Comment on “Effects of nanobubble on physical properties of water: A coarse-grained molecular dynamics analysis” by Yuhang Du, Fanqi Pei, Zhi Wen, and Guofeng Lou [J. Mol. Liq. 437 (2025) 128445]

Abstract

In the referenced article the authors employ a coarse-grained molecular dynamics approach to explore the influence of nanobubbles on the viscosity and surface tension of water. Surface tension is computed via the pressure-tensor method, and the simulations indicate that nanobubbles systematically reduce the surface-tension coefficient. A linear empirical correlation is proposed for the ratio $\gamma /{\gamma }_f$, where $\gamma$ is the surface tension of nanobubble water and ${\gamma }_f$ is that of pure water under identical conditions. However, this linear relation is calibrated only within $T\in [300,350]$ K, despite the pressure coefficients spanning up to $100$ bar, where liquid water remains stable at temperatures significantly above $350$ K. A direct extension of the linear form leads to unphysical predictions outside the calibration interval. In this Comment, by linking the molecular-dynamics results of Du et al. with classical interfacial thermodynamics—originating in Gibbs’ adsorption theory and later strengthened by Frenkel’s molecular-kinetic framework—it is demonstrated that the temperature dependence of $\gamma /{\gamma }_f$ should follow an exponential attenuation law. This form naturally reduces to the authors’ linear correlation via a first-order expansion, while remaining physically consistent at higher temperatures. Moreover, the resulting model predicts a molecular Gibbs free energy in excellent agreement with independent values reported in the literature, thus lending further confidence to the MD simulations of Du et al.