Molecular dynamics simulations of template-assisted nucleation: Alcohol monolayers at the air-water interface and ice formation

Perhaps the best characterized template nucleation systems are the alcohol monolayer assemblies at the air-water interface. These monolayers, consisting of aliphatic alcohols [C_nH_2n+1OH, n = 16-31) have been the subject of hexagonal ice (I_h) induction experiments over a range of temperatures. To learn more about the molecular basis of alcohol monolayer template-induced ice nucleation phenomena, we performed canonical molecular dynamics (MD) simulations (1 ns) of pure C29, C30, and C31 alcohol monolayer-water droplet systems, using the reversible reference system propagation algorithm (r-RESPA). We find that the MD-determined monolayer physical parameters (at 278.15 K) exhibit reasonable agreement with experimentally determined values for crystalline C30 and C31 monolayers at 278.15 K. More importantly, as the simulation temperatures approach monolayer-specific freezing points, the water layer immediately below the monolayer surfaces adopted "ice-like" lattice parameters and hexagonal or c-centered rectangular geometries that are characteristic of the (I_h) {001} plane. The analysis of monolayer -OH headgroup orientations and surface topologies reveal that odd-carbon monolayers, and C31 in particular, are more effective nucleation templates, due to the following factors: (1) ab-plane alcohol -OH headgroup geometries which provide a closer ab-plane lattice match to the {001} I_h interface and (2) the presence of smoother monolayer headgroup topologies (i.e., azimuthal positioning), which permit a larger percentage of water molecules to form hydrogen bonds with the monolayer alcohol headgroups.