Thermodynamics of hydronium and hydroxide surface solvation
Hub, J., Wolf, M., Caleman, C., Maaren, P., Groenhof, G., & Spoel, D. (2014). Thermodynamics of hydronium and hydroxide surface solvation. Chemical Science, 2014 (5), 1745-1749. doi:10.1039/C3SC52862F
Published inChemical Science
DisciplineEpäorgaaninen ja analyyttinen kemia
© Royal Society of Chemistry 2014. This is a final draft version of an article whose final and definitive form has been published by Royal Society of Chemistry.
[Introduction] The concentration of hydronium and hydroxide at the water-air interface has been under debate for a long time. Recent evidence from a range of experiments and theoretical calculations strongly suggests the water surface is somewhat acidic. Using novel polarizable models we have performed potential of mean force calculations of a hydronium ion, a hydroxide ion and a water molecule in a water droplet and a water slab and we were able to rationalize that hydronium, but not hydroxide, is slightly enriched at the surface for two reasons. First, because the hydrogen-bond acceptance capacity of hydronium is weaker than water it is more favorable to have the hydronium oxygen on the surface. Second, hydroxide ions are expelled from the surface of droplets, due to the entropy being lower when a hydroxide ion is hydrated on the surface. As a result, the water dissociation constant pK w increases slightly near the surface. The results are corroborated by calculations of surface tension of NaOH solutions that are in reasonable agreement with experiment. The structural and thermodynamic interpretation of hydronium and hydroxide hydration provided by these calculations opens the route to a better understanding of atmospheric- and surface chemistry. ...