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Borah-Structure and Dynamics of Ionic Fluids Across Broad Temperature Regimes21.32 MB
Embargoed Access, Embargo ends: 06/29/2028
Dissertation
Structure and dynamics of ionic fluids across broad temperature regimes
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2026
DOI: 10.25820/etd.008361
Abstract
Ionic liquids and molten salts are two classes of ionic fluids composed entirely of charged species– cations and anions. They are primarily distinguished by their melting points: ionic liquids are salts with melting points below 100 ◦C, whereas molten salts typically melt only at much higher temperatures. These materials are central to technologies including electrochemical energy storage, separations, catalysis, and advanced nuclear reactor systems, where performance depends sensitively on how local structure and connectivity change with temperature. To address these points, this work combines molecular dynamics simulations with experimental comparisons to elucidate structure-dynamics relationships in ionic fluids over wide temperature ranges, with a particular focus on phosphonium-based ionic liquids and lanthanum-containing molten chlorides.
For ionic liquids, the central goal of my work is to understand how structural heterogeneity and molecular organization evolve on cooling toward the glass transition and how these changes affect dynamics. In phosphonium ionic liquids, combined molecular dynamics simulations, NMR, and scattering analyses reveal that polar and apolar regions do not slow down simultaneously on cooling. This behavior appears to be broadly general for systems with pronounced apolar character. Total-scattering experiments and simulations further reveal an additional intermediate-range feature in the structure function S(q) of [P666,14][NTf2] at q ≈ 0.5–0.6 Å−1 that emerges close to the glass transition and disappears at high temperature; analysis indicates that this peak arises primarily from correlations among short tails from neighboring cations which generate short-tail-driven intermediate-range order. Intrestingly, but not the focus of my work, this peak has been adscribed in experiments to a liquid-liquid phase transition.
Extending the idea of intermediate-range order and networks to the high-temperature inorganic melts, my collaborators and I have studied the network structure and speciation of La3+ in melts containing NaCl and KCl. When such multivalent metal ions are mixed with lower valency salts such as NaCl or KCl as “spacer salts”, a concept recently coined in our research group, a new peak (called a prepeak or a first sharp diffraction peak) appears in S(q). The results show that both the identity and concentration of the monovalent spacer strongly modulate structural heterogeneity and intermediate-range order, as reflected in changes to La–Cl coordination statistics and the low-q prepeak in S(q). In particular, KCl promotes a higher fraction of monomeric La-containing species and disrupts La–Cl networking more efficiently than NaCl, indicating that KCl acts as a stronger network breaker in these molten-salt mixture melts.
Details
- Title: Subtitle
- Structure and dynamics of ionic fluids across broad temperature regimes
- Creators
- Bichitra Borah
- Contributors
- Claudio J Margulis (Advisor)Johna Leddy (Committee Member)Scott K Shaw (Committee Member)Pere Miró (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Spring 2026
- DOI
- 10.25820/etd.008361
- Publisher
- University of Iowa
- Number of pages
- xx, 109 pages
- Copyright
- Copyright 2026 Bichitra Borah
- Language
- English
- Date submitted
- 04/24/2026
- Description illustrations
- illustrations, graphs, tables
- Description bibliographic
- Includes bibliographical references (pages 90-109).
- Public Abstract (ETD)
Ionic fluids are made entirely of positively and negatively charged ions rather than neutral molecules. Some of these fluids, known as ionic liquids, are in the liquid state at room temperature and have potential applications in batteries, capacitors, and catalysis. Others, known as molten salts, only become liquid at very high temperatures and are central to designs for next-generation nuclear reactors and large-scale thermal energy storage. Less is known about what happens to ionic liquids as they are cooled toward the glassy state, where ion motion becomes extremely slow. Likewise, in molten salts, it is not yet fully clear how mixing multivalent salts (salts of cations that have multiple positive charges) with simpler monovalent salts (like rock salt) reshapes their nanoscopic network structure and affects practical properties that can be relevant in applications.
This thesis tackles some of these open questions by investigating how the structure and relaxation dynamics of ionic liquids changes when these become cold and enter the glass regime, and how the structure of molten salts changes when multivalent components are mixed with monovalent “spacer” salts. By combining advanced computer simulations with experimental data derived from collaborations, the work explores how nanoscale organization and dynamics are linked within the same materials.
- Academic Unit
- Chemistry
- Record Identifier
- 9985176974402771
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