Absorbate -Induced Restructuring of Interfacial Water

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

2. Acknowledgements Collaborators Paul Fenter (ANL) Changyong Park (ANL) Zhan Zhang (ANL) Kevin Rosso (PNNL) Yun Luo (Wash U; now at GIA) KarynBlake (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 • 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 Occurrence, Fate, and Transport of Water Contaminants Nutrient Availability in Soils and Aquatic Systems Microbial Metabolic Activity Earth and Planetary Sciences • Washington University Figure from: USDA-NRCS; PNNL; Polizzotto et al. (2008) Nature

4. Effects of Interfacial Structure and Properties on Geochemical Processes Metal (hydr)oxide Mineral ψ0 ψβ ψd Distance 0 β d 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? Earth and Planetary Sciences • Washington University Figure after: Brown and Parks (2001) Int. Geol. Rev.43, 963-1073

6. Measuring Surface Structures with X-ray Reflectivity Bragg Peak (024) Bragg Peak (036) Bragg Peak (012) 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 thesurface 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 Earth and Planetary Sciences • Washington University Catalano et al. (2007) GCA71, 5313-5324

8. Variations in Water Structure Near Isostructural Surfaces Catalano et al. (2006) Langmuir Catalano et al. (2007) GCA Catalano et al. (2009) GCA Catalano (2010) J Phys Chem C Catalano (2011) GCA • 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 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) GCA66, 3037-3054

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 Clear changes in 3200 and 3450 cm-1 bands with pH in SFVS spectra SFVS shows that interfacial water molecules reorient as pH increases, suggesting that restructuring occurs Earth and Planetary Sciences • Washington University Zhang et al. (2008) JACS130, 7686-7694

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 • 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 pH 5 0.01 M NaCl 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