Insights from the Atomistic Simulations of a Ternary Nucleating System

The development of a few advanced simulation techniques in our group has enabled us to venture on the very first atomistic simulations of a threecomponent nucleating system, water/n-nonane/1-butanol, that provided results which are in good agreement with experimental data. This study has practical implications in aerosol formation, separation process, nano-materials engineering, and micro-emulsion research. In this investigation, we found that the three nonideal nucleation behaviors we observed in the three binary mixtures (mutual nucleation enhancement for water/1-butanol, reluctant co-nucleation for n-nonane/1- butanol, and independent nucleation for water/n-nonane) were carried forward and combined in this ternary mixture at all compositions. Such diverse spectrum of nonideal behavior implies that changing the gas-phase compositions could lead to various nucleation pathways. In addition, the “micro-phase diagrams” (or nucleation free energy landscape) constructed at intermediate vapor-phase compositions show coexistence of multiple “micro-phases” (clusters with distinct compositions). Visual inspection of the clusters that contain equal molar amounts of each component further reveals the existence of an internal phase separation into a multilayered structure. In particular, water and 1-butanol adopt a core/shell pattern with the nonpolar butyl chains pointing outwards that favors the deposition of n-nonane compared to the bare water surface. This explains the enhanced miscibility between water and n-nonane with the presence of 1-butanol. However, the tendency for n-nonane to deposit on one end of (rather than wrapping around) the core/shell structure suggests that 1-butanol is just too short to act effectively as a surfactant. Simulations for ternary mixtures involving either shorter or longer alcohols are currently under investigation to examine this speculation and to search for possible ways of improving the amphiphile. We will also present results for those mixtures at the meeting.

Keywords Ternary nucleation, nonideal nucleation behaviors, simulation, phase separation, micro-phase diagram, critical cluster, surfactant