Abstract
High energy electron scattering of liquid water (H2O) at near-ambient temperature and pressure was performed in a transmission electron microscope (TEM) to determine the radial distribution of water, which provides information on intra- and intermolecular spatial correlations. A recently developed environmental liquid cell enables formation of a stable water layer, the thickness of which is readily controlled by pressure and flow rate adjustments of a humid air stream passing between two silicon nitride (Si3N4) membranes. The analysis of the scattering data is adapted from the x-ray methodology to account for multiple scattering in the H2O:Si3N4 sandwich layer. For the H2O layer, we obtain oxygen-oxygen (O-O) and oxygen-hydrogen (O-H) peaks at 2.84 Å and 1.83 Å, respectively, in good agreement with values in the literature. This demonstrates the potential of our approach toward future studies of water-based physics and chemistry in TEMs or electron probes of structural dynamics.
Highlights
The majority of known chemical and biological processes occur in a liquid medium
Compared to the previous electron diffraction studies on water, the environmental liquid cell (ELC) is capable of producing a repeatable thin water layer, which allows image acquisition for a longer time
The information extracted from the radial distribution function for the intermolecular oxygen–oxygen bond matched extremely well to the x-ray scattering measurements and molecular dynamic (MD) simulations, with only 1% error in the obtained bond length, and as electrons are more sensitive to hydrogen than x-rays, we were able to extend the diffraction analysis to obtain a peak at 1.88 Å
Summary
The majority of known chemical and biological processes occur in a liquid medium. there is great interest in theoretical and experimental investigations of the liquid structure and its evolution at the molecular level. Water stands out in this regard owing to its ubiquity and many exceptional properties that have been extensively studied and reviewed. The idiosyncratic behavior of liquid water in particular, such as its negative thermal expansion coefficient below 4 ○C, arises from the intricate dynamics of extended hydrogen bond networks that have been investigated numerically, spectroscopically, and by diffraction techniques on picosecond and femtosecond timescales. Intricacies of water structure, such as the dynamic interplay of tetrahedral and ring or chain-like molecular formations and the problem of how the hydrogen bond exists well above its percolation threshold despite its short lifetime in the few picosecond (ps) regime, remain problems that are under active investigation. the hydrophobic interaction of water with biological molecules governs such fundamental aspects as the stability of protein conformations, and DNA structure in solution is an important field of study.. Since electrons as charged particles interact with atomic nuclei via the electrostatic potential associated with each atom as opposed to the interaction of x-rays with electronic polarization and associated electron orbitals, there is no lower limit for the atomic number of atoms acting as electron scatterers.41 This implies that electron scattering data are sensitive to O–O and to O–H and, in principle, even H–H pair distributions, allowing for a deeper and more direct view of the hydrogen bond structure. Electron scattering experiments on liquids pose significant experimental challenges, the problem of maintaining, in a high vacuum environment, sufficiently thin liquid layers (about 150 nm for 200 keV electrons in water) that are required to forego multiple scattering events The latter obscure the oscillations in the acquired radial distribution function from which structural information is extracted. This opens the door to windowed liquid cell based structural dynamics investigations of hydrogen bond dynamics in water and, more generally, solution phase chemistry in TEMs and tabletop electron diffractometers
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