Optical Spectroscopy in Hard to See Places

rawalkerchemistry.montana.edu

Our group develops and utilizes optical methods to study chemical structure, organization and reactivity at surfaces. While the systems studied are diverse and far ranging, our goal is always the same: we work hard to understand how asymmetric forces found at surfaces alter interfacial chemistry from bulk material limits. Projects currently underway fall into two general categories: 1) solvation at liquid surfaces and 2) high temperature surface chemistry in electrochemical devices.

Keywords:
Physical, Spectroscopy

Measuring the widths of liquid interfaces – Our original work in the area of surface chemistry was motivated by the simple question, “Do oil and water really not mix?” A more quantitative way of asking this question is “Over what lengthscales do the properties of one liquid converge to those of a second, immiscible liquid.” Using custom designed surfactants dubbed “molecular rulers” and resonance enhanced, second harmonic generation (SHG) spectroscopy we have measured the distances required for solvent polarity to transition from the aqueous to the organic limit across a wide variety of immiscible liquid/liquid interfaces. As a result of these studies, we discovered that interfacial asymmetry can force solvent species to organize differently compared to their long range structure in bulk solution. Consequently, liquid surfaces may acquire properties that can not be described simply by extrapolating contributions from the two individual phases. For example, water is a very polar solvent and long-chain alcohols have intermediate polarity based on their respective static dielectric constants. However, water/alcohol interfaces are dominated by a nonpolar, alkane-like region. These findings necessarily force one to reconsider proposed mechanisms of solvent extraction, interfacial electrochemistry and colloid stability.

Selected Publications

W. H. Steel and R. A. Walker:
Measuring Liquid/Liquid Interfacial Width with Molecular Rulers
Nature, 424, 296-299 (2003).

W. H. Steel1, Y. Y. Lau, C. L. Beildeck and R. A. Walker:
Solvent Polarity Across Weakly Associating Liquid/Liquid Interfaces
J. Phys. Chem. B, 108, 13370-13378 (2004)

C. L. Beildeck, W. H. Steel, and R. A. Walker:
Surface Charge Effects on Solvation across Liquid/Liquid and Model Liquid/Liquid Interfaces
Faraday Discussions, 129 69 80 (2005)

M. R. Brindza and R. A. Walker :
Evaluating Solvation Mechanisms at Polar Solid Surfaces with Nonlinear Optical Spectroscopy
J. Am. Chem. Soc. 131 6207-6214 (2009)

A. R. Siler, M. R. Brindza, and R. A. Walker :
Hydrogen bonding molecular ruler surfactants as probes of specific solvation at liquid/liquid Interfaces
invited article in Analytical and Bioanalytical Chemistry DOI: 10.1007/s00216-009-2957-8 (on-line, print copy available Nov. 2009)

Keywords:
Spectroscopy, Physical

Structure and organization at liquid interfaces - Our studies of polarity across liquid/liquid interfaces raise questions about how molecules having different structures organize themselves when constrained to two dimensions. Irregularly shaped, organic molecules are ubiquitous throughout biology, chemistry and environmental science, yet very little is known about how these species assemble spontaneously to form organic films at aqueous interfaces. To answer these questions, we use vibrational sum frequency spectroscopy (VSFS) to acquire vibrational spectra of species adsorbed to the aqueous-vapor interface. These data coupled with careful surface pressure measurements provide the quantitative information needed to model the competing forces that control surfactant concentration and conformation at air/aqueous interfaces.

Selected Publications

S. Z. Can, D. D. Mago, and R. A. Walker:
Structure and Organization of Hexadecanol Isomers Adsorbed to the Air/Water Interface
Langmuir 22 8043-8049 (2006)

O. Esenturk and R. A. Walker:
Surface Vibrational Structure at Alkane Liquid/Vapor Interfaces
J. Chem. Phys. 125 Art. No. 174701 (2006).

S. Z. Can, D. D. Mago, O. Esenturk and R. A. Walker :
Balancing Hydrophobic and Hydrophilic Forces at the Water/Vapor Interface: Surface Structure of Soluble Alcohol Monolayers
J. Phys. Chem C 111, 8739-8748 (2007).

J. T. Fourkas, R. A. Walker, E. Gershgoren, and S. Z. Can :
Effects of Reorientation in Vibrational Sum Frequency Spectroscopy
J. Phys. Chem. C 111, 8902-8915 (2007).

S. Z. Can, C. F. Chang and R. A. Walker :
Spontaneous formation of DPPC monolayers at aqueous/vapor interfaces and the impact of charged surfactants
Biochmica et Biophysica Acta – Biomembranes, 1778 2368-2377 (2008)

Keywords:
Physical, Spectroscopy

The overall goal of this project is to identify the mechanisms responsible for electrochemical oxidation in solid oxide fuel cells (SOFCs). Due to the high activation energy needed to catalyze molecular oxygen dissociation and the small diffusion constants associated with oxide ion transport through doped, metal-oxide ceramics, SOFCs must operate at elevated temperatures – typically 650˚C or higher. Traditional studies of SOFC operation use electrochemical techniques to report on system performance, but these data can not differentiate the chemical species responsible for observed behavior. Samples used in these studies are often subjected to exhaustive, ex situ, post mortem analyses. Thus, researchers are left to infer how chemical and structural changes observed after operation correspond to electrochemical performance measured during operation. To overcome the challenges associated with making measurements at high temperatures and under strongly reducing or oxidizing conditions, my group has built and adapted instrumentation to acquire vibrational Raman spectra from metal and metal oxide surfaces at temperatures in excess of 750˚C (!).

Selected Publications

M. B. Pomfret, J. C. Owrutsky, and R. A. Walker:
High Temperature Raman Spectroscopy of Solid Oxide Fuel Cell Materials and Processes
J. Phys. Chem. B 110 17305-17308 (2006).

M. B. Pomfret, J. C. Owrutsky, and R. A. Walker :
In-situ Studies of Fuel Oxidation in Solid Oxide Fuel Cells
Analytical Chemistry 79 2367-2372 (2007)

M. B. Pomfret, J. Marda, G. S. Jackson, B. W. Eichhorn, A.M. Dean and R. A. Walker :
Hydrocarbon fuels in solid oxide fuel cells: In-situ Raman studies of graphite formation and oxidation
J. Phys. Chem. C 112, 5232-5240 (2008)

M. B. Pomfret, B. C. Eigenbrodt, and R. A. Walker:
Interfacial Resistivity of Yttria Stabilized Zirconia in Operating Solid Oxide Fuel Cells
Electrochemical Society Transactions, 11 (27) 111-120 (2008)

Keywords:
Physical, Spectroscopy

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