Crystal formation is one of the most common processes you can think of. Every time you freeze water into ice cubes, for example, you create crystalline structures. There is even a fun experiment you can do to grow salt crystals – nothing more than table salt and water.
But on the atomic level, we have a poor understanding of how crystals form, especially nucleation – the first step in the crystalline process. That’s partly because it’s such a dynamic process that happens on such small blades, and partly because it’s something random, and both make it difficult to study.
That is what makes the work of a team of researchers led by chemist Takayuki Nakamuro of the University of Tokyo in Japan so inspiring. Using a unique technique in development since 2005, they have filmed salt on the atomic scale for the first time.
Since crystallization has been used for a number of applications – from medicine to industrial manufacturing – this is a step toward better control over how we create products, the researchers said.
Technically known as single-molecular real-time electron microscopy, or SMART-EM, is used to study molecules and molecular aggregations. By combining it with a newly developed sample preparation method, the team captured the creation of real salt crystals.
(University of Tokyo)
“One of our master’s students, Masaya Sakakibara, used SMART-EM to study the behavior of sodium chloride (NaCl) – a salt,” Nakamuro said.
“To hold samples in place, we use atom-thick carbon nanohorns, one of the innovations we did before. With the amazing Sakakibara videos captured, we immediately realized the opportunity to study the sides structural and statistical precision of crystal crystal precisely never seen before. “
At a rate of 25 frames per second, the team recorded as water evacuating from sodium chloride solution. From the liquid chaos, caused by the formation of a shaking nanohorn carbon eliminating molecular dispersion, an order emerged as tens of salt molecules emerged and arranged themselves in cube-shaped crystals.
These pre-crystalline assemblages have never been seen or identified before, the researchers said.
Nine times the researchers watched the process, and nine times the molecules arranged themselves in a browser switching between featureless and half-order states before they suddenly became crystals: four atoms of a width of six atoms long. These states, the team noted, are very different from the real crystals.
They also observed a statistical pattern in the frequency at which crystals formed, grew and vibrated. They found that, during each of the nine nucleations, the time of the circulation process largely followed a normal rotation, with an average time of 5.07 seconds; this has been theorized, but this is the first time it has been experimentally proven.
Overall, their results showed that both the size of the molecular assembly and the structural dynamics play a role in the circulation process. By understanding this, it is possible to control the circulation process by controlling the place in which it occurs. They could even control the size and shape of crystals.
The next step in the research is to try to study more complex crystallization, with broader practical applications.
“Salt is just the first model material we have to study the basics of nucleation events,” said chemist Eiichi Nakamura of the University of Tokyo.
“Salt crystallizes only one way. But other molecules, such as carbon, can crystallize in a number of ways, leading to graphite or diamonds. This is called polymorphism, and one has not seen the early stages of nucleation that I hope our study provides the first step in understanding the mechanism of polymorphism. “
The research was published in the Journal of the Chemical Society of America.