Factors Influencing Poor Ice Nucleation by Model Fatty Acid Monolayers

Organic ice-nucleating substances (INSs) are relevant to atmospheric, biological, and industrial systems. Fatty acid and fatty alcohol monolayers have been used as model surfaces to investigate ice nucleation by organic materials. Laboratory studies show that fatty acid monolayers are inefficient ice nucleators, whereas fatty alcohol monolayers with similar alkyl-chain lengths can nucleate ice at temperatures up to approximately 20 °C warmer. Here, we use molecular dynamics simulations to investigate the molecular origin of this difference for a model fatty acid monolayer. Our results show that a major factor inhibiting ice nucleation by fatty acid monolayers is the presence of partially charged carbonyl oxygen atoms. The electrostatic interactions of these atoms with interfacial water molecules is consistent with strong carbonyl-water hydrogen bonding, and disrupts the formation of ordered, ice-like bilayers that are critical for nucleation on fatty alcohol monolayers. This conclusion is supported by simulations in which removing the partial charges on the carbonyl oxygen atoms restores ice nucleation. In contrast, differences in lattice mismatch and headgroup structure play a lesser role. These findings provide molecular-level insight into how interfacial functional groups control the ice nucleating ability of organic surfaces.