Mechanisms of heterogeneous ice nucleation for organic surfaces : molecular dynamics simulations and laboratory experiments

In the atmosphere, water freezes homogeneously at temperatures below -38°C. At warmer temperatures, freezing occurs heterogeneously, facilitated by ice nucleating substances (INSs). These substances influence the climate by modifying the properties of mixed-phase and ice clouds, but their underlying ice-nucleation mechanisms remain poorly understood. Furthermore, during atmospheric transport, INSs become coated in inorganic solutes like ammonium sulfate, which alter their ice nucleating efficiency. To investigate freezing phenomena, we employ Molecular Dynamics (MD) simulations and freezing droplet experiments. We used simulations to study the ice nucleation mechanisms of amphiphilic monolayers that expose -OH and -COOH groups at the water interface, and experiments to examine how ammonium sulfate exposure affects the nucleation temperatures of selected organic and inorganic INSs. Our simulations show that the ice nucleating ability of alcohol monolayers is sensitive to the C-O bond orientation at the water interface. Previous simulations have shown that alcohol monolayers consisting of molecules with an odd number of carbon atoms are more flexible than those with an even number of carbon atoms. We suggest that their greater flexibility allows odd-numbered monolayers to more easily adopt C-O bond orientations that favor ice nucleation than even-numbered monolayers, possibly accounting for their better ice nucleating ability. Experimentally, hydroxy ester and bromo alcohol monolayers nucleate ice at unexpectedly warm temperatures, despite their poor lattice matches to ice. Our simulations show that nucleation occurs when the lattice is constructed with dimensions that are smaller than the experimental values measured at 5°C. Consequently, we propose that contraction of these lattices at the colder ice nucleation temperatures could account for their ice nucleation efficiency. Fatty acid monolayers that expose -COOH groups to water exhibit poorer ice nucleation ability than alcohol monolayer of similar carbon chain length. Our simulations indicate that this discrepancy arises because -COOH surface groups disrupt the organization of ice at the acid monolayer interface. Our freezing droplet experiments demonstrate that the ice nucleation temperature of alcohol monolayers and microplastics remain unaffected by short-term ammonium sulfate exposure. In contrast, the response of α-alumina depended on particle size, suggesting that surface irregularities possibly mediate the influence of ammonium sulfate.