The Mechanisms of Heterogeneous Ice Nucleation by Amphiphilic Alcohol Monolayers Formed by Alkyl Hydroxy Esters and...

Molecular dynamics (MD) simulations are employed to examine ice nucleation by amphiphilic alcohol monolayers formed by two alkyl hydroxy esters and two bromo alcohols. These monolayers expose hydroxylated surfaces to water. Despite their poor two-dimensional (2D) lattice matches to hexagonal ice, these surfaces nucleate ice at surprisingly warm temperatures. In our simulations, monolayers with experimental lattice parameters did not nucleate hexagonal ice, but one alkyl hydroxy ester and one bromo alcohol surface nucleated cubic ice via its (001) face. However, cubic ice nucleation required particular surface conformations that were not available for the other two monolayers. We also consider the possibility that the lattice parameters measured at 5 °C do not give an accurate representation of the lattices at the lower (-5 °C to -9 °C) ice nucleating temperatures (INTs). We use measurements only available for a single alcohol monolayer (C₂₁H₄₃OH) to estimate changes in the lattice parameters of the alkyl hydroxy ester and bromo alcohol monolayers at their INTs. When cooled from 5 °C to their INTs, the monolayer lattice parameter contractions vary from 3.7% to 5.6%, substantially improving the 2D lattice matches to hexagonal ice. MD simulations performed by varying the relevant lattice parameter, a, showed that hexagonal ice nucleation occurs if a is in the range 5.0 Å - 5.6 Å. For the alkyl hydroxy ester monolayers, the lattice parameters estimated at their INTs (5.44 Å and 5.48 Å) lie well within the hexagonal ice-nucleating region. For the bromo alcohol monolayers, the estimates (5.67 Å and 5.61 Å) are borderline. Overall, our analysis suggests that the temperature dependence of lattice parameters is a significant factor contributing to ice nucleation by alkyl hydroxy ester and bromo alcohol monolayers, a possibility not previously considered.