The Mechanism of\nHeterogeneous Ice Nucleation by Fatty\nAlcohol Monolayers

Organic ice nucleating substances (INSs) are thought\nto play an\nessential role in cloud formation and, hence, precipitation and climate.\nOrganic INSs are an important but poorly understood class of INSs\nin the atmosphere. To study organic INSs with exposed hydroxylated\nsurfaces, researchers have previously used fatty alcohol monolayers\nas model systems. For alcohol monolayers, ice nucleation temperatures\nincrease with increasing alkyl chain length and show a high–low\noscillation following the number (odd–even) of carbon atoms\nin the alkyl chains. We employ atomistic models, together with molecular\ndynamics simulations, to investigate ice nucleation by C<sub>20</sub>H<sub>41</sub>OH, C<sub>30</sub>H<sub>61</sub>OH, and C<sub>31</sub>H<sub>63</sub>OH monolayers. As expected, we find that ice nucleation\nby alcohol monolayers depends on the lattice match to ice, and a poorer\nlattice match can at least partially account for the reduced ice nucleation\nability of C<sub>20</sub>H<sub>41</sub>OH monolayers compared to monolayers\nof the longer chain alcohols. More interestingly, our simulations\nidentify a limited range of alcohol configurations that readily nucleate\nice via the basal plane. For configurations outside this range, ice\nnucleation did not occur on the time scale of our simulations (i.e.,\n5000 ns). The configurational feature that crucially influences ice\nnucleation is the angle between the alcohol C–O bond and the\ninterfacial plane. C–O bonds directed sharply toward or away\nfrom the water phase strongly inhibit ice nucleation. In contrast,\nice nucleation is easily observed for a relatively narrow band of\nC–O bond orientations centered about the surface plane. For\ncomparable surface configurations, the ice nucleating abilities of\nC<sub>30</sub>H<sub>61</sub>OH and C<sub>31</sub>H<sub>63</sub>OH\nmonolayers are practically identical, but the existence of a narrow\nband of ice-compatible surface configurations can perhaps explain\nwhy odd-chain alcohol monolayers are better INSs than even-chain alcohol\nmonolayers. Earlier simulations have shown that for alcohols differing\nby a single carbon atom, the odd-chain monolayer is less rigid than\nthe even-chain monolayer. This suggests the possibility that for odd-chain\nalcohol monolayers, the orientation of the C–O bonds can more\neasily adjust into the ice-compatible range than their even-chain\ncounterparts, accounting for their enhanced ice nucleating ability.