  Nonempirical modeling of hydrated single-layer graphene oxide (GO) fragments
revealed that at least one, but substantial origin of the acidity of aqueous GO
solutions is the dissociation in closely grouped and favorably H-bonded
hydroxyls. The process can further be promoted by the adjacent $\pi$-conjugated
carbon segments at the edges of GO flakes or their inner defects. The apparent
$pK_a$ values of the groups fall in a range of ca. 2.0 to 5.3. Under certain
conditions the ionization may become spontaneous.
