Simulating vapor–liquid nucleation of n-alkanes

Abstract

A combination of the aggregation-volume-bias Monte Carlo algorithm, the configurational-bias Monte Carlo algorithm, and the umbrella sampling technique was applied to investigate homogeneous vapor–liquid nucleation in ethane, n-butane, and n-heptane. The simple transferable potentials for phase equilibria-united atom (TraPPE-UA) force field was used in this investigation. It was found that for the n-heptane case, the TraPPE-UA force field predicted a nucleation rate that is about three to four orders of magnitude higher than that measured by an upward thermal diffusion cloud chamber experiment. Comparison of the simulation results to the classical nucleation theory (CNT) shows that CNT consistently overestimates the barrier heights for all chain lengths investigated. The offset on the barrier heights was found nearly independent of the supersaturation for both ethane and n-butane, similar to a Lennard-Jones system previously studied. This also directly leads to a good agreement on the cluster sizes between the simulation and the CNT calculated from the nucleation theorem. For n-heptane, however, the offset was found to depend on the supersaturation. It appears that CNT predicts a slightly weaker dependence of the nucleation rate on supersaturation, which agrees with both density functional calculations and the experiments. Structural analysis demonstrates that the orientational order near the surface differs significantly between the critical nucleus and the bulk planar liquid–vapor interface for n-heptane systems, whereas the density in the interior of the critical nucleus is in good agreement with the bulk liquid density. The different surface ordering offers a microscopic explanation for the differences observed for n-heptane between the CNT on one side and experimental observations and simulations on the other side.