Public defence in Engineering Physics, M.Sc. Hossein Vahid Dastjerdi

Title of the thesis: Modeling of charged polymers in aqueous solutions in the presence of salt ions and substrates

Doctoral student: Hossein Vahid Dastjerdi
Opponent: Senior Research Associate Matej Kanduč, Jožef Stefan Institute, Slovenia
Custos: Professor Tapio Ala-Nissilä, Aalto University School of Science, Department of Applied Physics

Polyelectrolytes (PEs) are charged macromolecules. They interact with surrounding ions in aqueous solutions and are pivotal in various scientific and industrial applications. Examples of PEs include biologically relevant molecules such as DNA, RNA, and proteins, as well as synthetic PEs like poly(styrene sulfonate) and poly(diallyl dimethyl ammonium chloride). The PE interactions are governed by a complex interplay of electrostatic, steric, and interfacial dielectric effects, making it essential to develop a comprehensive understanding of these processes at the molecular level.

This thesis aims to advance our understanding of PE-ion interactions through theoretical models and molecular dynamics simulations. A soft-potential-enhanced Poisson-Boltzmann model was developed and optimized using coarse-grained and atomistic molecular dynamics simulations. The model effectively incorporates ion size effects in monovalent salt solutions, providing accurate predictions of ion distributions around rodlike PEs. Beyond mean-field theory, coarse-grained simulations reveal the impact of multivalent ions and varied ion shapes on PE interactions, demonstrating how ion properties and concentrations control interaction ranges. These findings offer insights for designing PE-based systems with applications in drug delivery, material science, and other fields.

Moreover, this research explored the role of ion valency in electrophoretic mobility (EM). While counterion valency is traditionally considered the dominant factor, we showed that coion valency significantly influences EM. Increasing coion valency in high-salt environments can even reverse EM.

Finally, we examined how dielectric discontinuities between solvents and solid substrates affect PE adsorption. Factors such as salt concentration, ion valency, and PE charge emerge as critical in determining whether PEs are attracted to or repelled from substrates. This understanding provides practical guidelines for optimizing PE adsorption and monolayer formation on neutral and polarizable surfaces.

Overall, this thesis contributes to a deeper understanding of the mechanisms governing PE interactions in complex environments, offering strategies for controlling and optimizing PE behavior in applications.

Key words: Polyelectrolyte, adsorption, salt, ion valency, ion shape, dielectric constant, electrophoretic mobility, Poisson-Boltzmann theory, molecular dynamics simulation

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact information:

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52 

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