  Nanoconfinement has been shown to drastically affect the physical properties
of water. Its effects on reactivity and dissociation, however, remain
controversial, despite their importance to understand aqueous chemistry at
interfaces, pores, aerosolos or protein cavities, and the growing evidence
indicating the acceleration of chemical kinetics in confined environments. The
dissociation constant $K_w$ in nanospaces has been assessed from experiments
and simulations in a few specific cases, leading to dissimilar conclusions.
Here, using carefully designed ab-initio simulations and data-science tools, we
demonstrate, challenging current misconceptions, that the energetics of bulk
water dissociation remains unchanged to surprisingly small length-scales
including clusters of only a dozen water molecules. This is rooted in the fact
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 or in droplets of nanometer size. The subsequent separation of the
hydroxide and hydronium species contributes a marginal fraction of the total
free-energy, for which reason it turns out that confinement exerts little
control on this process. The present work provides a definitive and fundamental
description of the mechanism and thermodynamics of water dissociation at
different scales with broader implications on water's self-ionization at the
air-liquid interface and on chemical reactivity under nanoconfinement.
