Science Thrust 1: Molecular and Solution Processes

Editor's Note: The IDREAM research described here represents the scientific focus from 2016 to Summer 2024.

Lead: Zheming Wang

Goal: Understand the solvent dynamics, solute–solvent organization, prenucleation species, and radiation-driven reactivity in concentrated alkaline electrolytes.

Solution-phase dynamics play a crucial role in the origin of the physical properties of fluids and influence kinetically hindered steps toward precipitation and dissolution. For aqueous Al(III) chemistry in basic solutions, these range from the local perturbation of Al(OH)4- coordination geometry and aggregation into dimers and oligomers to the formation of ion-pairing networks and cluster densification. These structures become especially evident in the highly concentrated "water-in-salt" solutions, where there is not enough water present to fully solvate each ionic species.

Task 1.1: Quantify the driving forces behind long-range solution organization and assembly of prenucleation species in highly organized ion–ion networks in the absence of ionizing radiation.

Current research in concentrated alkaline solutions focuses on understanding solution organizational structures. We seek to determine how the presence of variable cation and anion species promotes the formation of local polyhedral arrangements, ion–ion networks, and perturbation of the bulk water tetrahedral structure. These organizational structures are anticipated to significantly alter the chemical reactivity of these materials and lead to distinct reaction pathways specific to these extremely alkaline conditions.

In studies of alkaline sodium aluminate solutions, we are probing local aluminate structures, such as aluminate monomers and dimers, and how they relate to the solutions’ susceptibility toward gibbsite precipitation or enhanced metastability (Figure 1). Such knowledge will help unravel the underlying mechanism by which the tetrahedrally coordinated aluminate solution species transforms to the octahedrally coordinated aluminum in gibbsite.

These solutions are being investigated with geometrical fitting of neutron and X-ray pair distribution functions (PDF). The aluminate structures used in the geometrical fitting were obtained through ab initio and classical molecular dynamics simulations, featured within the IDREAM Cross-Cutting Theme 2 (XC2). Supplementing these studies, we employ Raman and Fourier transform infrared (FTIR) spectroscopies to quantify the relative abundances of aluminate monomer and dimer species. Through these studies, it has become evident that hydrogen bonding and proton transfer reactions greatly influence the aluminum speciation and metastability of these solutions. The role of hydrogen bonding is being further investigated through studies comparing the effects of substituting deuterium for protium.

Scientific figure showing how IDREAM researchers are investigating the role of aluminate dimerization on solution metastability.
Figure 1. The role of aluminate dimerization on solution metastability is being investigated by systematic control of the solution composition. The solutions shown contain similar [Al3+] but variable [Na+] with a range of stabilities toward gibbsite precipitation. In the case of solution I, the solution precipitates within hours, while solution II does not precipitate even over the span of months, and solution III instead forms a semi-crystalline gel after a few months. As investigated by Raman spectroscopy, as well as FTIR spectroscopy and neutron total scattering (not shown), the solutions show the presence of monomeric and dimeric aluminate. As the solution increases in [Na+] relative to [Al3+], the abundance of aluminate dimers increases at the expense of the aluminate monomer. Figure adapted from “Solution and Interface Structure and Dynamics in Geochemistry: Gateway to Link Elementary Processes to Mineral Nucleation and Growth,” Crystal Growth & Design.

 

In addition to probing the local structures in sodium aluminate solutions, long-range solution structures are being investigated in systems containing alkali nitrates and alkali nitrites. These solutions have been investigated through X-ray PDF analysis, small angle X-ray scattering, Raman spectroscopy, and FTIR spectroscopy. Molecular dynamics (MD) simulations (XC2) are being performed to investigate the complex O‧‧‧O correlations from approximately 3–8 Å in the PDF, as observed initially in our earlier work on solutions containing NaNO2 and NaOH. Initial MD results for concentrated NaNO2 reveal that solutions at these length scales reflect a polyhedral network consisting of nitrite anions, Na+, and water. As the concentration of the solution increases from undersaturation conditions to saturation, this network of ions becomes increasingly connected. We seek to connect the solution structures to the observed differences in solubilities between the various alkali nitrates and nitrites, as well as to the increase in solubility in their mixtures.

Scientific figure showing IDREAM research comparing the pair distribution functions (PDF) for concentrated NaNO2 and that of water.
Figure 2. Comparison of the PDF for concentrated NaNO2 and that of water reveals significant structural changes between 3 and 8 Å. Contributions to these length scales occur outside of the first solvation shell in solution and consist of complex O‧‧‧O correlations. MD simulations are being used to determine the species and structures that give rise to these features in the PDF. Initial MD simulations reveal a polyhedral network consisting of a variety of motif linkages and lifetimes. Continued experimental–computation synergy will pursue concentration effects as well as the role of the cation and anion species in solution.
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Task 1.2: Discover the radiation-driven chemical dynamics governing the formation of nonequilibrium steady-state species and their role in prenucleation speciation.

Current efforts to understand the effects of radiation on concentrated solutions spans multiple timescales (Cross-Cutting Theme 1 [XC1]: Radiolysis and Radiation Dynamics) that can be subdivided into stages of radiolysis (Fig. 3). A focus of IDREAM is to determine the processes occurring on the physico-chemical and chemical stages to understand the underlying mechanisms controlling the long-term effects on materials exposed to ionizing radiation, such as H2 gas production, precipitation, and aggregation behaviors.

Initial studies are unraveling the formation and reactivity of chemically reactive species in the radiolysis of water on the physico-chemical timescale using X-ray pump–probe experiments at the Linac Coherent Light Source. Following the investigations on water, the effect of solute species such as NaOH, NaNO2, and NaNO3 will be delineated. These species not only allow an additional experimental probe (such as Na k-edge or N k-edge) but also look at the effects of radiolysis and the subsequent reaction pathways when water is not necessarily the next nearest neighbor. These ultrafast radiolysis experiments benefit from a comprehensive computational team. Ab initio Ehrenfest dynamics simulations (XC2) are being utilized to determine the ultrafast processes following ionization in bulk water and aqueous electrolytes based upon the solution structures determined through ab initio and classical MD signatures in Science Thrust 1 Task 1.1, and X-ray absorption near edge structure (XANES) signatures are being computed for potential species that form after ionization.

Scientific figure showing IDREAM research into the radiolysis of water.
Figure 3. The various stages of radiolysis of water. IDREAM is utilizing X-ray pump–probe experiments to understand the formation and reaction pathways of chemically reactive species on the physico-chemical timescale. These experiments will be followed by studies of NaNO3, NaNO2, and NaOH solutions on the same timescales. The X-rays can be tuned to ionize specific species in solution.

 

Scientific figure showing IDREAM research into the very fast processes following photoionization of water.
Figure 4. Ab initio Ehrenfest dynamics simulations revealed the very fast processes following photoionization of water. A cationic hole was created upon sudden ionization from the highest occupied molecular orbital (HOMO) of the water nanodroplet. It quickly localized within 10 fs via hole trapping or hole transfer, leading to large OH oscillations. This was followed by proton transfer, which results in an OH radical after 30~40 fs in our simulations. The resulting vibrationally hot hydronium (H3O+) eventually shuttles one of its protons to another nearest-neighboring water 10–20 fs after its formation. During this stage, a metastable Zundel complex is formed.

 

Scientific figure showing how IDREAM researchers are using XANES spectroscopy.
Figure 5. XANES spectroscopy is a promising candidate for investigating the time-resolved structural changes triggered by ionizing radiation that occur in complex alkaline aluminate solutions. Using radiation as a tool for promoting formation of metastable aluminate species will enable investigation of the roles of various aluminate species in gibbsite precipitation and the underlying mechanisms.