Crystals formation is one of the most common processes you can probably imagine. 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 how crystals are formed, especially nucleation – the very first step in the crystallization process. That’s partly because it is a dynamic process that takes place on such a small scale, and partly because it is somewhat random, making both difficult to study.
That’s what makes the work of a team of researchers led by chemist Takayuki Nakamuro at the University of Tokyo in Japan so exciting. Using a special technique in development since 2005, they filmed the crystallization of salt on an atomic scale for the first time.
Since crystallization is used for a wide variety of applications – from medicine to industrial manufacturing – this is a step toward better control over how we make materials, the researchers said.
The technique is called single-molecule atomic resolution real-time electron microscopy, or SMART-EM, used to study molecules and molecular aggregates. By combining it with a newly developed sample preparation method, the team recorded the formation of salt crystals.
(The 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 samples in place, we use atom-thick carbon nanohorns, one of our earlier inventions. With the stunning videos Sakakibara made, 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 as water evaporated from a sodium chloride solution. Out of the fluid chaos caused by the shape of a vibrating carbon nanohorn suppressing molecular diffusion, order emerged as dozens of salt molecules emerged and arranged themselves in cubic crystals.
These precrystallization aggregates had never been observed or characterized before, the researchers said.
The researchers observed the process nine times, and nine times the molecules arranged themselves in a cluster that fluctuated between characterless and semi-ordered states before suddenly forming into a crystal: four atoms wide and six atoms long. These states, the team noted, are vastly different from the actual crystals.
They also noted a statistical pattern in the frequency with which crystals formed, grew, and shrinked. They found that during each of the nine nucleations, the timing of the nucleation process followed approximately a normal distribution, with an average time of 5.07 seconds; this was theorized, but this is the first time it has been experimentally verified.
Overall, their results showed that molecular assembly size and 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 determine 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 just our first model substance to explore the fundamentals of nucleation events,” said University of Tokyo chemist Eiichi Nakamura.
Salt only crystallizes one way. But other molecules, such as carbon, can crystallize in multiple ways, leading to graphite or diamond. This is called polymorphism, and no one has seen the early stages of nucleation that leads to it. Our study provides the first step to understand the mechanism of polymorphism. “
The research is published in the Journal of the American Chemical Society.