Figure 4: Schematic representation of the distinct proton hole migration mechanisms.

Schematic representation of the distinct proton hole migration mechanisms in the narrow and wide slit pores, that is, system N in (a) and system W in (b–e). The top and bottom confining surfaces are schematically included as brown bars, whereas the high/low-density water regions are represented using light/dark blue shading, to visualize the water monolayer/bilayer structure of the water film in system N/W, respectively. Only key species and hydrogen bonds (dotted lines) are shown in terms of chemical formulae, thus omitting the full solvation shell of the OH⁻ defect (with the negative charge symbol attached to it, see also f). In the narrow pore limit, the preferred zig–zag migration path of OH⁻ is depicted in a. In the bilayer pore W, one can distinguish proton (hole) rattling relative to the ‘exposed’ state E without net charge migration (b), structural diffusion via the ‘exposed’–‘buried’–‘exposed’ E–B–E (c) and ‘tilted’–‘buried’–‘exposed’ T–B–E (d) mechanisms, as well as structural diffusion via alternating ‘tilted’ and ‘buried’ configurations (e), which stabilizes the trapped Zundel intermediate Z (see text) as symbolized in e using square brackets; see Fig. 5 for definitions. The standard notation employed for the solvated OH⁻(aq) complex is depicted in f using a representative snapshot of the preferred hypercoordinated resting state as sampled at the interface. Here, O* and H′ denote the site of the hydroxide anion, whereas H* and Õ are the sites of the particular water molecule, which donates the most active hydrogen bond along which proton (hole) transfer will eventually occur.