Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Rm. E19-502, Cambridge, Massachusetts 02144, USA.
J Chem Phys. 2013 Apr 7;138(13):134707. doi: 10.1063/1.4798458.
The process of water's evaporation at its liquid/air interface has proven challenging to study experimentally and, because it constitutes a rare event on molecular time scales, presents a challenge for computer simulations as well. In this work, we simulated water's evaporation using the classical extended simple point charge model water model, and identified a minimum free energy path for this process in terms of 10 descriptive order parameters. The measured free energy change was 7.4 kcal/mol at 298 K, in reasonable agreement with the experimental value of 6.3 kcal/mol, and the mean first-passage time was 1375 ns for a single molecule, corresponding to an evaporation coefficient of 0.25. In the observed minimum free energy process, the water molecule diffuses to the surface, and tends to rotate so that its dipole and one O-H bond are oriented outward as it crosses the Gibbs dividing surface. As the water molecule moves further outward through the interfacial region, its local density is higher than the time-averaged density, indicating a local solvation shell that protrudes from the interface. The water molecule loses donor and acceptor hydrogen bonds, and then, with its dipole nearly normal to the interface, stops donating its remaining hydrogen bond. At that point, when the final, accepted hydrogen bond is broken, the water molecule is free. We also analyzed which order parameters are most important in the process and in reactive trajectories, and found that the relative orientation of water molecules near the evaporating molecule, and the number of accepted hydrogen bonds, were important variables in reactive trajectories and in kinetic descriptions of the process.
在液态/气相间，水的蒸发过程在实验上很难研究，而且由于它在分子时间尺度上构成了罕见事件，因此对计算机模拟也提出了挑战。在这项工作中，我们使用经典的扩展简单点电荷模型水模型模拟了水的蒸发，并根据 10 个描述性序参数确定了该过程的最小自由能途径。在 298 K 下，测量的自由能变化为 7.4 kcal/mol，与实验值 6.3 kcal/mol 相当合理，单个分子的平均首次通过时间为 1375 ns，对应的蒸发系数为 0.25。在观察到的最小自由能过程中，水分子扩散到表面，并倾向于旋转，使其偶极子和一个 O-H 键在穿过吉布斯分界面时向外定向。当水分子进一步向外穿过界面区域时，其局部密度高于时间平均密度，表明从界面突出的局部溶剂化壳。水分子失去供体和受体氢键，然后，随着其偶极子几乎垂直于界面，停止提供其剩余的氢键。在那一刻，当最后一个接受的氢键断裂时，水分子就自由了。我们还分析了哪些序参数在过程中和反应轨迹中最重要，并发现蒸发分子附近水分子的相对取向以及接受氢键的数量是反应轨迹和过程动力学描述中的重要变量。
