Molecular Level Structure of the Liquid-Vapor Interface of Hydrogen Fluoride

Using a Drude-based polarizable model, we investigate the liquid-vapor interface of hydrogen fluoride (HF) via molecular dynamics simulations. The interface displays a complex morphology with vapor-phase clusters, large internal bubbles in the liquid phase, and extended chains of hydrogen-bonded molecules protruding from the surface into the vapor phase. These features arise from the dual cohesive mechanisms in HF, i.e., strong directional hydrogen bonds and weaker van der Waals interactions, which act at distinct length and energy scales. The resulting surface structure resembles that of polymer melts and deviates markedly from that of conventional molecular liquids. We show that the anomalously low surface tension of HF and the associated emergence of interfacial heterogeneity can be understood in terms of the reduced van der Waals coordination imposed by the strong hydrogen bonding, showing the opposite effect as in other associative liquids like water.