Cluster Structure and Ordering in the Nucleation and Growth of Binary Molecular Mixtures.

Abstract

A complete understanding of aerosol formation remains elusive due to the microscopic scale and transient occurrence of nucleation. This process is further complicated by the multicomponent nature of atmospheric nucleating systems in which the properties of conucleating compounds influence the affinity of molecules to cluster. Molecular dynamics simulations were performed to investigate homogeneous vapor-liquid nucleation and growth of six binary mixtures composed of water, n-nonane, 1-butanol, and methanol. Structural analyses were performed to understand the dynamic configurations generated from binary nuclei. Geometric structure analysis revealed that clusters were found to be more spherical with increasing cluster size, while composition analysis revealed that more miscible species had less mole fraction variability from an equimolar composition. Radial density profiling and cluster snapshots revealed structural features that were dependent on the miscibility of the nucleating pairs. Homogeneous mixing was observed in n-nonane/1-butanol and water-methanol due to their miscibility. Meanwhile, systems with partial miscibility (water/1-butanol, water/methanol, 1-butanol/methanol, n-nonane/methanol), exhibited preferential ordering into core-shell structures. In water/n-nonane, simultaneous unary nucleation was observed, leading to lens-on-sphere configuration. Microstructure analysis also revealed internal fragmentation within core-shell motifs of water/1-butanol and 1-butanol/methanol. These findings have serious implications in nucleation theories, which lead to valuable insights for the nucleation of naturally occurring multicomponent systems in the atmosphere.