Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H34: Environmental Interfaces IV |
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Sponsoring Units: DCP Chair: Bruce Kay, PNNL Room: LACC 511A |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H34.00001: Organic Structure/Reactivity Relationships for Probing the Reactivity of Mineral/Water Interfaces Invited Speaker: Physical organic chemists have long used systematic variations in the functional groups and molecular structure of organic reactants to explore reaction mechanisms. To extend this approach towards investigating ligand-assisted dissolution of iron(III) and chromium(III) (hydr)oxides, it is useful to employ the following reaction scheme: M(surf) + L(aq) = ML(surf) = ML(aq). Capillary electrophoresis enables us to simultaneously monitor L(aq) and ML(aq), which in turn enables us to distinguish the adsorption step from subsequent metal atom detachment. The placement of amine Lewis Base groups is crucial. With iminodiacetate, an amine group is part of the linkage between two carboxylate groups. As a consequence, adsorption is low but overall dissolution rates are high. With aspartate, the amine group is not part of the linkage - it resides on a side arm of the molecule. As a result, adsorption is high but overall dissolution rates are low. Structure-reactivity relationships can also be used to investigate reductive dissolution reactions. Here, we are interested in how the placement of carbonyl groups relative to one another affects reactivity towards manganese(III,IV) (hydr)oxides. Extents of adsorption are low and electron transfer rates high, making it difficult to distinguish the two reaction steps. Structural changes that increase electronic delocalization (i.e. involving both carbonyl groups) give a substantial boost to overall reaction rates. For all types of surface reactions, structure/reactivity relationships provide the basis for making predictions. Knowing the functional groups and molecular structure of extracellular biochemicals (e.g. a siderophore) or synthetic organic contaminants (e.g. a herbicide) becomes the first step in evaluating reaction mechanisms, reaction timescales, and ultimate fate. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H34.00002: Probing the Structure of Natural Organic Matter Adsorbed on Carbonaceous Surfaces using Atomic Force Microscopy Howard Fairbrother, Justin Gorham Relatively little is known about the adsorbate layers formed by Natural Organic Matter (NOM) on carbonaceous surfaces in aquatic environments. Developing a microscopic understanding of these adlayers would contribute to the development of more accurate and realistic models of surfaces in the environment. Additionally, this information may have ramifications for water treatment strategies where the adsorptive capacity and effective lifetime of activated carbons (ACs) are reduced by adsorbed NOM. We report on the adsorbate structures formed by NOM from the Great Dismal Swamp on highly ordered pyrolytic graphite (HOPG) as a function of pH using Atomic Force Microscopy (AFM). At pH 3.1, the HOPG is covered with adsorbed NOM molecules. Despite the high density of species, spherical structures within the adsorbate layer with average widths on the order of 40 - 100 nm and heights approx. 1.5nm can still be resolved. Based on the 2-dimensional symmetry and the size of these adsorbates, we believe that these structures are individual NOM molecules. At higher pH discrete patches (or islands) of what are believed to be aggregated NOM molecules can be resolved. Within these NOM aggregates, numerous voids or ``holes'' are also evident, indicating regions where the HOPG substrate remains exposed at the liquid/solid interface. In other NOM aggregates, singular ``lighthouse'' features are observed. Additional results on the effects of increasingly higher pH on the structure of the adlayer formed by NOM on HOPG will also be presented. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H34.00003: The Binding of Roxarsone at the Silica/Water Interface Studied with Second Harmonic Generation Christopher Konek, David Ostrowski, Franz Geiger Arsenic is a carcinogen that can also cause chronic poisoning when ingested via drinking water in quantities as low as 10 micrograms/L. In the US, organic arsenicals such as Roxarsone are commonly used as feed additives in the poultry industry. The use of poultry litter as fertilizer results in environmental arsenic deposition rates of up to 50 metric tons per year; the subsequent environmental fate of Roxarsone is unknown. We use second harmonic generation (SHG) to study the thermodynamics and kinetics of Roxarsone binding to environmentally relevant mineral oxide/water interfaces. Roxarsone binding to water/SiO2 interfaces is fully reversible, consistent with high Roxarsone mobility. Results from Langmuir isotherm measurements and surface SHG spectra are presented as well. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:36AM |
H34.00004: Surfactant Control of Gas Uptake into Supercooled Sulfuric Acid Invited Speaker: Surfactant molecules on sulfuric acid droplets can potentially alter the rates of heterogeneous reactions in the upper troposphere and lower stratosphere by blocking gas molecules from entering the acid. We perform molecular beam experiments with deuterated sulfuric acid solutions (56-68 wt percent D2SO4/D2O at 213 K) containing the surfactants 1-butanol or 1-hexanol, which segregate to the surface to form a nearly complete monolayer. A beam of a protic gas HX (X = Cl or Br) is directed at a continuously renewed film of deuterated sulfuric acid in vacuum and the fraction of thermalized HX molecules that undergo HX-DX exchange is measured. This HX-DX exchange fraction is equal to the HX entry probability into the acid. Our results appear to contradict the notion that surfactants generally impede gas transport. The presence of surface butanol does not alter the rate of D2O evaporation from the liquid surface, whereas surface hexanol slightly impedes D2O transport. The most striking result is that surface butanol molecules increase the HX-DX exchange fraction, implying that HX dissociates more readily at the interface when butanol is present. This enhancement may be caused by dilution of the acid near the surface by segregated butanol molecules, which provide additional OD groups for protonation by HX. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H34.00005: Direct Observation of Charge Inversion by Multivalent Ions as a Universal Electrostatic Phenomenon Serge Lemay, Koen Besteman, Marcel A. G. Zevenbergen, Hendrik A. Heering We have directly observed reversal of the polarity of charged surfaces in water upon the addition of trivalent and quadrivalent ions using atomic force microscopy. The bulk concentration of multivalent ions at which charge inversion reversibly occurs depends only very weakly on the chemical composition, surface structure, size, and lipophilicity of the ions. It is however very sensitive to the valence of the ions as well as the charge density of the surface and the dielectric constant of the solvent. These results support the theoretical proposal that spatial correlations between ions are the driving mechanism behind charge inversion. Ref: Phys. Rev. Lett. {\bf 93}, 170802 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H34.00006: Molecular Dynamics simulation of Oil-Water liquid-liquid interfaces Janamejaya Chowdhary, Branka Ladanyi Molecular Dynamics simulations are performed water-hydrocarbon liquid systems to study the effect of hydrocarbon branching on interfacial properties.The following two series of hydrocarbons are considered: 1) n-octane, 2-methyl heptane and 2,2,4 trimethyl pentane (constant molecular mass), and 2) n-pentane, 2-methyl pentane and 2,2,4 trimethyl pentane (constant chain length). The density profiles were constructed and analyzed in terms of the Capillary Wave model. With a simple algorithm for identification of surface molecules at the interface, intrinsic density profiles were obtained for different systems and found to resemble density profiles of a liquid in the presence of a soft wall. This supports the picture of a sharp interface broadened by capillary waves. Order parameters are used to study the orientation of water and hydrocarbon molecules corresponding to the total and intrinsic density profiles. At the interface, water preferably points one O-H bond towards the hydrocarbon, linear hydrocarbons stack parallel to, while the more branched hydrocarbons orient perpendicular to the interface. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:36AM |
H34.00007: Connecting Spectroscopy and Computational Chemistry to Study Interfacial Phenomena Invited Speaker: Interpretation of spectra in systems of environmental interest is not generally straightforward due to the lack of close analogs and a clear structure of some components of the system. Computational chemistry can be used as an objective method to test interpretations of spectra. This talk will focus on applying ab initio methods to complement vibrational, NMR, and EXAFS spectroscopic information. Examples of systems studied include phosphate/Fe-hydroxides, arsenate/Al- and Fe-hydroxide, fractured silica surfaces. Phosphate interactions with Fe-hydroxides are important in controlling nutrient availability in soils and transport within streams. In addition, organo-phosphate bonding may be a key attachment mechanism for bacteria at Fe-oxide surfaces. Interpretation of IR spectra is enhanced by model predictions of vibrational frequencies for various surface complexes. Ab initio calculations were used to help explain As(V) and As(III) adsorption behavior onto amorphous Al- and Fe-hydroxides in conjunction with EXAFS measurements. Fractured silica surfaces have been implicated in silicosis. These calculations test structures that could give rise to radical formation on silica surfaces. Calculations to simulate the creation of Si and SiO radical species on sufaces and their subsequent production of OH radicals will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H34.00008: Dynamic interfaces of manganese oxide and manganese carbonate substrate in the presence of cobalt Young-Shin Jun, Scot Martin The transport and fate of metal contaminants in water are often affected by manganese oxide coatings. Direct microscopic observations of such dynamic reactions are sparse. In the current work, cobalt adsorption/co-precipitation with manganese oxides is studied by atomic force microscopy at circumneutral pH. Three complementary experimental protocols are employed: (1) Mn oxide film, (2) Mn oxide film and Co$_{x}$Mn$_{1-x}$O$_{y}$ (mixed oxides), and (3) Co$_{x}$Mn$_{1-x}$O$_{y}$. The appearance of islands are flat top and 2-$D$ rhombohedral (thickness: 2.4 nm) for Mn oxide, a stratum structure (2.3 nm and higher) for the mixed oxides, and round top (indefinite growth) for Co$_{x}$Mn$_{1-x}$O$_{y}$. The island density (number/m$^{2})$ is Mn oxide film $>$ mixed oxides $>$ Co$_{x}$Mn$_{1-x}$O$_{y}$. The macroscopic Mn dissolution rates of Co$_{x}$Mn$_{1-x}$O$_{y}$ are slower than that of Mn oxide. In the case of the mixed oxides, the dissolution rates decreases by three times. Direct observations of interactions of cobalt with manganese oxide on manganese carbonate provide new insights into the interfacial reactions between coatings and substrate minerals in the environment. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H34.00009: Rapid assessment of anisotropic surface dissolution processes using atomic force microscopy (AFM) and electron backscattered diffraction (EBSD): A corrosion study of polycrystalline alloy 22 in various acidic environments Jeremy Gray, Bassem El-Dasher, Joel Hayes, Christine Orme We utilize atomic force microscopy (AFM) and electron backscattered diffraction (EBSD) to correlate crystallographic orientations with dissolution rates of polycrystalline alloy 22 surfaces exposed to hydrochloric, sulfuric and oxalic acids. Additionally, we utilize AFM to gain insight into the dissolution mechanism processes and to correlate the dissolution current with penetration depth. For all acids, the dissolution rates scale with the deviation of the angle with the surface normal from the (1 1 1) direction. In hydrochloric and oxalic acids, the alloy dissolution is approximately uniform across individual grains. In contrast, in sulfuric acid, the dissolution is inhibited at crystallographic step sites. [Preview Abstract] |
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