Crystal formation is one of the most common processes you can probably think of. 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 but table salt and water.
But, at the atomic level, we have a poor understanding of how crystals are formed, especially nucleation, the first step in the crystallization process. This is partly because it is a dynamic process that happens on such small scales and partly because it is a bit random, which makes study difficult.
This is what makes the work of a team of researchers led by chemist Takayuki Nakamuro of the University of Tokyo in Japan so exciting. Using a special technique in development since 2005, they have filmed the crystallization of salt on an atomic scale for the first time.
Because crystallization is used for a wide range of applications, from medicine to industrial manufacturing, this is a step toward better control of how we create materials, the researchers said.
The technique is called real-time atomic resolution electron microscopy of a single molecule or SMART-EM, which is used to study molecules and molecular aggregates. Combining it with a newly developed sample preparation method, the team captured the same salt crystal formation.
(The University of Tokyo)
“One of our master’s students, Masaya Sakakibara, used SMART-EM to study the behavior of sodium chloride salt (NaCl),” Nakamuro said.
“To keep samples in place, we use atomic-thick carbon nanohorns, one of our previous inventions. With the impressive videos captured by Sakakibara, we immediately noticed the opportunity to study the structural and statistical aspects of nucleation. of crystals with unprecedented details “.
At a rate of 25 frames per second, the team recorded how water evaporates from a sodium chloride solution. From the liquid chaos, induced by the shape of a vibrating carbon nanohorn that suppresses molecular diffusion, order arose as dozens of salt molecules emerged and arranged in 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 were arranged in a cluster that fluctuates between uncharacteristic and semi-ordered states before suddenly forming into a crystal: four atoms wide by six atoms long. These states, the team noted, are extremely different from real crystals.
They also observed a statistical pattern in the frequency with which crystals formed, grew, and shrank. They found that during each of the nine nucleations, the timing of the nucleation process followed approximately a normal distribution, with a mean time of 5.07 seconds; this had been theorized, but it is the first time it has been verified experimentally.
Overall, their results showed that the size of the molecular set and its structural dynamics play a role in the nucleation process. Understanding this, it is possible to precisely control the nucleation process by controlling the space where it occurs. They could even control the size and shape of the crystals.
The next step of the research will be to try to study the most complex crystallization, with broader practical applications.
“Salt is just our first substance model to investigate the fundamentals of nucleation events,” said chemist Eiichi Nakamura of the University of Tokyo.
“Salt only crystallizes in one way. But other molecules, such as carbon, can crystallize in various ways, giving rise to graphite or diamond. This is called polymorphism and no one has seen the early stages. of the nucleation that drive it. I hope our study provides the first step in understanding the mechanism of polymorphism. “
The research has been published in Journal of the American Chemical Society.