Surface anomalies in ethanol plus n-octane mixture: An effect of molecular orientations and hydrogen bonds

Mixtures of ethanol with alkanes show an anomalous surface tension behavior, which reaches a stationary point on concentration at isothermal conditions. This behavior can be attributed to microscopic phenomena of ethanol molecules, causing the shrinkage of the interface. The behavior can probably be explained by the preferential orientation of ethanol at the interface competing with its partial absorption caused by hydrogen bonds. However, these assumptions have not been confirmed. In this contribution, the properties of the intrinsic vapor–liquid surfaces have been analyzed to identify the truly interfacial molecules method. Therefore, the composition, width, roughness, and separation of the subsurface molecular layers, orientation, and hydrogen bonds with the bulk phase of the surface molecules have been analyzed in detail. In addition, the density gradient theory is used coupled with PC-SAFT and molecular dynamics simulations to provide physical insights into the structural and thermodynamic behavior at the liquid–vapor interface of the ethanol-octane system. The experimental data and the molecular dynamics simulations demonstrate that the minimum on the surface tension (aneotropy) does not occur precisely at the azeotropic conditions but in their proximity. On the one hand, it is observed that the azeotrope is a switching point at which the ethanol molecules accumulate at the interface. This behavior can partly be explained by changing the preferential orientation at the interface and partly by the breakage of hydrogen bonds. On the other hand, at the aneotropic conditions, the relative Gibbs absorption of both components approaches zero, which occurs at the first surface layer.