Absorbate-Induced Restructuring of Interfacial Water

Jeffrey G. Catalano

Spring 2013 ACS National Meeting

Acknowledgements

Earth and Planetary Sciences • Washington University

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)

ANL Named Postdoctoral Fellowship ProgramDOE/BES Geosciences Research ProgramNSF Geobiology and Low-Temperature GeochemistryACS Petroleum Research FundWashington University

Earth and Planetary Sciences • Washington University

• 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

Nutrient Availability in Soils and Aquatic Systems

Microbial Metabolic Activity

Occurrence, Fate, and Transport of Water Contaminants

Figure from: USDA-NRCS; PNNL; Polizzotto et al. (2008) Nature

Importance of Interfacial Reactions to Environmental Systems

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

0 β d

ψ0 ψβ

ψd

Distance

Met

al (

hydr

)oxi

de M

iner

al

Figure after: Brown and Parks (2001) Int. Geol. Rev. 43, 963-1073

Need to Obtain a Fundamental Understanding of the Mineral-Water Interface

Earth and Planetary Sciences • Washington University

• 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?

• 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

Measuring Surface Structures with X-ray Reflectivity

Earth and Planetary Sciences • Washington University

Bragg Peak(012)

Bragg Peak(024)

Bragg Peak(036)

Bragg rod=

X-ray Reflectivity

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, 5313-5324 Earth and Planetary Sciences • Washington University

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

Earth and Planetary Sciences • Washington UniversityCatalano et al. (2006) Langmuir Catalano et al. (2007) GCACatalano et al. (2009) GCA

Catalano (2010) J Phys Chem CCatalano (2011) GCA

Distinct Mineral Surface Charging Behaviors may Produce Two Classes of Water Ordering

■ 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, 3037-3054

>AlOH2+1/2

or>AlOH-1/2

>Al2OH0

Evidence for Water Reorientation as a Function of pH and Surface

Charging on Corundum (001)

■ SFVS shows that interfacial water molecules reorient as pH increases, suggesting that restructuring occurs

Earth and Planetary Sciences • Washington UniversityZhang et al. (2008) JACS 130, 7686-7694

Clear changes in 3200 and 3450 cm-1 bands

with pH in SFVS spectra

Phase-sensitive SFVS shows reorientation of “ice-like” water as pH increases

Water reorientation follows surface

charging behavior

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

As(V) Adsorption on the Corundum (001) Surface

■ 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 50.01 M NaCl

Full Interfacial Profile Interfacial Water Comparison

Earth and Planetary Sciences • Washington University

Changes in Interfacial Water Structure Following AsO4

3- 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

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

Earth and Planetary Sciences • Washington University

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

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