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# Autoionization-driven ordering of interfacial water into an anisotropic proton conductor
## The adsorbate and its field
At a hydrophilic surface, water adsorbs into an ordered honeycomb film, a few layers thick (3–4). Autoionization between this adsorbate and the adjacent liquid releases H⁺ into the liquid and retains OH⁻ at the surface. The loss of bridging hydrogens allows adjacent layers to shift and collapse together — the layers slide so that O from one layer meets H from the next, forming new bonds as they compress. The resulting structure is the most stable phase for the deprotonated network: a thin (~1 nm) adsorbate of composition (H₃O₂⁻)ₙ, with honeycomb layers shifted by half an oxygen.
Only one carrier is mobile: H⁺ leaves, OH⁻ stays. This is the strongest case of mobility asymmetry, and it produces an enormous diffusion-potential field.
## The exclusion zone crystal
The field organizes the adjacent liquid into a honeycomb crystal, ~300 µm thick, with layers shifted by a full oxygen. The stacking must be ABC — in AB stacking, each oxygen would need five bonds (three in-plane, one up, one down), which oxygen cannot form. ABC avoids this by placing the nodes above and below at different lateral positions. The hexagonal openings are cross-linked by bridging waters.
In the honeycomb, the ring oxygens (δ⁻) sit on the vertices while the hydrogens (δ⁺) point partly inward along the edges. The interior of each hexagon is a positive electrostatic pocket.
The bridging water sits at the center of this pocket. It can only bond vertically — to the layers above and below. In the ground state, the bridge is OH⁻ — stabilized by the ring's positive pocket. The layer oxygen below is H₃O⁺, with its extra H pointing toward the adsorbate, along the field. The chain alternates: OH⁻, H₃O⁺, OH⁻, H₃O⁺. Every vertical hydrogen points the same direction. This polarized configuration is the ground state — it is what makes the exclusion zone stable. The electrostatic field in the ring stabilizes the arrangement: a negative ion centered in a positive pocket.
Electron diffraction shows hexagonal symmetry with electron density in the hexagonal centers (McGeoch & McGeoch, 2008). In projection, this is what one expects from honeycomb layers shifted by a full oxygen — nodes from adjacent layers fall into the hexagonal openings of the layer in front. In ice-like stacking, the hexagonal centers form a vertical column. A chain of H₂O through this column is not possible — hydrogen would meet hydrogen between adjacent molecules. An OH⁻/H₃O⁺ alternation is also not possible — H₃O⁺ does not fit in the column. The full-oxygen-shift ABC structure resolves both problems: the honeycomb lattice accommodates H₃O⁺, OH⁻ sits in the hexagonal centers, and the diffusion-potential field stabilizes the alternation. Full-oxygen-shift stacking is required. AB full-shift would produce the same projection, but cannot exist (the five-bond problem). Only ABC can. The measured d-spacing of 3.73 Å is compressed relative to normal ice (3.9 Å), consistent with the ring being drawn inward by the negative charge at its center.
## An anisotropic proton conductor
A hydrogen bond is a shared proton. An ordered network of oriented hydrogen bonds is a proton conductor. These are the same statement in two languages.
The ABC stacking means that direct node-to-node connections between adjacent layers strictly alternate with passages through hexagonal openings — never two of the same in a row. With bridging waters in place, the proton chain runs: layer-O → bridge-O → layer-O → bridge-O, with all hops equivalent.
In bulk water, Grotthuss conduction is limited by two bottlenecks: rotation of the receiving molecule to orient the next bond, and reorganization of the solvation shell around the new and old proton positions. The crystal eliminates solvation/desolvation entirely — the lattice is the solvation shell, and it is fixed.
The conduction is anisotropic: along the chains perpendicular to the surface only. Within the honeycomb plane, the structure is an insulator.
## Conduction mechanism
Each OH⁻ in the chain is a hole — it can accept a proton. When H⁺ arrives at an OH⁻ bridge, the bridge becomes H₂O — and now has a hydrogen pointing directly at the H₃O⁺ below. H meets H. This repulsion cannot be sustained. The H₃O⁺ must release its H downward — but that hits the next OH⁻, which becomes H₂O with H pointing at the next H₃O⁺.
The entire chain must move as a single concerted motion — all bridges rotating and all protons hopping simultaneously. It is a collective mode, not a sequential cascade.
## References
McGeoch, J. E., & McGeoch, M. W. (2008). Entrapment of water by subunit c of ATP synthase. *Journal of the Royal Society Interface,* 5(20), 311–318.
Pollack, G. H. (2013). *The Fourth Phase of Water: Beyond Solid, Liquid, and Vapor.* Ebner & Sons.
Zheng, J., Chin, W.-C., Khijniak, E., Khijniak, E., Jr., & Pollack, G. H. (2006). Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact. *Advances in Colloid and Interface Science,* 127(1), 19–27.