Variation of Ionic Strength Reveals the Interfacial Water Structure at a Charged Mineral Surface
Bibliographic record
Abstract
We have used visible-infrared sum-frequency generation spectroscopy to study the structure of water molecules that form the interface with fused silica. Spectra throughout the OH-stretching region were collected in two different polarization schemes so that the orientation of interfacial water molecules could be studied. A depth profiling experiment was performed by varying the ionic strength, thereby peeling away the outskirts of this interfacial region, leaving water molecules progressively closer to the charged interface. As the ionic strength increases, the spectral intensity drops since the interfacial region is becoming thinner. However, we notice a change in the relative contribution of tetrahedrally coordinated molecules compared to those that make less hydrogen bonds. We have the opportunity to study these two water species individually; we conclude that the higher coordination species exists some distance away from the interface and is oriented close to the plane of the surface with a mean tilt angle ∼70° and a narrow tilt and twist distribution. We have evidence that lower coordination water molecules are close to the interface and offer two possibilities for their orientation that are consistent with our data. One has a twist angle distribution centered in the plane of the interface, tilted ∼35° from the normal. The other possibility is that they are twisted out of the plane of the interface and tilted ∼30° from the normal. As the electrolyte concentration increases, we observe a reduction of the mean tilt angle in all cases. This is consistent with a picture of molecules nearer to the surface better aligned in the presence of a stronger interfacial field.
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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".