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45276-GB5
Molecular Dynamics Modeling of the Gas-Liquid Interface Using Self-Assembled Monolayers

Henry J. Castejon, Wilkes University

A Au surface has been simulated using a 3-layer slab of 108 atoms. The computer code was actually written to allow for any number of atoms to be used. However, we have found that 108 atoms provide a thermal bath large enough to represent a realistic surface. By applying random and friction forces to the atoms in the lowest layer of the slab, the temperature of the surface can be set to any desired value. This allows to study the effect of the temperature in the energy exchange processes. The monolayer was initially simulated using unified-atom potentials. However, we have been able to write code that uses all-atoms potentials. These include bond, angular and torsional potentials for intramolecular interactions. The intermolecular interactions are calculated using a Lennard-Jones potential. All interactions include the Ewald sum method to account for the electrostatic forces within and between molecules.

Currently, our computer program can simulate a metallic surface and a liquid phase containing thiol molecules, both system in thermodynamic equilibrium at constant temperature. The program is ompleted and tested. It is worth mentioning that the code is written in FORTRAN 90 which will make it easier to be parallelized to run in any computer cluster.

Since the interaction potential within the monolayer determines its rigidity, its parametrization in important. A considerable amount of time has been invested in that parametrization. The next step is to simulate the thermodynamic systems interacting in a stable stationary state. Then, proceed with the scattering from the monolayer.

The main problem we are facing now is that to account realistically for the electrostatic interactions in the monolayer, we need to use the Ewald sum method. The method requires complete periodicity in all dimensions. Since we are dealing with an interface, it is physically impossible to have complete periodicity in all directions. We are exploring some possible solutions to this problem.

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