Energy of wall-particle interactions over time Computational Adsorption of CO2 and N2 in Graphite Emily Koivu Emory University This Photo by Unknown author is licensed under CC BY-SA. introduction Procedure results      The Van der Waals force is described as the distance- dependent combination of attractive and repulsive pulls on any two particles. We use the Lennard Jones potential,  as an empirical model for the potential from these intermolecular interactions.      The epsilon and sigma values are individually fit to the two particle types, for example CO2 to N2 or CO2 to carbon. This differing interaction strength leads to the phenomena studied: adsorption.       Particles want to minimize potential energy, so was we place a mixture of gases in a box with a substrate, the particles will adsorb into a monatomic film on top of a substrate. Energy of wall-particle interactions over time      To run the simulations, 50 CO2 and 50 N2 particles are placed in a box with soft walls in the z direction. The particles are given one million timesteps of fs to come into thermodynamic equilibrium.       After the gases are mixed, the bottom wall is given a strong LJ interaction coefficient (3 kcal/mol). Molecules are given three million timesteps to adsorb and then the simulation is stopped. We see that CO2 interacts strongly with the wall as particles are trapped. N2 has a much weaker interaction with the wall. RESULTS Further work                             Because the strength of interactions for CH4 and N2 are different, adsorption will be used to separate the gas mixture into its  two components. Instead of using hypothetical values for the soft wall, include values that correspond to interactions with graphite Analyze the selectivity of CO2 and N2 on the graphite slab Test different structures like nanoribbons to increase selectivity of target gases Potential to use different substrate materials such as gold Looking at output images show the thin layer of molecules adsorbed on the wall, as expected. Methods Number of Adsorbed Molecules at Different Temperatures References We will be doing Molecular Dynamics simulation  Simulations are run using the LAMMPS program by the Sandia National Labs CO2 is modeled as a rigid body  and N2 as a spherical atom Simulations assume a constant temperature, number of particles, and volume of simulation box Plimption,S. Et al, (2018). Sandia National Labs: LAMMPS. Available from www.lammps.sandia.gov Naeem, R. (2019, June 05). Lennard-Jones Potential. Retrieved June 17, 2019, from https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Map s/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/At omic_and_Molecular_Properties/Intermolecular_Forces/Specific_Interactions/Lennard- Jones_Potential Filer, D. (2017, February 03). Nitrogen Gas Molecule. Retrieved from http://nitrogengassanroso.blogspot.com/2017/02/nitrogen-gas-molecule.html Acknowledgements Financial support from the REU Site in Physics at Howard University NSF Award PHY 1659224 is gratefully acknowledged. Direction from PI, Dr. Silvina Gatica, has contributed to the success of this research. This figure shows the number if adsorbed molecules  at 150K, 260K, and 400K. The CO2 moleuces were more responsive than the N2 molecules at all temperature; temperature did not seem to afftect the CO2 adsorbtion rate. The N2 was more easily adsorbed at lower temperature.  18 June 2019