Pharoah Lian, Bertram Allan K, Patey G N
Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
J Phys Chem A. 2024 Aug 29;128(34):7214-7225. doi: 10.1021/acs.jpca.4c03912. Epub 2024 Aug 15.
Organic ice nucleating substances (INSs) are thought to play an essential role in cloud formation and, hence, precipitation and climate. Organic INSs are an important but poorly understood class of INSs in the atmosphere. To study organic INSs with exposed hydroxylated surfaces, researchers have previously used fatty alcohol monolayers as model systems. For alcohol monolayers, ice nucleation temperatures increase with increasing alkyl chain length and show a high-low oscillation following the number (odd-even) of carbon atoms in the alkyl chains. We employ atomistic models, together with molecular dynamics simulations, to investigate ice nucleation by CHOH, CHOH, and CHOH monolayers. As expected, we find that ice nucleation by alcohol monolayers depends on the lattice match to ice, and a poorer lattice match can at least partially account for the reduced ice nucleation ability of CHOH monolayers compared to monolayers of the longer chain alcohols. More interestingly, our simulations identify a limited range of alcohol configurations that readily nucleate ice via the basal plane. For configurations outside this range, ice nucleation did not occur on the time scale of our simulations (i.e., 5000 ns). The configurational feature that crucially influences ice nucleation is the angle between the alcohol C-O bond and the interfacial plane. C-O bonds directed sharply toward or away from the water phase strongly inhibit ice nucleation. In contrast, ice nucleation is easily observed for a relatively narrow band of C-O bond orientations centered about the surface plane. For comparable surface configurations, the ice nucleating abilities of CHOH and CHOH monolayers are practically identical, but the existence of a narrow band of ice-compatible surface configurations can perhaps explain why odd-chain alcohol monolayers are better INSs than even-chain alcohol monolayers. Earlier simulations have shown that for alcohols differing by a single carbon atom, the odd-chain monolayer is less rigid than the even-chain monolayer. This suggests the possibility that for odd-chain alcohol monolayers, the orientation of the C-O bonds can more easily adjust into the ice-compatible range than their even-chain counterparts, accounting for their enhanced ice nucleating ability.
有机冰核物质（INSs）被认为在云的形成中起着至关重要的作用，因此也与降水和气候有关。有机INSs是大气中一类重要但尚未被充分理解的冰核物质。为了研究具有暴露羟基表面的有机INSs，研究人员此前曾使用脂肪醇单分子层作为模型系统。对于醇类单分子层，冰核形成温度随着烷基链长度的增加而升高，并随着烷基链中碳原子数（奇数-偶数）呈现高低振荡。我们采用原子模型以及分子动力学模拟，来研究CHOH、CHOH和CHOH单分子层的冰核形成。正如预期的那样，我们发现醇类单分子层的冰核形成取决于与冰的晶格匹配，与长链醇单分子层相比，较差的晶格匹配至少可以部分解释CHOH单分子层冰核形成能力的降低。更有趣的是，我们的模拟确定了通过基面容易形成冰核的有限范围的醇构型。对于超出此范围的构型，在我们的模拟时间尺度（即5000纳秒）内未发生冰核形成。对冰核形成起关键影响的构型特征是醇的C-O键与界面平面之间的夹角。急剧指向或远离水相的C-O键强烈抑制冰核形成。相比之下，对于以表面平面为中心的相对较窄的C-O键取向带，很容易观察到冰核形成。对于可比的表面构型，CHOH和CHOH单分子层的冰核形成能力实际上是相同的，但存在与冰兼容的窄表面构型带或许可以解释为什么奇数链醇单分子层比偶数链醇单分子层是更好的冰核物质。早期的模拟表明，对于相差一个碳原子的醇类，奇数链单分子层比偶数链单分子层的刚性更低。这表明对于奇数链醇单分子层，C-O键的取向可能比偶数链醇更容易调整到与冰兼容的范围，这解释了它们增强的冰核形成能力。
