Modeling coacervation in charged macromolecular systems

Madinya, Jason

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https://hdl.handle.net/2142/116092 Copy

Description

Title

Modeling coacervation in charged macromolecular systems

Author(s)

Madinya, Jason

Issue Date

2022-07-13

Director of Research (if dissertation) or Advisor (if thesis)

Sing, Charles E

Doctoral Committee Chair(s)

Sing, Charles E

Committee Member(s)

• Leckband, Deborah E
• Peters, Baron G
• Gruebele, Martin

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Polymer Physics
• Computational
• Soft Matter
• Polyelectrolytes

Abstract

Complex coacervation is a charge-driven liquid-liquid phase separation that occurs in aqueous solutions of charged macromolecules such as polyelectrolytes, polyampholytes, charged surfactant-micelles, and charged proteins. This phase separation leads to the formation of macromolecule-rich phase known as the coacervate phase and a macromolecule-dilute phase known as the supernatant phase. Complex coacervation has been studied for the development of materials for applications such as personal care products, food prodcucts, and biotechnology. They are also used as motif for understanding liquid-liquid phase separation in biological systems. Recently there has been considerable interest in understanding the formation of biomolecular condensates, which are phase separated droplets of biomacromolecules that form membrane-less organelles in the cell. These condensates serve to organize and compartmentalize the cellular environments and are crucial for the chemical processes that occur in the cell. Coacervates are used as a proto-model for phase separation in the cell, as it typically involves charged biomacromolecules. Coacervates can be made from a wide variety of different types of macromolecules, and the properties of the resulting phase-separated solutions can be engineered through modification of molecular-level properties such as charged surfactant fraction, polyelectrolyte linear charge density, degree of polymerization, inclusion of functional groups, charge sequencing, and many other features. Engineering coacervate solution properties through tuning of the physical molecular features requires knowledge of the relationship of those features on phase separation. The modeling efforts presented in this dissertation focuses on understanding how molecular-level features impact the bulk solution behavior in a variety of macromolecular systems. Most theoretical models and simulations focus on coacervation in polyelectrolytes, however there are many other macromolecules with more complex architectures and features that can undergo coacervation. In this work, we present models for coacervation in two macromolecular systems. The first system involves charge-sequenced polyampholytes in solution, where the polyampholytes undergo self-coacervation as they contain both positive and negative charges to allow for coacervation. The second system involves polyelectrolytes in solution with surfactants, where the surfactants have self-assembled into disordered wormlike micelles. Our work makes use of a hybrid simulation and theory approach, where we use established theoretical models that are informed by simple Monte Carlo simulations. For the polyampholyte system, we sought to model the effects of the charge sequence on phase separation. We used the transfer matrix theory that was developed in previous work from the group, and we modified the theory to account for sequenced charges along the polyampholyte. This required the use of MC simulations to evaluate the terms within the transfer matrix that were sequence-specific and had no analytical approximation. We showed that polyampholyte sequences with atleast 8-12 adjacents like-charges are able to under self-coacervaton. Sequences with more consecutive like-charges, or blockier sequences, show a stronger driving force for coacervation. These blocks of like-charges are able to localize more ions from solution to reduce the net-charge of the polyampholyte, therefore there are more ions released into the solution upon complexation with another polyampholyte, leading to an increase in entropy. We have identified that charge-sign interfaces, where a monomer is of the opposite charge of its neighbor, disrupts the ion localization onto that part of the chain, which reduces the driving force for coacervation. This effects is similar, however more pronounced, than the chain-end effects that also reduce ion localization. In the surfactant-micelle and polyelectrolyte solution model, we used Self-Consistent Field Theory to model the polyelectrolytes in solution. The polyelectrolyte interact with the surfactant-micelles through external potentials that correspond to the polymer-micelle surface interaction as well as the polymer-micelle interior interaction. To construct these external potentials, the surfactant micelle structures were predetermined using Monte Carlo simulations. Equilibrated snapshots of these simulations were discretized where the gridpoints were evaluated to determine their location on the surfactant micelles, including the micelle surface, micelle interior and micelle exterior. With the external potentials, the SCFT equations were evaluated numerically and free energy manifolds for the solution were constructed. Phase diagrams were determined from the free energy manifolds, and the effects from several molecular parameters on the solution behavior were evaluated. The molecular parameters include the degree of polymerization of the polyelectrolyte, the surfactant micelle surface concentrations, as well as the polymer-micelle binding energy. The surfactant-micelle and polyelectrolyte solution model was modified to evaluate the effects of micelle surface charge density as well as introduce steric effects that can be induced experimentally by grafting PEG chains on to the micelle surfaces. Our collaborators at the Perry Group from the University of Massachusetts in Amherst, measured the effects of micelle surface charge density and the effect of the number of repeat units in the grafted PEG chains on the phase separating behavior. Results from our model showed qualitative consistency with the results obtained through experiments. The hybrid MC - SCFT model can be modified to account for other surfactant-micelle architectures, and it can also be used to model solution behavior in solutions with other types of charged macromolecules. The hybrid simulation and theory approach has provided models that are able to qualitatively match experiments, provide physical insight into the systems, and are computationally accessible.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

© 2022 Jason Madinya

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