Water self-dissociation in slit pores displays non-monotonic behavior as a function of water filling.

Although nanofluidics and chemistry in nanoconfined liquids has emerged as an exciting field, the quantitative impact of confinement on fundamental properties remains often unclear. Currently, there is not yet consensus on the impact of slit pore confinement on water self-dissociation, namely if this ubiquitous elementary reaction is enhanced, unaltered or suppressed by nanoconfinement. We address this question for well-defined water/graphene slit pore systems that allow us to carefully establish the appropriate thermodynamic conditions for different pore fillings. Anticipating our key results, we show that the energetics of the self-dissociation reaction is very sensitive to even subtle changes of the confinement conditions, leading even to non-monotonic behavior depending on water filling. This effect is found to correlate with the different capabilities of different nanoconfined water lamellae to solvate the nascent hydroxide defect beyond its second hydration shell parallel to the confining walls.