arXiv subjects

Alec A. Beaton

Publications and source records attributed to Alec A. Beaton.

3 recordsLinked to original sources

arXiv2024

Direct Observation of the Translational Immobilization of Water Molecules Under Nanoscale Confinement

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.

cond-mat.soft↗

arXiv2024

Rapidly Screening the Correlation Between the Rotational Mobility and the Hydrogen Bonding Strength of Confined Water

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.

cond-mat.mtrl-sci↗

arXiv2022

A Modernized View of Coherence Pathways Applied to Magnetic Resonance Experiments in Unstable, Inhomogeneous Fields

Over recent decades, the value of conducting experiments at lower frequencies and in inhomogeneous and/or time-variable fields has grown. For example, an interest in the nanoscale heterogeneities of hydration dynamics demands increasingly sophisticated and automated measurements deploying Overhauser Dynamic Nuclear Polarization (ODNP) at low field. The development of these methods poses various challenges that drove us to develop a standardized alternative to the traditional schema for acquiring and analyzing coherence pathway information employed by the overwhelming majority of contemporary Nuclear Magnetic Resonance (NMR) research. Specifically, on well-tested, stable NMR systems running well-tested pulse sequences in highly optimized, homogeneous magnetic fields, traditional hardware and software quickly isolate a meaningful subset of data by averaging and discarding between 3/4 and 127/128 of the digitized data. In contrast, spurred by recent advances in the capabilities of open-source libraries, the domain colored coherence transfer (DCCT) schema implemented here builds on the long-extant concept of Fourier transformation along the pulse phase cycle domain to enable data visualization that more fully reflects the rich physics underlying these NMR experiments. In addition to discussing the outline and implementation of the general DCCT schema and associated plotting methods, this manuscript presents a collection of algorithms that provide robust phasing, avoidance of baseline distortion, and the ability to realize relatively weak signals amidst background noise through signal-averaged correlation alignment. The methods for visualizing the raw data, together with the processing routines whose development they guide should apply directly to or extend easily to other techniques facing similar challenges.

physics.chem-ph↗