Surface, ice

Low-temperature spectroscopy is indispensable for the studies of processes on the ice surface, illustrated by ozone adsorption and ethylene ozonolysis. Such results are important to clarify the mechanism of atmospheric pollutant elimination and air purification in the nature. 

Laboratory experiments by our group showed that reaction 13 occurs very slowly in the gas phase (10). However, in the presence of ice surfaces the reaction proceeds very efficiently the product CI2 is immediately released to the gas phase, whereas HNO3 remains frozen in the ice (11). Other groups also found that this heterogeneous (i.e., multiphase) process occurs efficiently (12,13), and that a similar reaction also occurs with N2O5 as a reactant ... 

The mechanism for reaction 13 is not well established yet, but it is likely to proceed through ionic intermediates (11) the Cl atom in chlorine nitrate is slightly electropositive, so that it readily combines with negative chloride ions to produce CI2 the HQ on ice is expected to be at least partially ionized. We have found that HCl has a very high mobility on the ice surface, so that even small amounts of HCl will enable reaction 13 to occur. It is also possible for this reaction to proceed in two steps the initial step is the reaction of chlorine nitrate with ice it is followed by the reaction of the product HOCl with HCl on the ice substrate ... 

Laboratory experiments have shown that reaction 15 occurs on ice in the absence of HCl (11-13) furthermore, the product HOCl appears on a time scale of minutes, in contrast to CI2 in reaction 13, which is produced on at most a millisecond time scale (11). Thus, in this mechanism HOCl serves as an intermediate if there is enough HQ on the ice, HOCl will react with HCl while still on the ice surface otherwise the HOCl will desorb, eventually finding an HCl molecule in the ice, perhaps after several adsorption-desorption cycles. 

FIGURE 26.16 The frictional force and the relative rating of the seven compounds of figure 14 as a power function of the ice surface temperature at a load of 75 N and a speed of 1.5 km/h. 

Friction Coefficients and Relative Ratings of Four Tire Tread Compounds at 4.5°C Ice Surface Temperature and a Speed of 0.5 km/h... 

A further piece of evidence for the presence of an ocean below the ice surface was found by Carr et al. (1998) during their analysis of pictures with resolutions of 1.2km, 180m and 54m per pixel local icebergs are visible. A more... 

Sea ice is represented in the model as a two-dimensional surface covered with a snowpack. Ice advection, rheology and snow cover are calculated from the sea-ice model embedded in MPIOM [Hibler (1979)]. The only source of pollutants for the ice compartment is deposition from the atmosphere. Once pollutants enter the ice compartment they can diffuse into the snow pore space air, dissolve in the interstitial liquid water or adsorb to the ice surface. Together with the sea ice the pollutants undergo advection. Sinks considered for the ice compartment are volatilisation to the atmosphere and release into the ocean with melt water. 

Heterogeneous catalysis is also proposed for the formation of the ice mantels around the particles. Co-adsorption of H, O and N atoms leads to the formation of water and ammonia-water ice on the surface, as deduced from ISO spectra. Adsorption of CO onto the ice surface provides a carbon source to initiate organic synthesis, for example, in the simple sequence of reactions ... 

Figure 5.19 Formation of amino acids on ice surfaces irradiated in the laboratory (Nature Nature 416, 403-406 (28 March 2002) doi 10.1038/416403a-permission granted). Data were obtained from analysis of the room temperature residue of photoprocessed interstellar medium ice analogue taken after 6 M HCl hydrolysis and derivatization (ECEE derivatives, Varian-Chrompack Chirasil-L-Val capillary column 12 m x 0.25 mm inner diameter, layer thickness 0.12 pirn splitless injection, 1.5 ml min-1 constant flow of He carrier gas oven temperature programmed for 3 min at 70°C, 5°C min-1, and 17.5 min at 180°C detection of total ion current with GC-MSD system Agilent 6890/5973). The inset shows the determination of alanine enantiomers in the above sample (Chirasil-L-Val 25 m, single ion monitoring for Ala-ECEE base peak at 116 a.m.u.). DAP, diaminopentanoic acid DAH, diaminohexanoic acid a.m.u., atomic mass units.

The temperature difference between inlet and outlet temperature at the coil(s) of the refrigerant should be smaller than 1 °C (AT < 1 °C), to ensure a uniform condensation on the total coil. On warmer areas no ice will condense until the temperature at the ice surface has increased to the warmer temperature on the coil. For large surfaces it is necessary to use several coils or plates in parallel, each of which must be separately temperature controlled. If the condenser is operated in an overflow mode, the weight of the liquid column should not change the boiling temperature of the liquid at the bottom of the column measurably. 

The flow of water vapor should deviate as little as possible before the first condenser surface. The condenser design has to ensure that the water vapor is completely frozen and the remaining water vapor pressure is practically equal to the vapor pressure at the ice surface. This can only be achieved if the vapor passes over several condenser surfaces in series. 

With pharmaceuticals the vapor flow (kg/h) during MD is usually smaller than for food therefore the ice layer produces a relatively smaller temperature difference between coil and ice surfaces, the ice layer can be thicker than for food plants. 

Moudrakovski, I.L. Sanchez, A.A. Ratcliffe, C.I. Ripmeester, J.A. (2001). Nucleation and Growth of Hydrates on Ice Surfaces New Insights from 129Xe NMR Experiments with Hyperpolarized Xenon. J. Phys. Chem. B, 105, 12338-12347. 

This is probably the key heterogeneous reaction for Antarctic stratospheric ozone depletion, and serves as a useful focus for the discussion of the theoretical challenges that must be addressed in dealing with fairly complex chemistry in a complex environment, challenges enlivened — as will be seen below — by the evident chemical involvement of the ice surface environment. 

A complete high level eleetronie stmeture ealeulation of reaetion (1) on an ice surface is currently impossible. Aeeordingly, in considering the appropriate strategy and which finite model eluster system to adopt to study the title reaetion or other heterogeneous reaetions — whieh are deeidedly complex by traditional vacuum electronic stmeture ealeulation standards — it is important to exploit all available experimental information. 

In contrast, in most heterogeneous reactions, we really do not even understand what one of the reactants, the surface, looks like on a molecular level i.e., the condensed-phase molecule and its environment that the incoming gaseous reactant encounters is not well characterized. An example of our incomplete understanding of the nature of surfaces is controversy about effective surface areas for ice and whether ice surfaces prepared in the laboratory for example, are porous or not (e.g., see Keyser et al., 1993 and Hanson and Ravishankara, 1993a). 

Kirchner, U., Th. Benter, and R. N. Schindler, Experimental Verification of Gas Phase Bromine Enrichment in Reactions of HOBr with Sea Salt Doped Ice Surfaces, Ber. Bunsenges. Phys. Chem., 101, 975-977 (1997). 

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