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 C20H41OH, C30H61OH, and C31H63OH 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 C20H41OH 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
C30H61OH and C31H63OH
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.
