Energetic and entropic components of the Tolman length for mW and TIP4P/2005 water nanodroplets

Abstract

The surface free energy of a droplet is approximately γ(R)=γ(∞)(1-2δ/R), with R being the droplet radius and δ being the Tolman length. Here we use the mitosis method to compute δ=-0.56±0.1Å at 300 K for mW water, indicating that γ(R) increases as the droplet size decreases. The computed Tolman length agrees quite well with a previous study of TIP4P/2005 water. We also decompose the size-dependent surface free energy into energetic and entropic contributions for the mW and TIP4P/2005 force fields. Despite having similar Tolman lengths, the energy-entropy decompositions are very different for the two force fields. We discuss critical assumptions which lead to these findings and their relation to experiments on the nucleation of water droplets. We also discuss surface broken bonds and structural correlations as possible explanations for the energetic and entropic contributions.