Pulling Simulations for Predicting the Interaction Properties Between Molecules
Open Access
- Author:
- Mkandawire, Wezi
- Area of Honors:
- Chemical Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Scott Thomas Milner, Thesis Supervisor
Scott Thomas Milner, Thesis Honors Advisor
Michael John Janik, Faculty Reader - Keywords:
- pulling simulation
Flory-Huggins
molecular dynamics
free energy
mixing - Abstract:
- Quantifying the free energy of chemical mixtures by simulations is hard. It often requires complex computational techniques and isn’t applicable to several systems of interest. Here, we present a simple and experimentally validated molecular dynamics (MD) simulation method used to compute the interaction parameter χ. In addition, the system’s chemical potential, mixing free energy, and osmotic pressure can be obtained. Using an applied harmonic potential, molecules within a system (A and B) are pulled to either the middle or periphery of a simulation box. In essence, this “pulling” method shows how well two molecules mix. The harmonic potential also reveals a concentration profile that can subsequently be used to compare simulation data with the regular solution and Flory-Huggins free energy models. Two systems are utilized to highlight the power of the method: (1) PEO chains in solution and (2) a benzene and pyridine regular solution mixture. The PEO chains are placed under a harmonic potential that pulls them into the middle of a box while the chains are stretched at varying end-to-end chain tensions. By then computing the simulated osmotic pressure, the χ parameter at different concentrations and tensions is calculated. This predicts the phase separation of PEO chains in solution when placed under strong flow. Likewise, benzene and pyridine are placed under equal and opposite harmonic potentials. By minimizing the free energy, we obtain the harmonic potential as a function of concentration U(X), or the chemical potential, which is used to obtain χ by comparison to simulated data. The χ value is then compared to previous simulated and experimental values. Thus, the pulling method proves to be a rather fast and general method to calculate the mixing free energy.