2023 Theses Doctoral

Exploring Reactivity in Aqueous Solutions and at Mineral-Water Interfaces Through the Lens of Rare Event Theory Atomic Scale Simulations

Rampal, Nikhil

This research aims to use advanced classical molecular dynamics simulations (CMD) and rare event simulation methods to study the reactive processes that govern solid phase nucleation and growth in aqueous solutions and at mineral-water interfaces. The rationale is that there is a lack of molecular-level understanding of how these processes influence nucleation. In addition, Classical Nucleation Theory (CNT) is modeled as a phase transition with no consideration or inclusion of chemical reactions. This is true even for ‘simple’ salt solutions (during homogenous nucleation) and reaction mechanisms at mineral-water interfaces (heterogeneous nucleation). Of specific focus are ion pairing/cluster formation, solvent exchange rates, and the effect of impurities. These mechanisms have distinct activation barriers that are often neglected in the classical nucleation theory.

First, to understand the role of ion association/pairing as precursors on the free energy landscape of nucleation, CMD simulations are performed using ZnCl₂(aq) as a model system. Results show increasing barriers at higher concentrations, indicating hindered reaction dynamics with ions assuming locally favored molecular entities with no evidence of extended networks of ions (or DOLLOPs: Dynamically ordered liquid-like oxyanion polymers). This, in turn, controls the solution structure and alters the reaction kinetics in highly concentrated solutions, thereby frustrating nucleation. To explore the formation of these precursors in low-solubility minerals, this approach is expanded from a fundamental model of solution-phase precursor formation to the nucleation and formation of solid-phases using constant chemical potential MD simulations along with well-tempered metadynamics.

Transitioning to heterogeneous nucleation, it becomes imperative to decipher the interfacial structure, dynamics, and thermodynamics of impurity incorporation on mineral-water surfaces to understand the underlying chemical reactions responsible for ion exchange, mechanism of adsorption, and mineral dissolution and growth. The solvent exchange rates have been observed to correlate to the rates of mineral reactions. However, due to the complexity of surface sites on (geo)chemical surfaces, multiple distributions of solvent exchanges are present that need to be probed. The uncertainty lies in how this would change at a complex interface during heterogeneous nucleation. Thus, knowing solvent exchange rates might be a valuable baseline for heterogeneous nucleation. In addition, the ambiguity is significant even for simple ions (~ 2 orders of magnitude for calcium ions). Therefore, the solvent exchange mechanisms on the calcite (CaCO₃)-water interface were resolved by benchmarking CMD forcefields to Quasi-Elastic Neutron Scattering spectra. Combining simulations and scattering data enhances our predictive capability in recognizing and linking the rate-determining steps to the solvent exchange rates on minerals and ions. This serves as a baseline for the dynamics that control heterogeneous nucleation.

The energetics of impurity incorporation on mineral surfaces is studied by employing a multistep free energy perturbation technique, which is applied on barite (BaSO₄) mineral using lead (Pb²⁺) and selenate (SeO₄²⁻) as impurities. When incorporated into the mineral, these impurities can either limit or enhance the rates by altering the activation barriers attributed to nucleation and growth. Our results show a more favorable incorporation of Pb²⁺ than SeO₄²⁻ into the barite lattice. Furthermore, there is a synergistic relationship in the incorporation of impurities where the Pb2⁺ enhances SeO₄²⁻ incorporation and vice-versa, which is reflected in the experimental measurements. Linking impurity incorporation to nucleation can either enhance or diminish the rates of nucleation/growth; for example, strontium (Sr) impurities have been shown to enhance the rate of BaSO₄ nucleation while simultaneously reducing the growth rates. This can variably change in complex natural systems, where abundant ions can affect nucleation. Therefore, it becomes imperative to understand how these individual ions affect nucleation.

In summary, the shift in paradigm from a thermodynamics ensemble average-based approach to a process-based framework to nucleation is highlighted in this work by probing critical processes, such as ion association, solvent exchanges mechanisms, and the effect of impurities in homogenous and heterogenous nucleation using a multifaceted set of advanced CMD techniques. This work provides a more holistic understanding of nucleation from a molecular perspective.