Abstract
The hydroxide anion OH−(aq) in homogeneous bulk water, that is, the solvated proton hole, is known to feature peculiar properties compared with excess protons solvated therein. In this work, it is disclosed that nanoconfinement of such alkaline aqueous solutions strongly affects the key structural and dynamical properties of OH−(aq) compared with the bulk limit. The combined effect of the preferred hypercoordinated solvation pattern of OH−(aq), its preferred perpendicular orientation relative to the confining surfaces, the pronounced layering of nanoconfined water and the topology of the hydrogen bond network required for proton hole transfer lead to major changes of the charge transport mechanism, in such a way that the proton hole migration mechanism depends exquisitely on the width of the confined space that hosts the water film. Moreover, the anionic Zundel complex, which is of transient nature in homogeneous bulk solutions, can be dynamically trapped as a shallow intermediate species by suitable nanoconfinement conditions.
Highlights
The hydroxide anion OH À in homogeneous bulk water, that is, the solvated proton hole, is known to feature peculiar properties compared with excess protons solvated therein
Alkaline aqueous solutions in reduced dimensionality and nanoconfinement are becoming highly relevant in view of the very distinct properties of nanoconfined water compared with the bulk
The water dynamics in this case is liquid-like and again this is consistent with our previous simulations of neutral and acidic nanoconfined water[33,34]
Summary
The hydroxide anion OH À (aq) in homogeneous bulk water, that is, the solvated proton hole, is known to feature peculiar properties compared with excess protons solvated therein. It is disclosed that nanoconfinement of such alkaline aqueous solutions strongly affects the key structural and dynamical properties of OH À (aq) compared with the bulk limit. Its layered structure can be intercalated by water and it has been suggested that a primordial ‘pyrophosphate synthetase nanoengine’ could have emerged, thanks to the charge gradients that can be established along these nanochannels[29,30] Because of these ramifications and as a prologue for an upcoming study of prebiotic reactions[31,32] in such environments, we set out to investigate the properties of nanoconfined water in mackinawite at the relevant elevated temperature and pressure conditions[33], and those of the solvated excess proton therein[34]. Anticipating our core result, it is shown that these confinements imprint stark and unexpected differences on the structural diffusion mechanism of OH À (aq) unknown from H þ (aq)
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