Effects of pH on Ice Nucleation by the alpha-Alumina (0001) Surface

Mineral dust particles can cause heterogeneous ice nucleation in cloud droplets, and alpha-alumina (alpha-Al2O3) is one component of mineral dust particles found in the atmosphere. Single surface droplet freezing experiments conducted for alpha-alumina (0001) found that ice nucleation is most efficient at neutral pH and decreases for both acidic and basic deviations from neutral. We employ classical molecular dynamics (MD) simulations to examine possible microscopic origins of the influence of pH on ice nucleation by the alpha-alumina (0001) surface. The (0001) surface of alpha-alumina is hydroxylated in neutral aqueous solution, and MD simulations have revealed that the basal plane of hexagonal ice is stabilized by hydrogen bonding of water molecules with hydroxy groups on the alpha-alumina (0001) surface. Under acidic or basic conditions, the surface hydroxy groups can become dual-protonated or deprotonated, respectively. Therefore, simulating model alpha-alumina (0001) surfaces with different compositions of mono-protonated (OH groups), dual-protonated, and deprotonated sites, and relating surface composition to pH using previously reported pKa values, allows for direct comparison with single surface experiments. We find that the ice nucleating efficiency decreases nearly symmetrically with acidic and basic pH changes away from the neutral point. This is in qualitative agreement with experiments for the alpha-alumina (0001) surface. The two water layers comprising a perfect basal face bilayer of hexagonal ice contain equal numbers of water molecules. We show that as the surface acquires additional charge due to dual-protonation (positive charge) or deprotonation (negative charge), the increasing surface-water attraction increases the number of water molecules in the inner layer (adjacent to the surface) of the surface ice bilayer, while the number in the outer layer remains unchanged. This violates the equal number requirement for a perfect ice bilayer and reduces the ice nucleating efficiency of the surface. This effect depends mainly on the magnitude of the surface charge and is only weakly influenced by its sign. This explains the nearly symmetric freezing response to both acidic and basic pH deviations from the neutral value.