  Nanoconfinement effects on water dissociation and reactivity remain
controversial, despite their importance to understand the aqueous chemistry at
interfaces, pores, or aerosols. The pKw in confined environments has been
assessed from experiments and simulations in a few specific cases, leading to
dissimilar conclusions. Here, with the use of carefully designed
\textit{ab-initio} simulations, we demonstrate that the energetics of bulk
water dissociation is conserved intact to unexpectedly small length-scales,
down to aggregates of only a dozen molecules or pores of widths below 2 nm. The
reason is that most of the free-energy involved in water autoionization comes
from breaking the O-H covalent bond, which has a comparable barrier in the bulk
liquid, in a small droplet of nanometer size, or in a nanopore in the absence
of strong interfacial interactions. Thus, dissociation free-energy profiles in
nanoscopic aggregates or in 2D slabs of 1 nm width reproduce the behavior
corresponding to the bulk liquid, regardless of whether the corresponding
nanophase is delimited by a solid or a gas interface. The present work provides
a definite and fundamental description of the mechanism and thermodynamics of
water dissociation at different scales with broader implications on reactivity
and self-ionization at the air-liquid interface.
