Nucleation Simulation Studies of Propane Vapour Condensation

Molecular dynamics (MD) simulations are used to study the nucleation kinetics of propane condensation. Using the TAMie potential for propane, equilibrium MD simulations are performed first to calculate the pressure-volume equation of state for the model in the metastable region over a range of temperatures. The resulting data shows good agreement with the predictions of the Peng equation of state (EOS), which makes the Peng EOS suitable to calculate the theoretical predictions of the binodal and spinodal. The nucleation behaviour, including the rate, free energy barrier, Zeldovich factor, and critical cluster size calculations, are obtained using the mean first passage time (MFPT) method over a range of decreasing temperatures that approach the calculated mean field spinodal, at a fixed system volume. As the temperature decreases, the barrier decreases and the sigmoidal MFPT behaviour associated with classical nucleation crosses over to a linear, growth-dominated regime. The MFPT calculations are compared with the classical nucleation theory (CNT), which shows that all the CNT critical sizes are smaller than those calculated from simulation. The nucleation rate and nucleation lag times are also obtained using the survival probability (SP) method. The nucleation rate calculated using the MFPT, CNT and the SP methods are all in very good agreement. The effect of the initial cluster distribution on the measured nucleation rate is also examined by selecting two ensembles of starting configurations from T = 800K and T = 340K. There is no distinguishable difference in the nucleation rates obtained using the different initial conditions. However, the nucleation lag times obtained from T=340K initial condition are consistently smaller. We also compared the results obtained from two commonly used cluster criteria, the Stillinger criterion and the Frenkel criterion. No significant difference in nucleation rates is detected given the statistics obtained in this study