The Journal of Physical Chemlstry, Vol. 87, No. 27, 1983 4335
(“quasi-liquidlayer”). When the relative amount of the fluctuating mobile phase with respect to the bulk ice is higher, a cooperative fluctuation from the surface goes more easily into the interior of the bulk ice. The fluctuation on the surface must increase in this fashion. B. R. Stauffer: At what partial COz pressure are the monomolecular layer and the formation of clathrates observed? Ocampo: It depends on the temperature. In general, it appears for reduced pressures x = P/Psgreater than 0.2. In some cases a coverage of one monolayer is attained before the clathrate begins to nucleate. All the pressures in adsorption measurements were up to 1bar. We cannot use our results to compare with the COz in polar ice cores where the partial pressure is very low.
D. E. Barnaal: How did you establish that a C02 clathrate appeared on the ice surface? Ocampo: In powder samples the pressure falls to the decomposition value.
Optimal Conditions of the Homogeneous Nucleation of Cubic Ice (IC)from Concentrated Solutions of LiCl 0 D20 (J. Dupuy) J. Perez: Do your results mean that the nucleation process implies the formation of only cubic ice? That seems evident with your results obtained at low temperature; for higher temperatures, growth might be too rapid, preventing the observation of cubic ice as, above a certain size, there is a transformation IC Ih. Dupuy: If the concentration “c” of LicP in D20 is 9.75 < c < 12 mol %, the nucleation process implies the formation of only cubic ice a t low temperatures near Tr If the size of cubic ice crystallites exceeds 180 A, there is a contamination of cubic ice by hexagonal ice, and the transformation IC Ih occurs.
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Cubic Ice in Nature (E. Whalley) F. Stillinger: I suppose you have assumed that cubic ice in nature has the same refractive index as that of the hexagonal form. Since we cannot be certain to what extent that natural ice ICis proton disordered (or partially ordered), is it not possible that the refractive index is correspondingly uncertain? Whalley: Ice ICas made in the laboratory is certainly proton disordered, as is shown by its infrared’s2 and bans spectra and by its dielectric proper tie^.^ There is no reason to expect natural ice ICto differ from artificial ice in ita degree of order. The density of ice Ih and IC are identical within experimental uncertainty. The refractive index of ice Ih is almost isotropic, and so the refractive index of ice IC can be estimated to well within the accuracy needed for predicting halos. In any case, if ice ICwere partly ordered its symmetry would probably be degraded from face-centered cubic and it would not crystallize in octahedra. Bertie, J. E.; Whalley, E. J. Chem. Phys. 1964, 40, 1657. Bertie, J. E.; Whalley, E. Ibid. 1967, 46, 1271. Taylor, M. J.; Whalley, E. Ibid. 1964, 40, 1660. Gough, S. R.; Davidson, D. W. Ibid. 1970,52, 5442. J. W. Glen: Relative to Stillinger’s question, is ice IC were proton ordered its relative stability with ice Ih would vary strongly with temperature which does not seem to happen. As regards the tendency to observe it in Doriis, could this be due to the presence there of some mineral aerosol, possibly from industrial pollution, which is favorable for epitaxial nucleation of ice IC? Whalley: The industrial pollution would have to be in Paris from at least 1629 to 1920, with possibly a gap of 160 years. In any case, the cirrus clouds that usually form the halos may be too high to be much affected by emissions immediately below them. G . P. Johari: Do you think that the possibility of Scheiner’s halos as arising from octahedral clathrate hydrate crystals in the atmosphere could be completely eliminated? Whalley: The equilibrium vapor pressure of the guest molecule over any clathrate that might form from the atmosphere is much higher than its partial pressure in the atmosphere. A clathrate could, therefore, be formed only under highly metastable conditions. I do not know of any evidence that this might occur.
E. Hindman: What can you say about the atmosphere conditions which give rise to cubic ice, temperature, pressures, etc. Whalley: Halos are usually caused by ice crystals in cirrus clouds, and presumably the octahedral crystals of cubic ice that appear to cause the 28’ halo are formed there. The pressure and temperature appear to be roughly 0.2 bar and 230 K.
Molecular Dynamics Study of Water Clathrates (P. L. M. Plummer) R. Smoluchowski: (1) How do the hydrogens change their positions as function of time and temperature? (2) Do you think that a structure made of several cages would show different stability? Plummer: (1)At the lower temperatures (below 200 K), the hydrogens remain localized between the same two oxygens throughout the simulation although substantial deviation in 0-H- - -0angle occurs. As the temperahre increases,more proton “switching”reactions and double hydrogen bonds between a pair of oxygens occur. The lifetime of a specific hydrogen bond decreases. (2) I believe the basic trends would be the same. However, I would expect the temperature of the transition to be somewhat higher. With multiple cages, fewer of the molecules would be in the surface and the internal structure would persist longer.
Molecular Dynamics Studies of Ice ICand the Structure I Clathrate Hydrates of Methane (J. S. Tse) F. Stillinger: How did you decide which way to orient water molecules in the two networks, and was there any attempt to average over such canonical orientations? Tse: Since the water molecules in both the clathrate hydrate and ice ICare rotationally disordered, initial proton configurations were selected in a random fashion to conform to the Bernal-Fowler rules. Only one configuration was considered in each simulation and we make no attempt to average over different canonical orientations. P. Halleck: When you say that the empty hydrate is stable at high pressure, do you mean thermodynamically stable relative to high pressure ice (say ice VII)? Tse: No. I make no attempt to compare the stability of the hydrates with other forms of ice.
R. Smoluchowski: To my knowledge the empty clathrate has never been observed. How can you claim that it is stable below 230 K? With respect to what? Tse: Within the time scale of our molecular dynamic simulation, the fluctuations in the total energy and the pressure of the system from their mean values are very small. We observe no indication that the empty hydrate structure is transforming into other phases and it appears to be mechanically stable. G. P. Johari: I think you can determine the heat capacity of an empty clathrate lattice by extrapolating to zero occupancy the data obtained for partially occupied cages. Tse: Yes, in principle you can do so. Unfortunately, it is very difficult to prepare hydrate samples within a wide range of guest occupancies.
Ice-Rich Moons and the Physical Properties of Ice (C. J. Consolmagno) J. Klinger: Why did you exclude C02 when you enumerated primary molecules that formed in the primitive nebula? Consolmagno: In a gas of solar composition, rich in hydrogen and relatively poor in oxygen and carbon, it can be shown that CO is thermodynamically stable relative to COz. The relevant reaction is CO + H2 F? C02 + H2;so long as the [HI/ [H20]ratio is fixed, by cosmic abundances, to be much greater than unity, then [CO]K,![CO,] must also be much greater than unity. For the P-T conditions in the solar nebula, K , is always close enough to unity that [CO] will be greater than [CO,]. S. Warren: You showed pictures of bright white moons and suggested that the high reflectivity was an indication of ice. But ice is actually highly transparent at visible wavelengths and
