AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Novel Methods for Determining Free Energies of Molecular Clusters of Water and Sulphuric Acid
Gabriel Lau, Jake Stinson, Shawn Kathmann, IAN FORD, University College London
Abstract Number: 543 Working Group: Advances in the Physics and Chemistry of New Particle Formation and Growth
Abstract A microscopic model of the nucleation of atmospheric particles must involve the following elements: (a) the identification of the molecular species that form the critical cluster under prevailing conditions; (b) a force field that is capable of representing the interactions between these species; and (c) a procedure for calculating growth and evaporation rates of clusters, and combining them in a suitable kinetic framework, bearing in mind that their structures are disordered, and their evaporative lifetimes are relatively short. With regard to (a), the consensus view is that sulphuric acid is the central species in the process, condensing in association with water, ammonia, amines and perhaps other molecules. But the practical implementation of elements (b) and (c) is less settled. We shall describe the development of a new empirical valence bond (EVB) force field suitable for sulphuric acid/water mixtures that allows for the possibility of proton transfer. The scheme is classical and therefore computationally cheap: it is essentially an empirical fit to higher level force fields but crucially includes mixing between patterns of molecular bonding. Furthermore, we have developed a mechanical procedure of cluster disassembly in molecular dynamics that provides an estimate of the cluster excess free energy. This is a generalisation of a surface term and is a thermodynamic equivalent of the relevant growth and evaporation rates that lie at the heart of nucleation kinetics. The excess free energy may be related to the nonquasistatic mechanical work of disassembling the cluster into separate molecular constituents, by way of the Jarzynski equality. In contrast, it is more usual to determine free energy differences associated with individual growth and evaporation events, or to employ a high temperature reference state for each cluster, in comparison with which the disassembly method is more direct. We shall describe the essential ideas behind these developments.