The unique behavior of hydrated excess protons in mixed dielectric environments will be presented. Such environments include the water-vapor and water-hydrophobic interfaces. The implications of this behavior for spectroscopic, thermodynamic, and dynamical measurements will be presented, as well as the unusual solvation behavior of hydrophobic species in concentrated acid solutions. Similarly, the structure and dynamics of hydrated hydroxide anions at the water-vapor interface will be characterized. The contrasting behavior of hydrated protons and hydroxide ions can be understood on a common footing by determining the free energy profile of moving the respective ion to the interfacial region and further analyzing the enthalpic and entropic contributions to the free energy. A local decomposition approximation to the enthalpy is employed, which examines the contributions from the coordinating environment around the ion, the interfacial region, and the bulk-like region, thus providing further insight into various competing effects to the enthalpy. For both the hydrated proton as well as the hydroxide ion the charge is delocalized to differing extent over the solvating water molecules. The contribution of these delocalization effects to the differences in the enthalpy between the bulk and the interfacial region will be examined. Although a local decomposition scheme is not possible for the entropy, the fluctuations in the instantaneous interface will be presented to help understand the origin of the entropic differences, at the aqueous interface, between the hydrated proton and hydroxide ions.

Understanding the behavior of hydrated protons and hydroxide anions at aqueous interfaces

246th National Meeting (2013)

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