Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
J Chem Phys. 2011 Jun 14;134(22):224704. doi: 10.1063/1.3595260.
Evaporation and condensation at a liquid/vapor interface are ubiquitous interphase mass and energy transfer phenomena that are still not well understood. We have carried out large scale molecular dynamics simulations of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to investigate these processes with molecular detail. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. We confirm that mechanical equilibrium plays a key role in determining the evaporation rate and the density and temperature profiles across the liquid/vapor interface. The velocity distributions of evaporated molecules and the evaporation and condensation coefficients are measured and compared to the predictions of an existing model based on kinetic theory of gases. Our results indicate that for both monatomic and polyatomic molecules, the evaporation and condensation coefficients are equal when systems are not far from equilibrium and smaller than one, and decrease with increasing temperature. For the same reduced temperature T/T(c), where T(c) is the critical temperature, these two coefficients are higher for LJ dimers and trimers than for monomers, in contrast to the traditional viewpoint that they are close to unity for monatomic molecules and decrease for polyatomic molecules. Furthermore, data for the two coefficients collapse onto a master curve when plotted against a translational length ratio between the liquid and vapor phase.
蒸发和冷凝是普遍存在于液-气界面的相间传质和传热现象，但目前仍未被很好地理解。我们对由单体、二聚体或三聚体组成的 Lennard-Jones（LJ）流体进行了大规模分子动力学模拟，以从分子细节研究这些过程。对于与真空接触的 LJ 单体，蒸发速率非常高，具有显著的蒸发冷却和伴随的液体域中靠近液-气界面的密度梯度。将链长增加到仅仅二聚体，显著降低了蒸发速率。我们证实机械平衡在确定蒸发速率以及液体/气体界面处的密度和温度分布方面起着关键作用。蒸发分子的速度分布以及蒸发和冷凝系数被测量并与基于气体动理论的现有模型的预测进行了比较。我们的结果表明，对于单原子和多原子分子，当系统远离平衡且小于 1 时，蒸发和冷凝系数相等，并随温度升高而降低。对于相同的约化温度 T/T(c)，其中 T(c)是临界温度，对于 LJ 二聚体和三聚体，这两个系数高于单体，这与传统观点相反，传统观点认为对于单原子分子，它们接近于 1，而对于多原子分子则降低。此外，当将两个系数相对于液相和气相之间的平移长度比绘制时，数据会收敛到主曲线。
