title: Propensity of Hydrated Excess Protons and Hydroxide Anions for the Air-Water Interface.
abstract: Significant effort has been undertaken to better understand the molecular details governing the propensity of ions for the air-water interface. Facilitated by computationally efficient reactive molecular dynamics simulations, new and statistically conclusive molecular-scale results on the affinity of the hydrated excess proton and hydroxide anion for the air-water interface are presented. These simulations capture the dynamic bond breaking and formation processes (charge defect delocalization) that are important for correctly describing the solvation and transport of these complex species. The excess proton is found to be attracted to the interface, which is correlated with a favorable enthalpic contribution and consistent with reducing the disruption in the hydrogen bond network caused by the ion complex. However, a recent refinement of the underlying reactive potential energy function for the hydrated excess proton shows the interfacial attraction to be weaker, albeit nonzero, a result that is consistent with the experimental surface tension measurements. The influence of a weak hydrogen bond donated from water to the protonated oxygen, recently found to play an important role in excess hydrated proton transport in bulk water, is seen to also be important for this study. In contrast, the hydroxide ion is found to be repelled from the air-water interface. This repulsion is characterized by a reduction of the energetically favorable ion-water interactions, which creates an enthalpic penalty as the ion approaches the interface. Finally, we find that the fluctuation in the coordination number around water sheds new light on the observed entropic trends for both ions.
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Ying-Lung Steve
authors.initials: YL
authors.lastname: Tse
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.|College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University , Wuhan 430072, China.
authors.forename: Chen
authors.initials: C
authors.lastname: Chen
authors.affiliation: Department of Chemistry and Biochemistry, Northern Arizona University , Flagstaff, Arizona 86011, United States.
authors.forename: Gerrick E
authors.initials: GE
authors.lastname: Lindberg
authors.affiliation: Department of Chemistry, Louisiana State University , Baton Rouge, Louisiana 70803, United States.
authors.forename: Revati
authors.initials: R
authors.lastname: Kumar
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Voth