Thermodynamics and mass transfer at vapor-liquid interface of liquid ammonia: A molecular dynamics study

Ammonia serves as a highly promising “zero‑carbon” fuel and hydrogen carrier due to its high energy density and carbon-free characteristics. However, the extreme volatility of liquid ammonia inevitably leads to complex evaporation and condensation at the vapor-liquid interface, posing potential threats to safe storage and pipeline transportation. Existing investigations into liquid ammonia phase change predominantly adopt macroscopic hydrodynamic simulations. Microscopic transport mechanisms at the nanoscale vapor–liquid interface of ammonia remain insufficiently explored, limiting the accurate determination of condensation coefficients and consequently compromising macroscopic calculation accuracy. In this study, a molecular dynamics (MD) simulation utilizing the TraPPE force field is employed to investigate the thermodynamics and mass transport of the ammonia vapor-liquid interface under equilibrium. The atomic-position statistics method calculates the collision flux by tracking molecular trajectories and collision events, while the characteristic time method, coupled with double-exponential fitting of molecular retention curves, accurately distinguishes molecular collision, rapid reflection, and prolonged condensation behaviors. Results show that the average vapor-liquid interface thickness increases from 1.21 nm to 2.33 nm, and surface tension decreases from 39.29 mN/m to 16.65 mN/m as temperature rises from 240 K to 320 K. High temperatures shorten the characteristic residence time of condensed molecules and reduce the condensation coefficient from 0.905 to 0.662. At 280 K, incident normal velocity is associated with molecular interfacial behavior, and molecules with higher incident normal kinetic energy are statistically more likely to penetrate the interface and condense, whereas molecules with lower incident normal kinetic energy are more likely to be reflected.