Probing the nucleation mechanism for the binary n-nonane/1-alcohol series with atomistic simulations

The AVUS-HR approach, which combines histogram reweighting with aggregation-volume-bias Monte Carlo nucleation simulations using self-adaptive umbrella sampling, was extended to multicomponent nucleation systems. It was applied to investigate the homogeneous vapor-liquid nucleation for the binary n-nonane/1-alcohol series, including the n-nonane/methanol, n-nonane/ethanol, n-nonane/1-propanol, n-nonane/1-butanol, n-nonane/1-hexanol, and n-nonane/1-decanol systems. The simple transferable potentials for phase equilibria-united atom force field was used in this investigation. It was found that the nucleation free energy (NFE) contour plots obtained for these binary n-nonane/1-alcohol nucleation systems exhibit rather interesting mechanistic features, some of which are distinct from other binary systems previously studied (such as water/ethanol and water/n-nonane). In addition, the NFE profiles show a subtle evolution with the increase in alcohol chain length, from a somewhat two-pathway type of shape as observed for the n-nonane/methanol system to a more normal single-pathway one for systems involving longer alcohols (1-hexanol and 1-decanol). In contrast, the NFE maps obtained for the other three binary systems involving those medium-length alcohols display the most striking feature with the saddle point stretched almost all the way from the n-nonane-enriched to the alcohol-enriched domain, implying that multiple pathways coexist on the nucleation map. These free energy profiles were shown to be consistent with the non-ideal nucleation behavior observed experimentally for this binary series, namely, a rather reluctant conucleation of the alcohols with n-nonane. In particular, this non-ideal behavior becomes more severe with a decrease in the alcohol chain length. Also, analysis of the compositions of the critical nuclei indicates a reluctant mixing behavior between these two species, i.e., depletion of the alcohol at low alcohol activity or depletion of n-nonane at low n-nonane activity, in agreement with the experimental interpretations. Furthermore, a microscopic inhomogeneity is present inside these critical nuclei, that is, alcohols aggregate via hydrogen bonds forming alcohol-enriched domains.