Science Thrust 3: Dynamics of Confined Electrolytes

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

Lead: Andrew Stack

Goal: Understand the chemical and radiation-driven phenomena of nanoscale-confined electrolytes that lead to interactions between interfaces to form aggregates of particles.

Macroscale properties of slurries originate in part from the molecular interactions that occur between particles. In Science Thrust 3 (ST3), we are focused on understanding the structure and dynamics of alkaline electrolytes confined between particle surfaces. Current research is focused on determining the roles of interfacial water, hydroxide, dissolved impurities, and solutes confined between solid surfaces in controlling interparticle forces and discover how interfacial radiolysis affects these interactions

To probe the interactions between particles and confined fluids that drive particle interactions in high-salt environments, ST3 builds on knowledge generated in Science Thrust 2 (ST2) about interfacial structure and reactivity and on that in Science Thrust 1 (ST1) about solution structure and ion–ion organization in highly concentrated alkaline electrolytes. Advancing our understanding of confined electrolytes at the nanoscale and their influence on particle interactions will be accelerated through our Cross-Cutting Themes (XC) focused on radiation dynamics (XC1), new theory development (XC2), and materials synthesis (XC3).

Task 3.1: Discover the molecular-scale influence of particle interactions on aggregation in the absence of radiation

Current state-of-the-art science tools and expertise developed by IDREAM are being leveraged to probe particle geometry and electrolyte composition effects in situ. These include a complementary combination of amplitude-modulated atomic force microscopy (AFM)-based 3D fast force mapping (3D FFM) to determine interfacial structures (Figure 1), atomic force microscopy–dynamic force spectroscopy (AFM-DFS) to directly measure interaction forces, and the dynamics of neutron and X-ray scattering and nuclear magnetic resonance of particle slurries in highly concentrated and low-water environments. These experimental approaches are interpreted by and used to constrain simulations of particle aggregation in an iterative process using a combination of atomic-scale simulations and modified colloid theory of particle aggregation. These include rare event methods (metadynamics) in conjunction with classical molecular dynamics force fields (ClayFF) to observe the free energy of particle attachment under varying conditions in silico (XC2), with the goal of unraveling how interface composition (e.g., electrolyte composition, including the water–salt ratio) and interfacial structure affect aggregation.

Scientific figure showing how IDREAM researchers are investigating the boehmite(010)–water interface.
Figure 1. 3D FFM: At the boehmite(010)–water interface, the orthorhombic crystal lattice templates the distribution of water molecules within 1 nm, as shown by (a, c). MD simulations confirm that the highest water density occurs at sites adjacent to surface hydroxyls (b, d).

 

Task 3.2: Understand the impact of interfacial radiolysis on the dynamics of particle interactions.

Radiation fields can change bulk solution (e.g., ionic strength, pH, and speciation) and surface chemistries (e.g., surface charge distributions), leading to significant impacts on particle interactions. Currently, synchrotron, AFM, and transmission electron microscopy (TEM) based techniques are being utilized to measure irradiation effects both ex situ and in situ. Synchrotron-based in situ small-angle hard X-ray (≥ 10 keV) scattering measurements simultaneously generate radical species and small-angle scattering to measure particle distributions, which are compared to X-ray-free small-angle neutron scattering (SANS) experiments to quantify the effects of radical species on aggregation. Tumbler- and rheo-(ultra)small-angle X-ray scattering (U)SAXS)/SANS measurements under radiation (i.e., X-ray beam) extend these results to show ensemble averages of aggregation behavior. Scattering results demonstrate that boehmites exposed to gamma radiation have different aggregation behavior than pristine materials. To obtain direct observations of aggregation behavior in the presence of a radiation field, a combination of complementary liquid-phase (LP-TEM) and cryo-TEM complement the scattering-based studies (Figure 2). The combination of LP-TEM and cryo-TEM simulates the effects of radiation on particle dynamics and interactions by modulating the dose of the electron beam in LP-TEM and minimizing the effect of the electron beam in cryo-TEM. In the same fashion, AFM-DFS of irradiated boehmites directly measures interaction forces at the nanoscale between irradiated particles. This technique is complemented by our specialized rad-AFM capability, which enables measurements of particle interactions within a radiation field. Again, to connect these experimental observations with atomic-scale structure, dynamics, and reactivity, these experimental techniques are interpreted by computational methods such as rare event classical molecular dynamics simulations, which are used to understand how oxygen and aluminum vacancies on the surfaces of boehmite particles affect interfacial and confined fluid structure. These approaches, along with modified colloid theory (based off Deriaguin, Landau, Verwey & Overbeek [DLVO]), will establish unprecedented knowledge of correlations between radiation, chemistry, and particle interactions over different length scales due to transient species and provide the opportunity to compare with results from Task 3.1 where radiation is absent.

 

Scientific figure showing how IDREAM researchers are investigating boehmite particles.
Figure 2. Directly imaging the trajectory of individual boehmite particles using LP-TEM reveals pH-dependent modes of aggregation. pH 3.3: particles are immobile (a). pH 8.5: aggregation along the boehmite(010) plane (b). pH 11.0: aggregation along edge-to-edge planes (c).