Modeling Macroions – Filling the Gap Between Ions and Colloids

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With the support of the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program of the Chemistry Division, Mesfin Tsige of the University of Akron is using computer simulation to model the self-assembly of very large molecules with multiple charges, called macro-ions. When placed in solution, coulombic forces between charge sites cause macro-ions to self-assemble into large complex structures. Dr. Tsige and his students will study the assembly process using computer models that track the motion of hundreds of thousands to millions of atoms. Their discoveries help interpret experimental observations and could also impact our understanding of the self-assembly of other important macro-ions, such as proteins and DNA. The project will help to train the next generation of scientists by providing research opportunities for graduate students, as well as undergraduate students through the NSF-REU center at the School of Polymer Science and Polymer Engineering, and high school students from the local St. Vincent-St. Mary School. In addition, Dr. Tsige and his group visit local schools that serve large numbers of minorities, introducing their students to scientific concepts and encouraging them to pursue careers in STEM. Computer simulation of macro-ion self-assembly is challenging due to the large numbers of atoms involved and long-range nature of coulombic forces. A typical simulation tracks the self-assembly of 25-50 macro-ions, which are surrounded by thousands of counterions and millions of solvent molecules, into large 'blackberry-like' structures. Dr. Tsige uses molecular dynamics simulations with a versatile coarse-grained model in which groups of atoms are combined and represented by beads. Their approach reduces the size of the system, making the simulation of large assemblies possible, while retaining molecular-level detail. They will observe the initial stages of assembly formation to develop a fundamental understanding of how self-assembly is affected by macro-ion size, charge density, and solvent polarity, as well as the presence of counter ions and co-ions. The project will complement experimental studies of macro-ion assembly by providing insights that are difficult or sometimes impossible to get from laboratory measurements.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.