Water autoionization reaction 2H2O 3 H3O ؊ ؉ 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 H3O ؉ and OH ؊ ions, density functional studies of (H2O)48H ؉ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O ؉ does, but OH ؊ does not, display preference for surface sites, the H2O 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.density functional theory ͉ IR spectroscopy ͉ molecular dynamics ͉ water autoionization ͉ ice nanocrystals I n room-temperature liquid, one in 6 ϫ 10 8 water molecules is autoionized, yielding the standard value of pH ϭ 7. Autoionization in crystal ice should be less favorable, because, in contrast to water, ice is a very poor solvent of ionic and polar substances (1). As recently realized (2-5), the chemistry and composition of aqueous surfaces are quite distinct from that of the bulk; therefore, autoionization behavior should be reexamined at the surface.A number of recent computations (6-8) indicated the preference of hydronium cations for surface positions. Surface propensity of H 3 O ϩ was also deduced from vibrational spectroscopy of large protonated water clusters (6), as well as vibrational sum frequency generation (8, 9) and second harmonic generation (10) spectroscopic experiments probing extended aqueous interfaces. Interestingly, zeta potential measurements and titration experiments on oil droplets dispersed in water indicated the presence of negative charges at the interface, interpreted as adsorbed OH Ϫ ions (11). Similar conclusions have also been drawn from zeta potentials of air bubbles in water (12). More work is clearly needed to reconcile this apparent discord between predictions of surface-selective spectroscopies and molecular simulations on one side and electrochemical measurements on the other side.H 3 O ϩ forms three strong proton-donor bonds to H 2 O, but acts as a poor proton acceptor. A surface position with only H atoms hydrogen-bonded is preferred to interior positions, because the latter are associated with disruption of the approximately tetrahedral hydrogen-bond network in water (10). The present work focuses on the effect of surface stabilization of hydronium on water autoionization and surface pH. CalculationsOverview. Modeling of proton-tranfer systems is a nontrivial problem, because standard (empirical) potential energy surfaces do not include a possibility of proton hopping between different water molecules or transitions between the two li...

2004) Solid water clusters in the size range of tens-thousands of H 2 O: a combined computational/spectroscopic outlook, A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n ¼ 20-931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasispherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mostly crystalline subsurface layer are stretched by the interaction with the disordered component. The emergence of the (strained) crystal core occurs at a size of a few hundred H 2 O. Smaller clusters are described as compactamorphous.

Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl(-)) and hydrated protons (H(3)O(+) or H(5)O(2)(+)). This acid ionization process also occurs in small water clusters and on ice surfaces, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H(3)O(+) and Cl(-), and an ionic surface phase rich in Zundel ions, H(5)O(2)(+).

The study focuses on acid adsorption on cold ice particle surfaces. The investigation encompasses HCl, DCl, and HBr adsorbate spectroscopy, Monte Carlo simulations of molecular HCl adsorbate on a model ice particle, and ab initio studies of HCl solvation and ionization in mixed acid−water clusters. It is shown that ice nanocrystal surfaces offer a range of adsorption sites, in which HCl freezes in different recognizable solvation stages. These stages were identified spectroscopically and assigned, with the help of calculations, to weakly and strongly stretched HCl molecules, and to Zundel and hydronium ions that are products of proton transfer. At moderate submonolayer coverages in the 50−60 K range, the acid adsorbate is predominantly molecular. Heating promotes formation of contact hydronium − chloride ion pairs. Near 90 K, an ionization burst is observed, resulting in an ionic surface hydrate layer rich in Zundel cations.

No abstract