MOLECULAR SIMULATION OF IONIC SPECIES: CORRELATING IONIC CHARACTERISTICS TO STRUCTURE, TRANSPORT AND MECHANICAL PROPERTIES

Azmain Faek Islam

Ionic materials, whether disordered liquids or ordered crystalline solids, exhibit properties that arise from the structure, charge distribution, and interactions of their constituent ions. This dissertation develops a unified atomistic framework to understand how ionic species control solvation, transport, cohesion, elasticity, and deformation across two technologically significant classes of materials: pyrrolidinium based ionic liquids (ILs) designed for extraterrestrial metal extraction within In Situ Resource Utilization (ISRU), and rare earth doped cerium oxide (CeO2) ceramics intended for high performance structural and functional applications. Although these systems differ in phase and degree of order, both require a clear linkage between local ionic environments, including charge density, ionic radius, defect topology, and molecular or lattice symmetry, and the macroscopic properties that govern their overall behavior.In the first part of this dissertation, extensive classical molecular dynamics simulations are used to investigate the solvation structure and dynamics of Na⁺, Mg2⁺, and Al3⁺ ions in two ILs, [mppy][TFSI] and [mppy][HSO4]. Radial distribution functions, coordination statistics, potentials of mean force, and residence time analyses collectively reveal that cation charge density and anion geometry strongly influence the compactness, rigidity, and temporal stability of the solvation shells. The trivalent Al3⁺ ion forms the most tightly bound and persistent coordination environment, while monovalent Na⁺ exhibits the greatest structural flexibility. Between the two ILs, the rigid tetrahedral HSO4- anion produces stronger binding, higher free energy barriers, and longer residence times compared to the more flexible TFSI anion. These results provide molecular level insight into the factors governing ion encapsulation and mobility in ILs and establish design rules for selecting ILs capable of efficient metal extraction from lunar and Martian regolith under the extreme and water limited conditions relevant to ISRU. The second part of the dissertation extends this ion centered perspective to the ordered fluorite lattice of CeO2 and examines how aliovalent dopants such as Eu3+, Gd3+, and Y3+, as well as isovalent Zr4+, modify the mechanical properties and strain driven phase behavior of the solid. Large scale molecular dynamics simulations show that the elastic constants C11, C12, and C44 and the bulk modulus decrease systematically with increasing dopant concentration. This trend is driven by the formation of oxygen vacancies, local lattice distortion, and reductions in cohesive energy. Analysis of local potential energy distributions demonstrates significant heterogeneity arising from dopant vacancy clustering and the resulting charge redistribution. Complementary density functional theory calculations confirm vacancy induced bond asymmetry and its influence on local stiffness. Under uniaxial tension, doped systems display distinct pathways of stress localization, defect mediated softening, and strain induced structural transitions. These behaviors are captured using Polyhedral Template Matching and strain dependent radial distribution analysis. Higher dopant concentrations reduce ductility, while moderate concentrations broaden the plastic regime and alter the evolution of low symmetry phases relative to pure CeO2. These studies provide a coherent mechanistic description of how ionic species, whether solvated within complex liquids or embedded within crystalline solids, govern structural organization, energetics, and macroscopic mechanical response. By integrating multiscale simulation methods, this dissertation advances fundamental understanding that supports the rational design of ILs for extraterrestrial metal extraction and rare earth doped oxide ceramics for advanced mechanical and functional technologies.

MOLECULAR SIMULATION OF IONIC SPECIES: CORRELATING IONIC CHARACTERISTICS TO STRUCTURE, TRANSPORT AND MECHANICAL PROPERTIES

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