The standard description of water structure consists of a
random tetrahedral network in which every molecule is
H-bonded to its four nearest neighbors, an arrangement similar
to that in hexagonal ice. In recent years, a number of x-ray absorption
spectroscopy (XAS) and related experiments have begun to characterize
the H-bond structure in detail by assigning the observed spectral
features to specific H-bonding configurations. However, in order
for such an analysis to yield a quantitative description of the
H-bond structure, it is necessary to establish an experimental
definition of what actually constitutes an H-bond, or in other
words, what degree of distortion of the H-bond network leads to
a measurable change in the XAS.
A liquid microjet about 30 μm in diameter.
The nozzle is formed from a fused silica capillary with an inside
diameter of 100 μm, which is elongated by means of a commercial
CO2 laser pipette puller
in order to obtain the final nozzle diameter.
Previously, only a geometric criterion had been established, which
was derived by density functional theory (DFT) calculations of
a model cluster [Ph. Wernet et al., Science 304,
995 (2004)]. Applying their theoretical H-bonding criterion to
their x-ray Raman measurements, the authors had to invoke a picture
of a liquid composed of monocyclic rings and linear chains—radically
different from a tetrahedral network—to obtain agreement
with theoretical calculations.
This latest experiment begins with injection of a small liquid
microjet (10–30 μm diameter) into a high-vacuum chamber.
The small size of the volatile sample makes it possible to maintain
a relatively good vacuum (10-5 torr), which enables
windowless coupling to both Beamlines 8.0.1 and 11.0.2.
Furthermore, such low pressures allow for efficient transport of
electrons generated by the x-ray absorption process away from the
jet surface to a detector placed about 1 mm away from the interaction
region.
The Berkeley group measured x-ray absorption by the total electron
yield (TEY) method. To obtain the temperature dependence, they
recorded successive TEY spectra as a function of distance from
the liquid jet nozzle. The microjet undergoes rapid evaporative
cooling upon injection into the high-vacuum chamber, and therefore
the liquid jet temperature continually decreases with distance
away from the nozzle. The cooling rate had been thoroughly characterized
in previous experiments by the group.
Comparison of the TEY x-ray absorption spectra taken at two different
temperatures. The solid black curve was recorded at 288 K and the
gray curve at 254 K.
Upon cooling, a sizeable decrease in the pre-edge intensity, an
XAS spectral feature previously assigned to broken or distorted
hydrogen bonding configurations in water, was observed. A simultaneous
increase in the post-edge intensity, a spectral feature assigned
to highly symmetric ice-like configurations, was also observed.
A plot of the logarithm of the ratio of peak areas (Ipost-edge/Ipre-edge)
vs. inverse temperature should be linear, with a slope proportional
to the energy difference between the two configurations (distorted
and ice-like). From its temperature dependence measurements, the
group found the difference in energy to be 1.5 ± 0.5 kcal/mol.
This is the experimentally derived energetic hydrogen bond criterion.
Plots of the log of the ratio of the areas of the post-edge and pre-edge features versus inverse temperature from three separate experiments. The solid lines represent the linear fit with a slope proportional to the difference in energy between the distorted and ice-like H-bonding distributions. The energy difference is determined to be 1.5 ± 0.5 kcal/mol (average of the three measurements).
By this experimental criterion, the x-ray Raman measurements reported
by Wernet et al. are consistent with the “standard model” for
water after all. The Berkeley group would like to study the nature
of the transition from the symmetric ice-like structure to the
distorted structure described above via molecular dynamics simulations.
Research conducted by J.D. Smith, C.D. Cappa, B.M. Messer, R.C.
Cohen, and R.J. Saykally (University of California, Berkeley, and
Berkeley Lab) and K.R. Wilson (Berkeley Lab).
Research funding: U.S. Department of Energy, Office of Basic
Energy Sciences (BES). Operation of the ALS is supported by BES.
Publication about this research: 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 (2004). |