Qiu Yuqing, Lupi Laura, Molinero Valeria
Department of Chemistry , The University of Utah , 315 South 1400 East , Salt Lake City , Utah 84112-0850 , United States.
J Phys Chem B. 2018 Apr 5;122(13):3626-3634. doi: 10.1021/acs.jpcb.7b11476. Epub 2018 Feb 1.
Graphitic surfaces are the main component of soot, a major constituent of atmospheric aerosols. Experiments indicate that soots of different origins display a wide range of abilities to heterogeneously nucleate ice. The ability of pure graphite to nucleate ice in experiments, however, seems to be almost negligible. Nevertheless, molecular simulations with the monatomic water model mW with water-carbon interactions parameterized to reproduce the experimental contact angle of water on graphite predict that pure graphite nucleates ice. According to classical nucleation theory, the ability of a surface to nucleate ice is controlled by the binding free energy between ice immersed in liquid water and the surface. To establish whether the discrepancy in freezing efficiencies of graphite in mW simulations and experiments arises from the coarse resolution of the model or can be fixed by reparameterization, it is important to elucidate the contributions of the water-graphite, water-ice, and ice-water interfaces to the free energy, enthalpy, and entropy of binding for both water and the model. Here we use thermodynamic analysis and free energy calculations to determine these interfacial properties. We demonstrate that liquid water at the graphite interface is not ice-like or vapor-like: it has similar free energy, entropy, and enthalpy as water in the bulk. The thermodynamics of the water-graphite interface is well reproduced by the mW model. We find that the entropy of binding between graphite and ice is positive and dominated, in both experiments and simulations, by the favorable entropy of reducing the ice-water interface. Our analysis indicates that the discrepancy in freezing efficiencies of graphite in experiments and the simulations with mW arises from the inability of the model to simultaneously reproduce the contact angle of liquid water on graphite and the free energy of the ice-graphite interface. This transferability issue is intrinsic to the resolution of the model, and arises from its lack of rotational degrees of freedom.
石墨表面是烟灰的主要成分，而烟灰是大气气溶胶的主要组成部分。实验表明，不同来源的烟灰在异质冰核形成方面表现出广泛的能力。然而，在实验中纯石墨形成冰核的能力似乎几乎可以忽略不计。尽管如此，使用单原子水模型mW进行的分子模拟，其中水 - 碳相互作用参数化以重现水在石墨上的实验接触角，预测纯石墨会形成冰核。根据经典成核理论，表面形成冰核的能力由浸没在液态水中的冰与表面之间的结合自由能控制。为了确定mW模拟和实验中石墨冻结效率的差异是源于模型的粗分辨率还是可以通过重新参数化来解决，阐明水 - 石墨、水 - 冰和冰 - 水界面对于水和模型的结合自由能、焓和熵的贡献非常重要。在这里，我们使用热力学分析和自由能计算来确定这些界面性质。我们证明，石墨界面处的液态水既不像冰也不像蒸汽：它具有与本体水中相似的自由能、熵和焓。mW模型很好地再现了水 - 石墨界面的热力学。我们发现，在实验和模拟中，石墨与冰之间结合的熵都是正的，并且主要由减少冰 - 水界面的有利熵主导。我们的分析表明，实验中石墨的冻结效率与mW模拟之间的差异源于该模型无法同时再现液态水在石墨上的接触角和冰 - 石墨界面的自由能。这种可转移性问题是模型分辨率所固有的，并且源于其缺乏旋转自由度。
