On the complex structural diffusion of proton holes in nanoconfined alkaline solutions within slit pores

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.

One-dimensional vs. two-dimensional proton transport processes at solid–liquid zinc-oxide–water interfaces

“…One sometimes uses a different perspective, namely that proton “holes”, i.e. , missing protons, are transferred from the hydroxide ion to the water molecule 2,3. The Grotthuss mechanism then becomes a series of proton hole transfer events.…”

“…For instance, recent molecular dynamics (MD) studies10,11 have shown that OH – diffuses both vehicularly (mass transport) and via a Grotthuss-like mechanism in anion exchange membranes. Further, Muñoz-Santiburcio and Marx3 explored OH – diffusion in nanoconfined slit pores, and concluded that the transport mechanism depends on the degree of confinement. Achtyl et al 12 showed that protons can diffuse through hydroxyl-terminated atomic defects in single-layer graphene.…”

Potential Role of Inorganic Confined Environments in Prebiotic Phosphorylation

“…Another consequence attributed to a confined environment is its effect on the dielectric constant of interfacial water in comparison to bulk water, where charged species are stabilized to a greater extent at the interfacial region in comparison to bulk-water [ 46 ]. This conforms to Davis’ principle- the importance of being charged [ 34 ], observed at a lower confinement scale.…”

“…Another interesting aspect of confined water is the efficient proton transfer at extreme confinement (by the Grotthuss mechanism) by retaining this property from bulk water [ 46 , 48 ]. Such efficient proton transfer in confinement might have preceded the efficient proton channels that are observed in contemporary cells and maintain a natural proton gradient.…”

“…Munoz-Santiburcio and Marx [ 46 ] have recently reported ab initio studies on peptide synthesis (by activation via N-carboxyanhydride) and their investigations of interfacial water between iron-sulphur minerals have revealed that charged species and transition states required lower activation energy in comparison to bulk water and water in extreme conditions. They attributed this observation to the enhanced dielectric properties of water in confinement, where the charged species are stabilized and the zwitterionic forms are prevalent, which, in turn, reinforces the importance of charged species in present-day cellular compartments.…”