Autoionization of water which gives rise to its pH is one of the key
properties of aqueous systems. Surfaces of water and aqueous electrolytes
solutions are traditionally viewed as devoid of inorganic ions, however, recent
molecular simulations and spectroscopic experiments show the presence of
certain ions including hydronium in the top-most layer. This raises the question
about what is the pH of the surface of neat water. Microscopic simulations and
measurements with atomistic resolution show that water surface is acidic due to
a strong propensity of hydronium (but not of hydroxide) for the surface. In
contrast, macroscopic experiments, such as zeta potential and titration
measurements, indicate a negatively charged water surface interpreted in terms
of preferential adsorption of OH-. Here we review recent simulations and
experiments characterizing autoionization at the surface of liquid water and ice
crystals in an attempt to present and discuss in detail, if not fully resolve, this
controversy.