Molecular Origin of Aneotropy and Related Surface Tension Anomalies in Hydrogenated and Fluorinated Alcohol Mixtures: New Experimental Data and Theoretical Molecular Modeling

Aneotropy is an intriguing interfacial phenomenon that is still poorly understood and displayed by several relevant mixtures. This work is a contribution toward the molecular-level interpretation of aneotropy and related surface tension anomalies, combining new experimental data, theoretical calculations, and atomistic molecular dynamics (MD) simulations, applied to carefully chosen systems. First, bulk and interfacial properties of the hexane/perfluorohexane mixture were modeled with the soft-SAFT and soft-SAFT-DGT approaches, respectively. This mixture, governed exclusively by dispersion forces, is well-known by the mutual phobicity between hydrogenated and perfluorinated chains. The theory captures the very positive excess volumes and enthalpies and predicts the negative aneotrope, revealing that the weak interactions between unlike molecules, which are responsible for the large deviations to ideality in the bulk liquid, are even more unfavorable at the interface, leading to large surface anomalies. The effect of introducing an associative OH group into molecules of an equivalent chain length was then addressed. Three mixtures of hydrogenated and fluorinated alcohols were studied, extending to longer chains a previous work involving mixtures of short chain alcohols. New experimental data were measured for the surface tension of butanol/1H,1H-perfluorobutanol, hexanol/1H,1H-perfluorohexanol, and decanol/1H,1H-perfluorooctanol mixtures at 298.2K. The bulk properties of the mixtures were modeled with MD simulations and soft-SAFT, relying on the description of the dispersive interactions previously determined, and the interfacial behavior was predicted with soft-SAFT-DGT. Although these components interact strongly through hydrogen bonding, the surface tension of the mixtures displays very large negative deviations from the weighted average and in some cases shallow negative aneotropes, shallower than that displayed by the hexane/perfluorohexane mixture. The theoretical predictions reproduce the experimental data and provide a molecular-level interpretation of these unusual interfacial anomalies, advancing our fundamental understanding of complex fluid interfaces.