The Hindu : Unusual chemistry on ice surfaces
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Unusual chemistry on ice surfaces
HAVE YOU ever thought that ice surface can be a subject of intense
research? As you pick an ice cube you may not have thought that
unusual processes occur right on its surface. But new chemistry
observed on ice surfaces suggest that there may be processes of
importance occurring at the surface of ices.
Don't worry, the results have no direct significance to ice in your fridge
or in your ice cream; the processes are observed at extremely cold
temperatures, in the range of -173 degree Celcius.
The subject refers to a recent report appeared in the Angewandte
Chemie International Edition (Angew. Chem. Int. Ed. 2001, vol. 40,
page 1497-1500). The study is on the well-known proton transfer
reaction, NH3 + H3O + NH4 + H2O performed on ice and concluded
that on its surface, the reaction is incomplete!
Note that this reaction occurs instantaneously both in liquid water and in
gas phase, and virtually all reactants convert to products. The rate of the
reaction is so fast that for all practical purposes, it is infinite. On ice,
however, the amount of conversion is finite and the researchers
quantitatively estimated this. The paper concludes that less stable
species can be stabilised on ice surfaces, within what is called, kinetic
barriers.
It is important to mention that the process that is studied is on ice and
not in ice. The emphasis is because the events occur on the very top of
the ice surface, of the order of the first one or two molecular layers. To
study this, there is a need to have extremely surface sensitive
techniques. The information derived from the top should not be lost due
to the infinite number of molecules below.
This surface sensitivity is extremely difficult to achieve in conventional
techniques and there are inherent limitations to achieving increased
sensitivity in several cases. Proton transfer reactions are central to
chemistry and biology and have been investigated in the solution phase
for a long time. Reactions on the surface of ice are gaining importance
in the recent past due to their role in upper atmospheric processes.
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The Hindu : Unusual chemistry on ice surfaces
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Some of the key reactions leading to the destruction of ozone in the
stratosphere are occurring on the surface of small ice particles called
polar stratospheric clouds (PSC's). These reactions are known to occur
only on the ice surface and not in water or in gas phase. The catalytic
role of ice surface has been investigated for nearly a decade. The fact
that ice can kinetically stabilise metastable species is a very important
observation which may have implications to the mechanisms of
atmospheric processes and processes in outer space.
The work came out of a collaboration between Prof. T. Pradeep of the
Indian Institute of Technology, Madras and Prof. Heon Kang of the
Pohang University of Science and Technology, South Korea. Two
graduate students of Prof. Kang, Mr. Seong-Chan Park and Mr. KeyWon Maeng performed the measurements. In the technique they
employed, low energy ions were scattered from monolayers of ice
mixed with HCl vapours in a an ultrahigh vacuum chamber at
temperatures of the order of 100 K (minus 173 oC).
The deposition of HCl on ice surfaces (instead of H2O, they used D2O,
ie. deuterated water) produced H3O+ (HD2O+ and other isotope
exchange products in this case) and a layer of NH3 was deposited on
this surface. The reaction products were studied by a technique called
low energy secondary ion mass spectrometry (SIMS). In order to
perform monolayer specific chemistry, it is important to do these in a
very high order of vacuum, similar to that exists in the moon.
Let us look at the general aspects of this work. Molecular processes at
the surface of ices and water ice in particular, are important issues of
atmospheric concern (not just in Stratosphere). Ice covers several
million square kilometers of the earth's surface. In Antarctica alone, the
ice surface is almost double the area of Australia, about 13 x 106 km2.
The area covered by ice is more than the hospitable land area of the
planet. Due to it's giant magnitude, the chemistry on ice will certainly
influence atmospheric processes. Apart from the chemistry in the
troposphere, many of the stratospheric processes are influenced by the
chemistry in clouds which are mostly ice particles. Other than water ice,
ices of varying forms are important in several planetary systems.
Methane ice in Uranus and Pluto, SO2 ice in Io, ammonia ice in Uranus
etc. are particularly important to mention. Ionic and molecular processes
at these surfaces are probably important to understand the atmospheres
of these planets.
In the past decade, there has been enormous interest in understanding
the chemistry of certain small molecules on the surface of water ice.
The actual choice of the molecule depends on the role of it in PSCs in
the ozone depletion processes. PSCs are small, 1 um particles
containing nitric acid hydrates and larger particles (10 um diameter) of
hexagonal ice. Stratospheric chlorine species are the stable molecules,
ClONO2 and HCl, and the less stable molecules, HOCl and Cl2.
http://www.hinduonnet.com/2001/06/07/stories/08070007.htm
6/18/2011
The Hindu : Unusual chemistry on ice surfaces
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These less stable ones can photochemically convert to Cl radicals
leading to the destruction of ozone. It has been suggested and has been
experimentally proven that Cl2 and HOCl can be formed on the surface
of ice at stratospheric temperatures (185 K). The problem with these
processes is that they occur on the surface of ice, and ice surface is a
tough for an experimentalist to look at. The problem is that even at 185
K, its vapour pressure is too high (104torr) for most of the surface
science experiments. How to solve these problems and look at the very
top of the surface is a challenging question. The desire is so intense that
many new techniques are being explored. The present study shows a
way to explore such surface processes on ice.
There are several aspects which make this work interesting.
Very top layers of water ice can create and stabilize metastable species,
which can be important in ice chemistry.
Such species can be quantitatively analyzed by reactive ion scattering.
The methodology illustrated can be adapted to a number of diverse
chemical systems where molecular surface chemistry is important.
The quantitative aspect of this investigation will make it possible to
determine thermodynamic parameters on ice surfaces. One may ask,
will the chemistry on the few layers of water be representative of ice
surface?
More data will have to be available to answer this question. For this to
happen additional tools have to be employed to look at a variety of
chemical processes.Efforts are continuing in the groups of Prof. Pradeep
and Prof. Kang in this direction.
T. Pradeep
Associate Professor
Department of Chemistry
IIT, Chennai
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