Lupi Laura, Hanscam Rebecca, Qiu Yuqing, Molinero Valeria
Department of Chemistry, The University of Utah , 315 South 1400 East, Salt Lake City, Utah 84112-0850, United States.
J Phys Chem Lett. 2017 Sep 7;8(17):4201-4205. doi: 10.1021/acs.jpclett.7b01855. Epub 2017 Aug 22.
The control of assembly and crystallization of molecules is becoming increasingly important in chemistry, engineering, and materials sciences. Crystallization is also central to understand natural processes that include the formation of atmospheric ice and biomineralization. Organic surfaces, biomolecules, and even liquid/vapor interfaces can promote the nucleation of crystals. These soft surfaces present significant structural fluctuations, which have been shown to strongly impact the rate of crystallization. This raises the question of whether degrees of freedom of soft surfaces play a role in the reaction coordinate for crystal nucleation. Here we use molecular simulations to investigate the mechanism of ice nucleation promoted by an alcohol monolayer. Our analysis indicates that while the flexibility of the surface strongly depresses its ice nucleation ability, it does not play a role in the coordinate that controls the transformation from liquid to ice. We find that the variable that drives the transformation is the size of the crystalline cluster, the same as that for the homogeneous crystallization. We argue that this is a general result that arises from the separation of time scales between surface fluctuations and the crossing of the transition state barrier for crystallization.
分子组装和结晶的控制在化学、工程和材料科学中变得越来越重要。结晶对于理解包括大气冰的形成和生物矿化在内的自然过程也至关重要。有机表面、生物分子甚至液/气界面都可以促进晶体的成核。这些柔软表面存在显著的结构波动，已表明这些波动会强烈影响结晶速率。这就提出了一个问题，即柔软表面的自由度是否在晶体成核的反应坐标中起作用。在这里，我们使用分子模拟来研究醇单分子层促进冰成核的机制。我们的分析表明，虽然表面的柔韧性强烈抑制了其冰成核能力，但它在控制从液体到冰的转变的坐标中不起作用。我们发现驱动转变的变量是晶体簇的大小，这与均相结晶的情况相同。我们认为这是一个普遍的结果，它源于表面波动与结晶过渡态屏障穿越之间的时间尺度分离。
