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Absorbate-Induced Restructuring of Interfacial Water

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Presentation on theme: "Absorbate-Induced Restructuring of Interfacial Water"— Presentation transcript:

1 Absorbate-Induced Restructuring of Interfacial Water
Jeffrey G. Catalano Spring 2013 ACS National Meeting

2 Acknowledgements Collaborators Financial Support Paul Fenter (ANL)
Changyong Park (ANL) Zhan Zhang (ANL) Kevin Rosso (PNNL) Yun Luo (Wash U; now at GIA) Karyn Blake (Wash U) Financial Support ANL Named Postdoctoral Fellowship Program DOE/BES Geosciences Research Program NSF Geobiology and Low-Temperature Geochemistry ACS Petroleum Research Fund Washington University Earth and Planetary Sciences • Washington University

3 Importance of Interfacial Reactions to Environmental Systems
Nutrient Availability in Soils and Aquatic Systems Occurrence, Fate, and Transport of Water Contaminants Reactions at the interface between mineral solids and natural waters affect: The behavior of natural and anthropogenic water contaminants, including radionuclides Nutrient bioavailability Electron transfer processes associated with microbial metabolism CO2 sequestration The composition of natural water Microbial Metabolic Activity Figure from: USDA-NRCS; PNNL; Polizzotto et al. (2008) Nature Earth and Planetary Sciences • Washington University

4 Metal (hydr)oxide Mineral
β d ψ0 ψβ ψd Distance Metal (hydr)oxide Mineral Effects of Interfacial Structure and Properties on Geochemical Processes Reactive sites on mineral surfaces control the chemical interactions with aqueous species Charging at the surface generates a local electric potential that substantial alters the favorability of interfacial reactions Mineral surfaces induce ordering of interfacial water that in turn may affect the stability of surface complexes Feedbacks may exist between surface charging, ion adsorption, and water ordering Earth and Planetary Sciences • Washington University

5 Need to Obtain a Fundamental Understanding of the Mineral-Water Interface
Reactions at mineral-water interfaces affect many important environmental and geological processes We must understand mineral-water interfaces at a basic level in order to predict the effect of interfacial reactions on natural and engineered systems Essential fundamental questions: What is the structure of a mineral- water interface? Do changes in pH and ionic strength alter the structure of interfacial regions? Does the adsorption of ions on mineral surfaces alter the arrangement of interfacial water? Figure after: Brown and Parks (2001) Int. Geol. Rev. 43, Earth and Planetary Sciences • Washington University

6 Measuring Surface Structures with X-ray Reflectivity
Bragg Peak (012) Bragg Peak (024) Bragg Peak (036) Bragg rod = X-ray Reflectivity Scattering from a crystal is observed as Bragg points Scattering from a surface is observed as Bragg rods X-ray reflectivity reveals the surface structure (relaxations, reconstruction), water and adsorbate structures, and surface roughness Earth and Planetary Sciences • Washington University

7 Hematite (012)-Water Interface
Surface relaxations are minor Structure is similar to bulk Surface atoms fully coordinated Surface perturbs local water structure Two resolved adsorbed water sites Layering in water farther from the surface Catalano et al. (2007) GCA 71, Earth and Planetary Sciences • Washington University

8 Variations in Water Structure Near Isostructural Surfaces
Mineral surface structure controls the arrangement of interfacial water Water structure is similar on isostructural surfaces Water generally shows greater positional disorder on hematite Possibly controlled by functional group exchange rates Water displays fundamentally weaker ordering on (001) surfaces The adsorbed water layer has a more disordered spatial distribution This surface displays bridging O functional groups with a limited ability to form H-bonds with water Catalano et al. (2006) Langmuir Catalano et al. (2007) GCA Catalano et al. (2009) GCA Catalano (2010) J Phys Chem C Catalano (2011) GCA Earth and Planetary Sciences • Washington University

9 Distinct Mineral Surface Charging Behaviors may Produce Two Classes of Water Ordering
>AlOH2+1/2 or >AlOH-1/2 >Al2OH0 Surface with functional groups that are neutral over a wide pH range [e.g., corundum and hematite (001), quartz (10-10) and (10-11)*] show distinctly weaker ordering of interfacial water Earth and Planetary Sciences • Washington University *Schlegel et al. (2002) GCA 66,

10 Evidence for Water Reorientation as a Function of pH and Surface Charging on Corundum (001)
Phase-sensitive SFVS shows reorientation of “ice-like” water as pH increases Water reorientation follows surface charging behavior SFVS shows that interfacial water molecules reorient as pH increases, suggesting that restructuring occurs Clear changes in 3200 and 3450 cm-1 bands with pH in SFVS spectra Zhang et al. (2008) JACS 130, Earth and Planetary Sciences • Washington University

11 pH Effects on the Interfacial Water Structure on α-Al2O3 (001) Surfaces
No large changes in XR observed between pH 3 and 9 in 0.01 M NaCl No evidence for water restructuring Few changes in structural parameters are statistically meaningful Only a small population of water molecules likely reorients as pH changes Earth and Planetary Sciences • Washington University

12 As(V) Adsorption on the Corundum (001) Surface
Full Interfacial Profile Interfacial Water Comparison pH 5 0.01 M NaCl Mixture of inner- and outer-sphere arsenate complexes Outer-sphere located above first water layer Inner-sphere coordinated to bridging oxygen groups As(V) adsorption substantially alters interfacial water Increases positional order of first water layer Modifies structure Creates a more hydrophilic surface Earth and Planetary Sciences • Washington University

13 Changes in Interfacial Water Structure Following AsO43- Adsorption
Adsorption of a charged ion induces a structural change in water much more substantial than produced through surface charging Positional ordering of adsorbed water increases and multiple layers shift closer to the mineral surface Earth and Planetary Sciences • Washington University

14 Charged Surface Sites and Adsorbates Produce Highly-Ordered Interfacial Water
The spatial arrangement of interfacial water is controlled by mineral surface structure Chemical properties or surface group exchange rates play a secondary role in controlling spatial ordering of water The charge states of surface functional groups determines the strength of water ordering Surfaces with uncharged groups produce weaker water ordering pH changes on weakly-ordering surface do not alter interfacial water structure, but adsorbates cause substantial restructuring What are the energetic contributions of water restructuring transitions to interfacial reactions and are these transitions discrete or continuous? Earth and Planetary Sciences • Washington University

15 Earth and Planetary Sciences • Washington University

16 pH Effects on Interfacial Water on (001) Surfaces
Nominal PZC is ~6 Subtle changes in interfacial structure occur away from the PZC Surface atoms relax inward Extended ordered water network shifts away from surface Bridging oxygen functional group shows increased vibrational motion Adsorbed water becomes more ordered Earth and Planetary Sciences • Washington University

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