Ramírez Rafael, Singh Jayant K, Müller-Plathe Florian, Böhm Michael C
Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain.
Eduard-Zintl-Institut für Anorganische und Physikalische Chemie and Center of Smart Interfaces, Technische Universität Darmstadt, Alarich-Weiss-Str. 4, 64287 Darmstadt, Germany.
J Chem Phys. 2014 Nov 28;141(20):204701. doi: 10.1063/1.4901562.
Ice and water droplets on graphite have been studied by quantum path integral and classical molecular dynamics simulations. The point-charge q-TIP4P/F potential was used to model the interaction between flexible water molecules, while the water-graphite interaction was described by a Lennard-Jones potential previously used to reproduce the macroscopic contact angle of water droplets on graphite. Several energetic and structural properties of water droplets with sizes between 10(2) and 10(3) molecules were analyzed in a temperature interval of 50-350 K. The vibrational density of states of crystalline and amorphous ice drops was correlated to the one of ice Ih to assess the influence of the droplet interface and molecular disorder on the vibrational properties. The average distance of covalent OH bonds is found 0.01 Å larger in the quantum limit than in the classical one. The OO distances are elongated by 0.03 Å in the quantum simulations at 50 K. Bond distance fluctuations are large as a consequence of the zero-point vibrations. The analysis of the H-bond network shows that the liquid droplet is more structured in the classical limit than in the quantum case. The average kinetic and potential energy of the ice and water droplets on graphite has been compared with the values of ice Ih and liquid water as a function of temperature. The droplet kinetic energy shows a temperature dependence similar to the one of liquid water, without apparent discontinuity at temperatures where the droplet is solid. However, the droplet potential energy becomes significantly larger than the one of ice or water at the same temperature. In the quantum limit, the ice droplet is more expanded than in a classical description. Liquid droplets display identical density profiles and liquid-vapor interfaces in the quantum and classical limits. The value of the contact angle is not influenced by quantum effects. Contact angles of droplets decrease as the size of the water droplet increases which implies a positive sign of the line tension of the droplet.
通过量子路径积分和经典分子动力学模拟研究了石墨上的冰和水滴。采用点电荷q-TIP4P/F势来模拟柔性水分子之间的相互作用，而水-石墨相互作用则用先前用于再现水滴在石墨上的宏观接触角的Lennard-Jones势来描述。在50 - 350 K的温度区间内，分析了含有10²至10³个分子的水滴的几种能量和结构性质。将结晶态和非晶态冰滴的振动态密度与冰Ih的振动态密度相关联，以评估液滴界面和分子无序对振动性质的影响。发现量子极限下共价OH键的平均距离比经典极限下大0.01 Å。在50 K的量子模拟中，OO距离延长了0.03 Å。由于零点振动，键距波动很大。氢键网络分析表明，液滴在经典极限下比量子情况下更有序。将石墨上冰滴和水滴的平均动能和势能与冰Ih和液态水的值作为温度的函数进行了比较。液滴动能的温度依赖性与液态水相似，在液滴为固态的温度下没有明显的不连续性。然而，在相同温度下，液滴势能比冰或水的势能明显更大。在量子极限下，冰滴比经典描述中更膨胀。液滴在量子和经典极限下显示出相同的密度分布和液-气界面。接触角的值不受量子效应的影响。水滴的接触角随着水滴尺寸的增加而减小，这意味着液滴的线张力为正。
