Interfacial thermodynamics of spherical liquid nanodroplets: Molecular understanding of surface tension via hydrogen bond network

Surface tension plays a important role in nucleation of atmospheric liquid droplets. Especially, understanding of interfacial thermodynamics of the 1 nm scale nucleus is essential for characterization of nucleation such as the activation energy barrier. Despite theoretical and experimental studies performed for decades, determination of surface tension of the nanodroplet is still a topic of ongoing controversies. Here, we investigate surface tension of spherical nanodroplets by both molecular dynamics and density functional theory, and find that surface tension decreases appreciably below 1 nm diameter, whose analytic expression is derived from the Tolman's equation. In particular, analysis of the free energy of nanodroplets shows that the change of surface tension originates from the configurational energy of molecules, which is evidenced by the disrupted hydrogen bond network of nanodroplet. Our results may further illuminate the molecular mechanisms of the interface-related phenomena associated with molecular fluctuations such as ice nucleation or biomolecule adsorption where the macroscopic thermodynamic quantities are ill-defined.