Lupi Laura, Kastelowitz Noah, Molinero Valeria
Department of Chemistry, The University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
J Chem Phys. 2014 Nov 14;141(18):18C508. doi: 10.1063/1.4895543.
Carbonaceous surfaces are a major source of atmospheric particles and could play an important role in the formation of ice. Here we investigate through molecular simulations the stability, metastability, and molecular pathways of deposition of amorphous ice, bilayer ice, and ice I from water vapor on graphitic and atomless Lennard-Jones surfaces as a function of temperature. We find that bilayer ice is the most stable ice polymorph for small cluster sizes, nevertheless it can grow metastable well above its region of thermodynamic stability. In agreement with experiments, the simulations predict that on increasing temperature the outcome of water deposition is amorphous ice, bilayer ice, ice I, and liquid water. The deposition nucleation of bilayer ice and ice I is preceded by the formation of small liquid clusters, which have two wetting states: bilayer pancake-like (wetting) at small cluster size and droplet-like (non-wetting) at larger cluster size. The wetting state of liquid clusters determines which ice polymorph is nucleated: bilayer ice nucleates from wetting bilayer liquid clusters and ice I from non-wetting liquid clusters. The maximum temperature for nucleation of bilayer ice on flat surfaces, T(B)(max) is given by the maximum temperature for which liquid water clusters reach the equilibrium melting line of bilayer ice as wetting bilayer clusters. Increasing water-surface attraction stabilizes the pancake-like wetting state of liquid clusters leading to larger T(B)(max) for the flat non-hydrogen bonding surfaces of this study. The findings of this study should be of relevance for the understanding of ice formation by deposition mode on carbonaceous atmospheric particles, including soot.
含碳表面是大气颗粒物的主要来源，并且可能在冰的形成过程中发挥重要作用。在此，我们通过分子模拟研究了无定形冰、双层冰和冰I从水蒸气在石墨和无原子的 Lennard-Jones 表面上沉积的稳定性、亚稳性和分子途径，作为温度的函数。我们发现，对于小簇尺寸，双层冰是最稳定的冰多晶型物，然而它可以在远高于其热力学稳定区域的情况下亚稳生长。与实验一致，模拟预测随着温度升高，水沉积的产物是无定形冰、双层冰、冰I和液态水。双层冰和冰I的沉积成核之前会形成小的液态簇，这些液态簇有两种润湿状态：小簇尺寸时为双层薄饼状（润湿），较大簇尺寸时为液滴状（非润湿）。液态簇的润湿状态决定了哪种冰多晶型物成核：双层冰从润湿的双层液态簇中成核，冰I从非润湿的液态簇中成核。双层冰在平面上成核的最高温度T(B)(max)由液态水簇作为润湿双层簇达到双层冰平衡熔点的最高温度给出。增加水-表面吸引力会稳定液态簇的薄饼状润湿状态，导致本研究中平面非氢键表面的T(B)(max)更大。这项研究的结果对于理解通过沉积模式在含碳大气颗粒物（包括烟灰）上形成冰应该具有相关性。
