Science Thrust 1: Molecular and Solution Processes
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.
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.
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.