Nature of Electrostatic Interactions in Macromolecular Systems

Author: Walker, Pierre J.

Year: 2027

Degree: Dissertation (Ph.D.)

Advisor: Wang, Zhen-Gang

Committee Members: Manthiram, Karthish; Greer, Julia R.; Fong, Kara D.; Wang, Zhen-Gang

Option: Chemical Engineering

Abstract

Electrostatic interactions govern the organization and motion of charged matter across molecular and macroscopic length scales. In macromolecular systems, their long-ranged and collective nature couples molecular conformation, mobile-ion organization, solvent response, chemical equilibrium, and material dynamics. Consequently, the behavior of charged materials cannot generally be inferred from charge magnitude, pair interactions, or static molecular structure alone. This thesis combines physically grounded theory with molecular simulations to isolate these couplings and establish transferable relationships between microscopic electrostatic interactions and macroscopic material behavior.

The first part maps the thermodynamic landscape of charge-containing systems. For polymer--ionic liquid mixtures, we show that electrostatic correlations produce phase-envelope asymmetries and molecular-weight scaling laws that differ from classical Flory--Huggins predictions, enabling the development of a minimal model with physically interpretable parameters. We then challenge the treatment of solvents as passive dielectric continua. An explicit-solvent liquid-state theory and molecular simulations demonstrate that dipolar polarization is an active thermodynamic degree of freedom: ion--dipole correlations alter dielectric screening, reverse the temperature dependence of effective electrostatic interactions, and generate both upper-critical-solution-temperature behavior and inverted salt partitioning in polyelectrolyte coacervates. The effects of molecular architecture are subsequently examined through charge sequence and polymer topology. For polyampholytic intrinsically disordered proteins, charge blockiness promotes salt-free coacervation, whereas net charge suppresses it through electrostatic repulsion and counterion entropy. Added salt can nevertheless screen these repulsions and induce phase separation over a broad region of sequence space. For branched polyelectrolytes, compact topologies display an enhanced propensity for coacervation, while differences in topology alone can drive multiphase separation.

The second part examines the dynamic pathways by which charge, matter, and stress move and relax within these thermodynamic states. In ionic-liquid--solvent mixtures, all-atom simulations and analytical theory reveal that ion dissociation is controlled by chemically specific short-range interactions rather than bulk dielectric constant alone. Moreover, ion-pair populations do not uniquely determine conductivity, which depends on collective relaxation, electrophoretic, viscous, and ion-asymmetry effects. In charge-regulating polyelectrolyte hydrogels, dynamic protonation can enhance multivalent-ion diffusion by an order of magnitude through a protonation-assisted hopping mechanism, thereby decoupling ionic mobility from equilibrium binding. Finally, a multiscale framework combining quantum chemistry, thermodynamic theory, and coarse-grained simulation connects metal--ligand coordination to the phase behavior and mechanics of metallo-polyelectrolyte complexes. Their mechanical response is governed not by binding strength alone, but by competition among interchain bridges, intrachain loops, crosslink persistence, and network relaxation.

Together, these results establish that the properties of charged materials emerge from the collective organization and motion of ions, solvents, and macromolecular architectures, providing practical design principles for ionic liquids, biomolecular condensates, responsive hydrogels, and other charge-containing soft materials.