Kinetic stability of complex molecular clusters

Abstract

This investigation is concerned with modeling the evaporation, or decay, of n-nonane molecular clusters. We use a unique cluster decay model that was first developed to estimate the decay time scale of argon clusters using molecular-dynamics simulations. In this study we seek to enhance the model so that it represents a more complex cluster decay dynamic, suitable for n-nonane clusters. Experimental measurements of nucleation rates of n-nonane droplets have been used to deduce the rate at which a molecule escapes from the cluster. Typically for an n-nonane cluster containing 40 molecules, at an experimental temperature of 225 K, the empirical decay time, which is the inverse of the decay rate, is estimated to be 50 ns. For this time scale, the direct observation of n-nonane cluster decay from a molecular-dynamics trajectory is not feasible, since decay events are so rare. However, the cluster decay model uses a combination of molecular dynamics and stochastic dynamics in order to resolve the problem associated with long decay time scales. The model is based on a Langevin treatment that views cluster decay as single-particle escape from a confining potential of mean force. It is used to predict kinetic decay times of n-nonane clusters. We discover this result differs significantly from a classically derived decay time scale determined from a continuum thermodynamic treatment of the population balance equations of clusters. However, the dynamically generated results obtained from the kinetic decay model compare more favorably than the classical results with the empirical decay times that are deduced from experimental measurements of n-nonane clusters.