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Published April 22, 1998 | Published
Journal Article Open

Binary nucleation of sulfuric acid-water: Monte Carlo simulation


We have developed a classical mechanical model for the H2SO4/H2O binary system. Monte Carlo simulation was performed in a mixed ensemble, in which the number of sulfuric acid molecules is fixed while that of water molecules is allowed to fluctuate. Simulation in this ensemble is computationally efficient compared to conventional canonical simulation, both in sampling very different configurations of clusters relevant in nucleation and in evaluating the free energy of cluster formation. The simulation yields molecular level information, such as the shape of the clusters and the dissociation behavior of the acid molecule in the cluster. Our results indicate that the clusters are highly nonspherical as a result of the anisotropic intermolecular interactions and that a cluster with a given number of acid molecules has several very different conformations, which are close in free energy and hence equally relevant in nucleation. The dissociation behavior of H2SO4 in a cluster differs markedly from that in bulk solution and depends sensitively on the assumed value of the free energy f(hb) of the dissociation reaction H2SO4+H2O-HSO4-. H3O+. In a small cluster, no dissociation is observed. As the cluster size becomes larger, the probability of having an HSO4-. H3O+ ion pair increases. However, in clusters relevant in nucleation, the resulting ion pairs remain in contact; about 240 water molecules are required to observe behavior that resembles that in bulk solution. If a larger value of f(hb) is assumed to reflect its uncertainty, the probability of dissociation becomes negligible. A reversible work surface obtained for a condition typical of vapor to liquid nucleation suggests that the rate-limiting step of new particle formation is a binary collision of two hydrated sulfuric acid molecules. The ion pairs formed by dissociation play a key role in stabilizing the resulting cluster. The reversible work surface is sensitive to the assumed value of f(hb), thus pointing to the need for an accurate estimate of the quantity either by ab initio calculations or experiments.

Additional Information

© 1998 American Institute of Physics. Received 11 November 1997; accepted 22 January 1998. The authors wish to express gratitude to Dr. Tsan H. Lay for helpful discussions regarding the free energy of the reaction Eq. (9) and performing ab initio calculation for us and to Dr. Markku Kulmala for providing them with the program to perform a calculation using the classical theory. This work was supported by National Science Foundation Grant No. ATM-9614105. Simulation is performed in large part using the parallel computer system operated by Caltech Center for Advanced Computing Research (CACR). Z.-G.W. acknowledges support from the Camille and Henry Dreyfus Foundation, the Alfred P. Sloan Foundation, and the National Science Foundation. We also acknowledge support by a grant from the Mobil Corporation.

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