CRYSTALS OF WATER ICE
The first crystals I studied were those of water. Both light and heavy (deuterium oxide) water ice crystals were recorded growing in the vapor phase condensation chamber (VPCC). To date, no significant differences in appearance between the two types of ice have been noticed. Different crystal structures would form depending on the growing conditions. As an example, if the temperature of the cold stage was at -20°C, the observed crystal growth would differ not only in speed of formation, as expected, but also in physical appearance as compared to crystals grown at higher temperatures. Traces of contaminants (e.g., organic liquids) have an effect on the morphology of the ice crystals as well.
Somewhere on the Moon and somehow missed by the Lunar Prospector spacecraft back in 1999 is a pool of ice. The pool may have been formed by the collection of water vapor from a comet that collided with the Moon in the remote past. The lunar surface temperature can rise above 200°C in the sun and drop below -200°C in the shade or at night. In the changing microclimate that the ice pool inhabits, as temperatures rise ice passes from the solid state to vapor and back again without liquefying (sublimation). As in this simulated depiction of the base of a lunar mountain range, sublimation may take place in numerous cracks and crannies that line every crater, mountain, and valley. The crystals, which form during sublimation, would probably adopt the appearance of hoarfrost. A climber in an ice valley such as this might be treated to a glittering display of diamond-like gems glowing in all colors of the spectrum as sunlight refracts off the minute, crystalline icy surfaces!
Hoarfrost crystals of the type that may be found on our Moon, as described above, are easily grown in the laboratory.
Various substrates, such as tungsten wire and a button-shaped layer of carbon nanotubes in epoxy, were used for growing the water ice crystals seen here. The complex substrate surfaces have numerous points, some of molecular dimensions, which can act as nuclei for the forming ice crystals. The crystal growth was recorded in time lapse using a transmission microscope. (Single click to activate the video.)
ORGANIC "ICE" CRYSTALS
After water, I next looked at a number of organic chemicals. One requirement was that these organic compounds needed to be liquid at room temperature with a freezing point not too far below. Also, ideally the liquid should have a boiling point that was not too high so that it would have a substantial vapor pressure at room temperature. These requirements needed to be met so that the VPCC would be successful in growing crystals. Of course, it was important not to use organic chemicals that could chemically attack the growth chamber and endanger the microscope (and microscopist). It quickly became necessary to use glass chambers for most organic experiments. The thermal conductivity of glass is higher than plastic. This has the negative effect of producing an adverse thermal environment for crystal growth. But, on the positive side, the addition of organic chemicals opened up tremendous possibilities for mixing vapors (organic and inorganic) and noting the effects of mixed crystal growth.
Bromoform, CHBr3, a heavy liquid with an odor similar to chloroform
This organic liquid solidifies at 7.5°C. Its boiling point is approximately 150°C. The relatively high freezing point and a reasonably low boiling point make bromoform ideal for these vapor phase experiments. Its vapor pressure is 5mm @ 20°C.
Dimethyl carbonate, C3H6O3, a liquid with a slight fragrance used in synthesis
This liquid has a boiling point of 90°C and solidifies between 2-4°C. It finds many uses as a solvent, an octane booster for gasoline, and in manufacturing as a substitute for phosgene. Its vapor pressure is 18mm @21.1°C. The liquid is excellent for vapor phase work!
Mixed vapors of bromoform and dimethyl carbonate
Cutting the felt ring in two places inside and near the top of the chamber allows vaporizing liquids without first mixing. Using this split-ring approach, small quantities of bromoform and dimethyl carbonate were allowed to evaporate and commingle vapors on the surface of a tungsten wire as freezing began. Note the dissolution or crystal melting at one point.
