Different Chemistry at an Icy Edge
Clearing up the atmospheric impact of salting roads and sidewalks is one possible outcome of recent research exploring the quasiliquid layer found at the interface between air and ice. Another possible outcome could be a better understanding of the chemistry inside high-altitude cirrus clouds, which has important implications for atmospheric science.
Researchers studied the interface between air and ice using two techniques simultaneously. Ultraviolet laser beams enter through quartz windows, providing the light for both glancing-angle Raman (355-nm excitation) and laser-induced fluorescence (337-nm excitation) spectroscopy. A single detector picks up both signals. Images courtesy of Tara Kahan, University of Toronto.
Investigators from the University of Toronto and from the University of Bristol in the UK used glancing-angle Raman and laser-induced fluorescence spectroscopy to study the air-ice interface and found evidence of enhanced hydrogen bonding of the surficial water molecules.
Previous work by the group had shown a dramatic difference in the photochemical degradation rates, and other scientists have reported chemically strange behavior at the interface.
“We are trying to understand how chemistry differs at the ice surface from that which takes place at the liquid water surface or in aqueous solution,” said D. James Donaldson, professor of chemistry at the University of Toronto.
He added that, in the course of such investigations, having a variety of probes to study what is happening is important. That is one reason that they used both glancing-Raman and laser-induced fluorescence spectroscopy, neither of which had been used widely for such research.
Applying these methods was not easy. Both fluorescence and Raman scattering signals generally are much stronger from the bulk substrate than from the surface. Several years ago, the researchers began using glancing-angle fluorescence to study chemical reactions on water. They discovered a strong Raman signal and eventually decided to use it to investigate the air-ice interface, after assuring themselves that they could distinguish surface signals from bulk ones.
This image shows the Raman spectra of the OH-stretch band of a pure ice surface (blue trace); the same ice surface after the deposition of gaseous HCl (green trace); and the surface of a frozen 0.01-M aqueous NaCl solution (red trace).
For this most recent study, their experimental setup consisted of a Teflon chamber with gas inlets and outlets, along with quartz windows on the sides. They cooled a copper plate inside the chamber to between –16 and –24 °C. For Raman scattering measurements, they used a frequency-tripled 355-nm Nd:YAG laser made by Continuum Inc. of Santa Clara, Calif., and for laser-induced fluorescence, they used a 337-nm nitrogen laser made by Spectra-Physics of Mountain View, Calif. The beams entered from the side and glanced off the sample.
The researchers collected both Raman scattering and fluorescence signals with a liquid lightguide suspended 1 cm above the sample surface. For Raman measurements, they sent this light through a long-pass filter that removed the excitation light and imaged it on a Spectra-Physics monochromator. They detected the intensity with a photomultiplier tube made by Hamamatsu. They did not use the long-pass filter for fluorescence readings.
When performing Raman measurements, the investigators froze highly pure deionized water onto the copper plate, turning the ice over manually so that the laser would strike a flat surface. For fluorescence, they added a fluorophore to the water. They deposited gas-phase acids and bases onto the ice or dissolved sodium chloride in the water before freezing.
They found that the dissolved salt disrupted the hydrogen bonding both in liquid water and at the air-ice interface. The same was not true for nitric or hydrochloric acid, which seemed to increase the hydrogen bonding, as indicated by Raman measurements. At the same time, the laser-induced fluorescence spectra of the pH probe acridine showed that the acids at the surface were dissociated and thus had produced the cation hydronium (H3O+).
These results suggest that the formation of hydronium enhances the hydrogen bonding of water molecules at the air-ice interface, and such information provides another clue about the quasiliquid layer. Donaldson, however, cautioned that these findings probably would not be the last word.
“The story may be different at lower temperatures. We are examining this now.”
Journal of Physical Chemistry A, Nov. 1, 2007, pp. 11006-11012.
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