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X-ray
Spectroscopy of Volatile Liquids and their Surfaces
Walter Drisdell, Andrew Duffin, Daniel Kelly, Craig Schwartz, Janel Uejio
The goal of this project is to explore and develop novel methodologies
for probing the nature of volatile liquids and solutions and their surfaces,
employing combinations of liquid microjet technology, with synchrotron
X-ray and laser Raman spectroscopies.
Utilizing the intense monochromatic soft X-Rays available at the LBNL
Advanced Light Source (ALS), we have investigated the temperature dependence
of the bulk structure of deeply supercooled liquid water by X-ray absorption
spectroscopy[1]. It was recently suggested that the hydrogen bond network
of liquid water comprised rings and chains, rather than the traditionally
accepted disordered tetrahedral network. Our temperature- dependent measurements
indicate that this interpretation greatly overestimates geometric distortions
within the hydrogen bond network. Using an experimentally determined "energetic
criterion" for a hydrogen bond, we find that the NEXAFS spectrum
of liquid water is actually in good agreement with the traditional view
of water as a locally tetrahedral liquid.
Using liquid microjets to avoid the problem of radiation damage to fragile
solutes, we have measured the pH-dependent NEXAFS spectra of several amino
acids, including glycine, proline, lysine, and the dipeptide diglycine[2,3].
We find that the nitrogen terminus of primary amino acids is sterically
shielded at high pH, and exists in an "acceptor-only" state,
wherein neither amine proton is involved in hydrogen bonding to the surrounding
solvent. The diglycine study characterized a similar behavior in this
first study of the peptide bond hydration.
We have carried out a systematic study of the perturbative effect of
a variety of inorganic salts on the unoccupied water orbitals. We conclude
that monovalent cations have a very small effect on the local electronic
structure, whereas simple anions are strongly perturbative[4].
We have recently addressed the long standing controversy over whether
continuum or a multi-component ("intact" or "broken bond,"
etc.) models best describe the hydrogen bond interactions in liquid water.
The temperature dependence of water's Raman spectrum has long been considered
to be among the strongest evidence for a multi-component distribution.
However, we have shown, using a combined experimental and theoretical
approach, that many of the features of the Raman spectrum considered to
be hallmarks of a multi-state system, including the asymmetric band profile,
the isosbestic (temperature invariant) point, and van't Hoff behavior,
actually result from a continuous distribution[5]. This work complements
the study of the structure of pure liquids by X-ray absorption spectroscopy
that has been ongoing. We have published a joint theory/experiment study
of liquid methanol that characterized the nature of H-bonded domains[6,7].
References (DOE supported papers 2003-present)
1. J.D. Smith, C.D.
Cappa, K.R. Wilson, B.M. Messer, R.C. Cohen, and R.J. Saykally, "Energetics
of Hydrogen Bond Network Rearrangements in Liquid Water," Science
306, 851-853 (2004). LBNL-56349
2. B.M. Messer, C.D.
Cappa, J.D. Smith, K.R. Wilson, M.K. Gilles, R.C. Cohen, and R.J. Saykally,
"pH Dependence of the Electronic Structure of Glycine," J. Phys.
Chem. B 109, 5375-5382 (2005). LBNL-56348
*Cover Article.
3. B.M. Messer, C.D. Cappa, J.D. Smith, W.S. Drisdell, C.P. Schwartz,
R.C. Cohen, R.J. Saykally, "Local Hydration Environments of Amino
Acids and Dipeptides Studied by X-Ray Spectroscopy of Liquid Microjets,"
J. Phys. Chem. B 109, 21640-21646 (2005). LBNL-59184
4. C.D. Cappa, J.D. Smith, K.R. Wilson, B.M. Messer, M.K. Gilles, R.C.
Cohen, and R.J. Saykally, "Effects of Alkali Metal Halide Salts on
the Hydrogen Bond Network of Liquid Water,"J. Phys. Chem. B 109,
7046-7052 (2005). LBNL-56812
*Cover Article.
5. J.D. Smith, C.D.
Cappa, K.R. Wilson, R.C. Cohen, P.L. Geissler, and R.J. Saykally, "Unified
description of temperature-dependent hydrogen-bond rearrangements in liquid
water," PNAS (in press 2005). LBNL-58789
6. K.R. Wilson, M.
Cavalleri, B.S. Rude, R.D. Schaller, T. Catalano, A. Nilsson, L.G.M. Pettersson,
and R.J. Saykally, "X-ray Absorption Spectroscopy of Liquid Methanol
Microjets: Bulk Electronic Structure and Hydrogen Bonding Network,"
J. Phys. Chem. B 109, 10194-10203 (2005). LBNL-56350
*Cover Article.
7. K.R. Wilson, B.S. Rude, J. Smith, C.D. Cappa, D.T. Co, R.D. Schaller,
M. Larsson, T. Catalano, and R.J. Saykally, "Investigation of volatile
liquid surfaces by synchrotron x-ray spectroscopy of liquid microjets,"
Review of Scientific Instruments 75, 725-736 (2004). LBNL-56347
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