Water autoionization reaction 2H ₂ O → H ₃ O ⁻ + OH ⁻ is a textbook process of basic importance, resulting in pH = 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H ₃ O ⁺ and OH ⁻ ions, density functional studies of (H ₂ O) ₄₈ H ⁺ clusters, and spectroscopic isotopic-exchange data for D ₂ O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H ₃ O ⁺ does, but OH ⁻ does not, display preference for surface sites, the H ₂ O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.

Autoionization of water which gives rise to its pH is one of the key properties of aqueous systems. Surfaces of water and aqueous electrolyte 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 topmost layer. This raises the question of what is the pH (defined using proton concentration in the topmost layer) of the surface of neat water. Microscopic simulations and measurements with atomistic resolution show that the 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.

Reaction mechanisms for the acidic dissociation of HCl in water clusters are considered. Intermediates in the reaction are obtained from stationary points on the potential energy surface of the systems ${\mathrm{HCl–(H}}_{\mathrm{2}}{\mathrm{O)}}_n$ with $\text{n=4}$ and 5. These points have been determined by the B3LYP density functional method in an aug-cc-pVDZ atomic orbital (AO) basis. The total energies of the stationary points are checked by the coupled cluster single-double-triple [CCSD(T)] method in the same AO basis. For the case of $\text{n=4}$ a multibody analysis of the interaction energies is performed by the CCSD(T) method as well as by symmetry adapted perturbation theory. The clusters have a completely dissociated form as their energetically lowest minimum.