Charge Patterning and Molecular Interactions in the Phase Behavior of Polyelectrolyte/Particle Solutions

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

NON-TECHNICAL SUMMARY This award supports theoretical and computational research and education in the fields of polymer physics, thermodynamics, and computer simulations of complex systems. Consumer products and biological structures within the cell both rely on the formation of liquid droplets within another liquid media, known as 'liquid-liquid phase separation'. In biology, this leads to compartments that can organize the interior of the cell. In industry, a similar effect is used to control the 'feel' or texture of personal care or food products. The physical reason for these phase separation phenomena is complicated but is often attributed to several types of large molecules that interact with electrostatic charge. Many of these systems include particles such as folded proteins, which contain nanometer-scale charged patches that interact with long-chain, flexible molecules. The PI's group will investigate how the surface of these charged particles affects liquid-liquid phase separation. This will consist of understanding the 'stickiness' between positively-charged molecular chains called polycations, and negatively-charged molecules that form nanoscale structures similar to those found in soap, called surfactant micelles. The polycations and surfactant micelles stick together in a dynamic, gel-like material known as a complex coacervate. This sticking will be highly dependent on the patchiness of the micelles, and the presence of molecules that get in the way of the charged interactions. The PI's group will establish how different patches, and molecular components will affect the propensity to undergo liquid-liquid phase separation. This work will show how complicated biological systems, such as patchy proteins, can form similar structures in the cell. This work will also inform the design of consumer products, such as shampoo, cosmetics, and perfumes, and will guide engineers in navigating a complicated design space as they seek to use greener and healthier formulations. The integrated education and outreach component of this project supports broader outreach to graduate and undergraduate research training and mentorship. Outreach efforts consist of using interactive computer simulation as the centerpiece of a PI-designed activity within the St. Elmo Brady STEM Academy at the University of Illinois. This project will build up demonstrations needed to understand more 'advanced' aspects of existing efforts, by developing modules that introduce the concepts of (macro)molecules and their corresponding materials. The overall activity will introduce the lifecycle of plastics and sustainability to elementary-age students. TECHNICAL SUMMARY This project will use simulation and polymer field theory to study how molecular interactions and charge patchiness affect phase separation in polyelectrolyte/particle solutions. A wide variety of materials systems, from consumer products to phase-separated regions in the cell, rely on charge-driven phase separation. This process of 'coacervation' can be driven by the attraction between oppositely-charged polyelectrolytes and particle species such as surfactant micelles or proteins. Taking cues from biology, the PI will establish how charged interactions and patchiness control phase behavior in PE-particle coacervates. In this work, the PI will systematically study the relationship between molecular structure, particle configuration, and non-uniform charge patterning. This investigation will build on an established hybrid field theory model, with parameters informed by particle-based simulations to understand the strong, polyelectrolyte-mediated electrostatics between nearby charged particles. This model will be used to first probe the effect of steric repulsion and hydrophobic interactions on surfactant-polyelectrolyte coacervates, and then charged patterns will be introduced onto particles to represent catchy particles or proteins. This project will establish the theoretical and computational basis for understanding coacervates formed between polyelectrolytes and a variety of particles that are not flexible polymers, yielding new insights into the role of charged patterning and surface interactions on bulk phase behavior. This will have impact on materials important for a wide range of consumer applications and biological systems. Outreach and education are also an integral part of this research project, which will support the interdisciplinary training of at least one graduate and one undergraduate researchers. The PI's group will develop outreach activities, further integrating computation into existing, successful programs such as the St. Elmo Brady STEM Academy. The goal will be to design interactive computational activities to increase student engagement with difficult and abstract concepts in plastic life cycles and recycling. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.