Molecular dynamics simulations of homogeneous nucleation of liquid phase in highly supersaturated propylene glycol vapors

Abstract

The homogeneous nucleation rate of the liquid phase from a supersaturated propylene glycol vapor is a key factor in aerosol formation from condensation aerosol generators. The droplet nucleation rate is difficult to measure experimentally at high supersaturations. This paper demonstrates the feasibility of using classical Molecular Dynamics (MD) simulations to describe the nucleation of propylene glycol from the vapor phase in air at 320 and 343 K. After an initial induction period, nucleation was observed in many of the scenarios studied here. The classical nucleation theory underestimates the nucleation rate compared to the MD simulation results at high supersaturations. However, a kinetic model that assumed no free energy barrier to nucleation had better agreement with the MD predictions at extremely high supersaturations. For systems where a quasi-steady state was reached during the simulation, it is possible to determine the nucleation free energy as a function of cluster size. From this analysis it is found that the MD simulations predict over a factor of two larger critical clusters and about 50% higher free energy barriers than the classical nucleation theory.