Effects of pH on Ice Nucleation by the α-Alumina (0001) Surface
Bibliographic record
Abstract
Mineral dust particles can cause heterogeneous ice nucleation in cloud droplets, and α-alumina (α-Al 2 O 3 ) is one component of mineral dust particles found in the atmosphere. Single surface droplet freezing experiments conducted for α-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 α-alumina (0001) surface. The (0001) surface of α-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 α-alumina (0001) surface. Under acidic or basic conditions, the surface hydroxy groups can become dual-protonated or deprotonated, respectively. Therefore, simulating model α-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 p K a 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 α-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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".