One of the most fundamental concepts in aqueous chemistry is the degree of dissociation of Bronsted acids and bases. For strong acids usually a complete dissociation and formation of independent solvated charged fragments is assumed. The existence of a contact ion pair even in low concentrated hydrochloric acid (2.5M) demonstrates that the counter ions do not behave merely as spectators. Through the use of modern X-ray absorption fine structure (EXAFS) measurements in conjunction with state-of-the- art density functional theory (DFT) simulations, we are able to obtain an unprecedented view into the molecular structure of medium to high concentrated electrolytes (2.5M�16M).These findings in the bulk are compared to recent experimental findings suggesting a very different dissociation behavior for strong acids at the air-water interface . The free energy barrier associated with the dissociation of strong acids, XH (HBr, HCl and HNO3)  and the formation of ionpairs, X����H3O+ is investigated using extensive density functional theory simulations. The free energy for HCl and HNO3 show significant difference at the air-water interface as compared to bulk solvation, leading to a picture in which the contact ion pair is a local minimum at the air-water interface.Parts of  the research described in this presentation was conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the U.S. Depart-ment of Energy under Contract DE-AC05-76RL01830.Parts of this work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.  Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Structure and dissociation of strong acids in bulk and at the air-water interface

246th National Meeting (2013)

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