Engineering topics

Baer, Marcel D

Publications and source records attributed to Baer, Marcel D.

3 records

DOE OSTI2013

An ab initio approach to understanding the specific ion effect

Although there is a consensus that large, polarizable anions will adsorb to the air-water interface, the precise interactions that give rise to surface enhancement are still being debated. Previously, we have demonstrated that there is a significant dependence on the choice of molecular interaction potential for iodide adsorption at the air-water interface. Specifically, density functional theory (DFT) based interaction potentials lead to significantly less adsorption than empirical interaction potentials that include polarization. We have also demonstrated that DFT based interaction potentials can accurately capture the structure of the first solvation shell of both simple and polyatomic anions as compared to multi-edge X-ray absorption fine structure (XAFS) experiments. We continue to push DFT by examining the surface propensity and structure of polyatomic anions that exhibit both chaotropic and kosmotropic characteristics as classified by the Hofmeister series. We will investigate the role of local solvation effects of SCN- and compare to previous studies on I- and IO3- under both bulk and interfacial conditions. A picture emerges where we can correlate local solvation stucture to the Hofmeister series.

Ion adsorption to the water/air Interface↗

DOE OSTI2012

Electrochemical Surface Potential due to Classical Point Charge Models Drives Anion Adsorption to the Air-Water Interface

Herein, we present research that suggests that the underlying physics that drive simple empirical models of anions (e.g. point charge, no polarization) to the air-water interface, with water described by SPC/E, or related partial charge models is different than when both ions and water are modeled with quantum mechanical based interactions. Specifically, we will show that the driving force of ions to the air-water interface for point charge models results from both cavitation and the negative electrochemical surface potential. We will demonstrate that we can fully characterize the role of the free energy due to the electrochemical surface potential computed from simple empirical models and its role in ionic adsorption within the context of dielectric continuum theory (DCT). Our research suggests that a significant part of the electrochemical surface potential in empirical models appears to be an artifact of the failure of point charge models in the vicinity of a broken symmetry. This work was supported by the U.S. Department of Energy‘s (DOE) Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory (PNNL) is operated for the Department of Energy by Battelle.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Does Nitric Acid Dissociate at the Aqueous Solution Surface?

Nitric acid is a prevalent component of atmospheric aerosols, and the extent of nitric acid dissociation at aqueous interfaces is relevant to its role in heterogeneous atmospheric chemistry. Several experimental and theoretical studies have suggested that the extent of dissociation of nitric acid near aqueous interfaces is less than in bulk solution. Here, dissociation of HNO3 at the surface of aqueous nitric acid is quantified using X-ray photoelectron spectroscopy of the nitrogen local electronic structure. The relative amounts of undissociated HNO3(aq) and dissociated NO3-(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we determine the degree of dissociation, αint, in the interface (the first ~3 layers of solution) as a function of HNO3 concentration. Our measurements show that dissociation is decreased by approximately 20% near the solution interface compared with bulk, and furthermore that dissociation occurs even in the top-most solution layer. The experimental results are supported by first-principles MD simulations, which show that hydrogen-bonds between HNO3 and water molecules at the solution surface stabilize the molecular form at low concentration, in analogy to the stabilization of molecular HNO3 that occurs in bulk solution at high concentration. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

54 ENVIRONMENTAL SCIENCES↗