Date of Award
2026
Document Type
Open Access Dissertation
Degree Name
Doctor of Philosophy in Physics (PhD)
Administrative Home Department
Department of Physics
Advisor 1
Issei Nakamura
Committee Member 1
Ranjit Pati
Committee Member 2
Raymond A. Shaw
Committee Member 3
Gregory M. Odegard
Abstract
Ionic liquids (ILs) and polymerized ionic liquids (polyILs) are charged liquids whose properties have spurred applications from energy storage to separations and catalysis. However, their high viscosities hamper ion transport and processing. Unravelling viscosity's molecular origins in these materials is challenging because experiments cannot separately tune electrostatic, steric, and chain-connectivity effects, and atomistic simulations are computationally expensive. In this dissertation, we present coarse-grained molecular dynamics simulations of ILs and polyILs based on the Stockmayer fluid (SF) model, which treats ions as spheres interacting via a Lennard-Jones potential with an embedded point charge and a permanent dipole moment. This description accounts for repulsion, dispersion, electrostatic interactions, and molecular polarization, enabling systematic exploration of the molecular factors governing viscosity.
We first parameterize a SF model for ethylammonium nitrate (EAN) IL to reproduce experimental density, viscosity, ionic conductivity, diffusion coefficient, and glass transition temperature. Using this optimized model, we explore how ionic charge, dipole moment, and ion size affect IL viscosity. Viscosity grows monotonically with charge and dipole moment, while its dependence on ion size is non-monotonic, reflecting competition among electrostatic interactions, molecular packing, and cation-anion size asymmetry. Ion-pair lifetimes track viscosity trends, showing that longer-lived associations yield higher viscosity.
We then extend the SF model to polyILs, tethering one ion species (cations) to the side chains of a flexible polymer backbone, and examine how cation size, dipole mobility, and chain length affect viscosity, ion-pair dynamics, and chain relaxation. Decreasing cation size and restricting dipole mobility enhance electrostatic correlations, raise viscosity, and prolong ion-pair lifetimes. Viscosity shows only weak chain-length dependence, whereas chain orientational relaxation scales strongly with molecular weight. Decomposing the Green-Kubo viscosity integral shows viscous behavior is dominated by ion-pair and slow stress-relaxation processes occurring before full chain reorientation, explaining why viscosity is largely insensitive to chain length despite strong chain-length dependence of orientational relaxation.
Overall, by systematically isolating individual molecular parameters in a coarse-grained SF model, this work clarifies the interplay between electrostatic interactions, steric constraints, and polymer chain dynamics underlying the high viscosity of ILs and polyILs, offering quantitative guidance for designing soft ionic materials with tailored viscosities.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Recommended Citation
Itliong, Jester N., "MOLECULAR ORIGINS OF VISCOSITY IN IONIC AND POLYMERIZED IONIC LIQUIDS: INSIGHTS FROM STOCKMAYER FLUID SIMULATIONS", Open Access Dissertation, Michigan Technological University, 2026.
https://digitalcommons.mtu.edu/etdr/2165