelles serve as excellent model systems, both for modeling variable confinement effects, and for interrogating how confined water molecules respond to changes in surface chemistry and dynamics. From a technique-oriented standpoint, reverse micelle systems offer the opportunity to describe measurements in highly phase-heterogeneous and confined systems, and allows us to include comparisons to molecular modeling simulations. We are developing a toolbox of robust techniques to highlight the change in translational and rotational dynamics of the water in response to different levels of confinement and surfactant concentrations. In particular, Overhauser Dynamic Nuclear Polarization (ODNP) isolates the translational motion of water at the nanoscale, and reads out the result through variable enhancement of NMR (Nuclear Magnetic Resonance) signal. ODNP does not rely on higher-order spectroscopic effects, requires only a simple, versatile, and non-perturbative labeling strategy, and functions seamlessly in opaque, viscous, and other “dirty” systems. ODNP still possesses many fundamental capabilities and a flexibility that have remained largely untapped. Similarly, past research employed 2H NMR solution state relaxometry to a limited extent, but we will present how modern equipment can automate this technique and deploy it across a wide range of samples. One current highlight of this multi-modal methodology is a new automated 2D 2H methodology that experimentally measures the correlation of average hydrogen bonding strength and rotational mobility in systems with dramatic water confinement. The dynamics and structure of water are found to respond to both changes in surfactant as well as to inclusions of solutes in the nanoconfined water pool. Simultaneously, an improved variant of ODNP can work synergistically with 2H NMR relaxometry and 1H NMR diffusometry to develop a complete picture of translational motion of the water, rotational motion of the water, and translational motion of the reverse micelle assemblies, respectively. We discuss how such a multi-modal relaxometry strategy offers unique insights into the dynamics of reverse micelles, and discuss applying the methodology to other confined environments and synthetic soft materials systems.

Characterizing nanoconfined pools of water with nuclear and electron-nuclear relaxometry

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.