Near-Surface Free Energy Profile by Monte Carlo Simulation
Separation of analytes by supercritical fluid chromatography (SFC) relies fundamentally on molecular-scale interaction between analytes and the stationary phase, both bathed in the mobile-phase fluid. This project will calculate free energy as a function of the analyte's distance from the stationary phase's solid surface. The solid surface will be modeled as silica terminated with a mixture of trimethylsilyl and eighteen-carbon alkylsilyl groups. The mobile-phase fluid will be a mixture of carbon dioxide and methanol. Analytes studied initially will be methane, which is unretained in SFC, and benzene and naphthalene. Force fields for silica, silyl caps, carbon dioxide, methanol and methane have recently been gathered and refined for the SFC environment; force fields for benzene and naphthalene will be optimized against thermodynamic data. The Gibbs-ensemble Monte Carlo method, including hybrid molecular dynamics moves, will be used to equilibrate the system at fixed (T,P) conditions. The free-energy profile will be calculated by Jarzynski-equality nonequilibrium pulling moves, pulling the analyte's center of mass normal to the surface. The result sought is a free energy profile at a range of supercritical (P,T) conditions.
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