The acid-base character of the surface of water is important in a wide range of chemical, biological, material, and environmental contexts. However, the question has proved vexing, with indications ranging all the way from a distinct excess of hydroxide according to electrophoresis studies of colloids to an excess of hydronium according to conventional computer simulations.1-5 An alternative is the LEWIS force field, in which valence electron pairs are treated as explicit, independently mobile, semi-classical particles that interact pairwise with each other and with semi-classical kernels that comprise nuclei and core electrons. The resulting water is amphiprotic and intrinsically polarizable, while the efficiency of the construct allows large and long simulations.6 A study of the self-ions in bulk water has shown hydroxide solvation that largely mirrors that of hydronium and ion diffusion rates relative to water that are consistent with experiment.7 We now turn to systems with reduced symmetry. Using LEWIS to simulate slabs, cylinders and droplets of 1000 molecules, we find that hydroxide consistently prefers the surface, hydronium consistently avoids the surface, and the two together form an electrical double layer until neutralization occurs. The behavior of hydroxide can largely be accounted for by the observation that hydroxide moving to the surface loses fewer hydrogen bonds than are gained by the water molecule that it displaces from the surface. No such information can be gleaned for hydronium as it is never found at the surface. (1) Petersen, P. B.; Saykally, R. J. Chemical Physics Letters 2008, 458, 255. (2) Beattie, J. K.; Djerdjev, A. M.; Warr, G. G. Faraday Discussions 2009, 141, 31. (3) Buch, V.; Milet, A.; Vacha, R.; Jungwirth, P.; Devlin, J. P. Proceedings of the National Academy of Sciences 2007, 104, 7342. (4) Zimmermann, R.; Freudenberg, U.; Schweiss, R.; Küttner, D.; Werner, C. Current Opinion in Colloid & Interface Science 2010, 15, 196. (5) Saykally, R. J. Nat Chem 2013, 5, 82. (6) Kale, S.; Herzfeld, J. J. Chem. Phys. 2012, 136, 084109. (7) Kale, S.; Herzfeld, J. Angew. Chem. Int. Ed. 2012, 51, 11029.

Surface propensity of the self-ions of water: A Lewis Study

251st National Meeting (2016)

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