Stunning video reveals the formation of salt crystals from individual atoms



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One of the most common processes you can possibly think of is crystal formation. Every time you freeze water in ice cubes, for example, you create crystalline structures. There’s even a fun experiment you can do to grow salt crystals – with nothing more than table salt and water.

But at the atomic level, we have a poor understanding of crystal formation, especially nucleation – the very first step in the crystallization process. This is partly because it is a dynamic process that occurs at such small scales, and partly because it is somewhat random, which makes studying difficult.

This is what makes the work of a team of researchers led by chemist Takayuki Nakamuro from the University of Tokyo in Japan so exciting. Using a particular technique in development since 2005, they filmed for the first time the crystallization of salt on an atomic scale.

Since crystallization is used for a large number of applications – from medicine to industrial manufacturing – this is a step towards greater control over how we create materials, the researchers said.

The technique is called single-molecule atomic-resolution real-time electron microscopy, or SMART-EM, which is used to study molecules and molecular aggregates. By combining it with a new method of sample preparation, the team captured the very formation of salt crystals.

salt crystals(University of Tokyo)

“One of our master’s students, Masaya Sakakibara, used SMART-EM to study the behavior of sodium chloride (NaCl) – salt,” Nakamuro said.

“To hold the samples in place, we use atomic-thick carbon nanhorns, one of our previous inventions. With the stunning videos captured by Sakakibara, we immediately noticed the opportunity to study the structural and statistical aspects of crystal nucleation in unprecedented detail.

At a rate of 25 frames per second, the team recorded it as water evaporated from a sodium chloride solution. From liquid chaos, induced by the shape of a vibrating carbon nano-horn suppressing molecular scattering, order emerged as dozens of salt molecules emerged and arranged themselves into cube-shaped crystals.

These precrystallization aggregates had never been observed or characterized before, the researchers said.

Nine times the researchers observed the process, and nine times the molecules arranged in a fluctuating cluster between featureless and semi-ordered states before suddenly forming into a crystal: four atoms wide by six atoms long. These states, the team noted, are vastly different from actual crystals.

They also noticed a statistical pattern of how often crystals form, grow and shrink. They found that during each of the nine nucleations, the timing of the nucleation process followed roughly a normal distribution, with an average time of 5.07 seconds; this had been theorized, but this is the first time that this has been verified experimentally.

Overall, their results showed that the size of the molecular assembly and its structural dynamics both play a role in the nucleation process. Understanding this, it is possible to precisely control the nucleation process by controlling the space in which it occurs. They could even control the size and shape of the crystal.

The next step in the research will be to try to study more complex crystallization, with broader practical applications.

“Salt is only our first model substance to probe the fundamentals of nucleation events,” said chemist Eiichi Nakamura of the University of Tokyo.

“Salt crystallizes only in one direction. But other molecules, like carbon, can crystallize in multiple ways, leading to graphite or diamond. This is called polymorphism, and no one has seen it. the early stages of nucleation leading to it. I hope our study provides the first step to understanding the mechanism of polymorphism. “

The research was published in the Journal of the American Chemical Society.

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