The formation of crystals is one of the most common processes you can probably think of. Every time you freeze water in ice cubes, for example, you are creating crystalline structures. There is also 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 how crystals are formed, especially nucleation – the very first step in the crystallization process. This is partly because it is a dynamic process that occurs on such a small scale, and partly because it is somewhat random, both are difficult to study.
This is why the work of a team of researchers led by chemist Takayuki Nakamuro of the University of Tokyo in Japan is so exciting. Using a special technique in development since 2005, he has filmed the crystallization of salt on the atomic scale for the first time.
Since crystallization is used for many applications – from medicine to industrial manufacture – it is a step towards better control of how we make materials, the researchers said.
The technique used to study molecules and molecular aggregates is called single-molecule atomic-resolution real-time electron microscopy or SMART-EM. By combining it with a newly developed sample preparation method, the team captured the very formation of salt crystals.
“One of our master’s students, Masaya Sakibara, used SMART-EM to study the behavior of sodium chloride (NaCl) -salt,” Nakamuro said.
“To keep samples in place, we use one of our previous inventions, the atom-thick carbon nanohorn. With the stunning video captured by Sakibara, we immediately unprecedentedly studied the structural and statistical aspects of crystal nucleation. Seen the opportunity to. “
At a rate of 25 frames per second, the team recorded water as evaporating from a sodium chloride solution. From liquid chaos, induced by the shape of a vibrating carbon nanohorn that suppresses molecular diffusion, tens emerged as salt molecules and arranged themselves into cube-shaped crystals.
The researchers stated that these pre-crystallization aggregates had never been seen before nor was it characterized.
Nine times the researchers observed this process, and nine times the molecules arranged themselves in a fluctuating cluster between featureless and quasi-ordered states before suddenly forming in a crystal: four atoms long by six atoms. These states noted, the team is extremely different from the actual crystal.
They also observed a statistical pattern in the frequency at which crystals formed, grew, and shrunk. They found that during every nine nucleations, the normal distribution of the time of the nucleation process occurs with an average time of about 5.07 seconds; This was theorized, but it is the first time it has been verified experimentally.
Overall, their results showed that both the size of the molecular assembly and its structural dynamics play a role in the nucleation process. Understanding this, it is possible to carry out the nucleation process properly by controlling the location in which it occurs. They could also control the size and shape of the crystals.
The next step in research will be to try and study more complex crystallizations with wider practical applications.
“Salt is just our first model substance to investigate the fundamentals of the phenomena of nucleation,” said Iichi Nakamura, a chemist at the University of Tokyo.
“Salt crystallizes only in one way. But other molecules, such as carbon, can crystallize in a number of ways, leaving graphite or diamond. This is called polymorphism, and none led to the initial nucleation of it. Haven’t seen the steps. I hope. Our study provides the first step in understanding the mechanism of polymorphism. “
Has been published in Journal of the American Chemical Society.