Monte Carlo Simulation of Near- and Supercritical Hexane Fluid and Physisorption Phase Behavior

Monte Carlo (MC) simulations were used to determine the near- and supercritical fluid phase behavior of hexane and the adsorption thermodynamics of hexane on cobalt. Canonical ensemble MC simulations were used to compute the equation of state and chemical potentials, and Gibbs ensemble MC simulations (GEMC) were used to compute fluid phase coexistence. Overall, MC results with the TraPPE-UA force field were successful in representing near and supercritical hexane thermodynamic behavior, and represent an improvement over treatments with a classical cubic equation of state. In addition, GEMC simulations predict a critical point for TraPPE-UA hexane of 509 K, 35 bar, and 0.22 g/mL, in excellent agreement with critical constants for real hexane. With regard to physisorption, hexane physisorption onto a (0001) cobalt surface at 523.15 K was modeled using grand canonical Monte Carlo (GCMC). GCMC excess adsorption results show crossover from adsorption to depletion around a bulk density of 0.1 g/mL and a global minimum in depletion at 0.44 g/mL. Energetically, GCMC results indicate that the energy of adsorption decreases with increasing bulk density. Both molecule loading and adsorption energy data suggest three distinct adsorption regions: (1) a low density (vapor -like) region, (2) a near-critical region with little to no change in loading or energy of adsorption, and (3) a high density (liquid -like) region.