Accidental synthesis of “sodium chloride superhydrate” Possibility of existence in ice bodies | sorae Portal site to space

「sodium chloride (NaCl)” is one of the substances familiar to us as a seasoning. After dissolving sodium chloride in various proportions in water, and then evaporating the water to form crystals, it is usually sodium chloride that appears. However, when crystals are formed at temperatures below 0.1°C,sodium chloride dihydrate (NaCl 2H2O)”, a crystal that combines sodium chloride and water molecules is generated.

【▲ Figure 1: Europa’s surface is covered with numerous streaks that appear colored. It is believed that this originates from various substances transported from underground. And although the spectral data suggested the existence of sodium chloride hydrate, there was a problem that the corresponding substance had not been found (Credit: NASA/JPL/Galileo)]

Sodium chloride dihydrate decomposes into water and sodium chloride at 0.1°C or higher, so it is not usually seen, but in cold regions, low-temperature and dry environments that generate sodium chloride dihydrate often exist. Since the same can be said for icy bodies such as Jupiter’s moon Europa and Saturn’s moon Enceladus, it has been estimated that there is a large amount of sodium chloride hydrate in the universe. For example, the color of the numerous streaks on Europa’s surface is thought to come from a variety of substances other than water brought from the subsurface. So it’s no surprise that sodium chloride hydrate exists.

However, there is a long-standing mystery that when we actually observe these icy bodies with space probes, we get different results than expected. Observational spectral data indicate the presence of a “watery” sodium chloride hydrate containing more than two water molecules, but this actually indicates the presence of a different sodium chloride hydrate than the dihydrate. It remained unclear whether or not the spectral data were erroneous.

A research team led by Baptiste Journaux of the University of Washington conducted an experiment in which salt water was subjected to high pressure in a laboratory. Salt water, which is water containing sodium chloride, has a lower freezing point (freezing point) in normal environments. This action is applied in snow-melting agents, etc., but the freezing temperature also changes with pressure. The research team was originally doing this experiment to see how the freezing temperature of salt water changes under high pressure.

[▲ Figure 2: Crystals of NaCl・8.5H2O, one of the sodium chloride hyperhydrates synthesized this time. Synthesis requires high pressure, but at low temperatures it remains stable even at atmospheric pressure (Credit: Journaux et al./PNAS)]

but,Contrary to expectations, crystals of sodium chloride hydrate formed instead of ice at a pressure as high as 25,000 times the atmospheric pressure.This surprised the research team. As a result of examining the structure and components of the obtained crystals, the research team revealed the existence of a previously unknown sodium chloride hydrate.this isDiscovery that rewrites the phase diagram of a mixture of water and sodium chloride for the first time in 150 yearshave become.

[▲ Figure 3: Crystal structure of sodium chloride hydrate. So far, only one type of sodium chloride hydrate has been known (left). Coincidentally, two new types of sodium chloride hyperhydrate were discovered this time (center and right) (Credit: Baptiste Journaux/University of Washington)]

「sodium chloride superhydrate It was also found that this crystal, called “hyperhydrated sodium chloride”, has two chemical components. One is”NaCl·8.5H2O (Sodium Chloride 8.5 Hydrate)”, the other is “NaCl・13H2O (sodium chloride 13-hydrate)”. Both have a larger proportion of water molecules compared to sodium chloride dihydrate, so they may correspond to the “watery” sodium chloride hydrates found in icy bodies. The high pressure inside the crust of icy bodies is consistent with the high pressure environment created in the laboratory. This result was an unexpected discovery, and it was found while the search for conditions had not progressed. By changing the experimental conditions, it is possible to find different sodium chloride superhydrates.

Of the two sodium chloride superhydrates, NaCl 8.5H2O is experimentally stable at -50°C or below, and theoretically at -38°C or below, even when the pressure is reduced to atmospheric pressure. was shown to exist as In other words, there is a possibility that sodium chloride hyperhydrate exists not only on the surface of icy bodies but also on the earth. For example, it is known that salt lakes with extremely high salinity exist within the thick ice sheets of Antarctica. If sodium chloride hyperhydrate is produced at the bottom of salt lakes, it is possible that there may be some that have been transported to the surface of the ice sheet.

In any case, it is not yet confirmed whether the sodium chloride hyperhydrate discovered this time exists in the natural world, including icy bodies. To that end, it is necessary to improve the accuracy of the data through both detailed observations by space probes and the production of larger crystals in the laboratory.

Source

• Baptiste Journals, et.al. “On the identification of hyperhydrated sodium chloride hydrates, stable at icy moon conditions” (Proceedings of the National Academy of Sciences)
• Hannah Hickey. “Newly discovered form of salty ice might exist on surface of extraterrestrial moons”. (University of Washington)

Text: Riri Ayae