Crystal formation is one of the most common processes you can think of. Every time you freeze water in ice cubes, for example, you create crystalline structures. There is 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 form, especially nucleation – the very 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 somewhat random, both of which make study difficult.
That makes the work of a team of researchers led by chemist Takayuki Nakamuro of the University of Tokyo in Japan so interesting. Using a special technique in development since 2005, they filmed the crystallization of salt on an atomic scale for the first time.
Because crystallization is used for a large number of applications – from medicine to industrial production – it is a step towards better control over how we create materials, the researchers said.
The technique is called real-time atomic resolution microscopy with a single molecule or SMART-EM, used to study molecules and molecular aggregates. Combining it with a newly developed sample preparation method, the team even captured the formation of 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,” said Nakamuro.
“To hold samples in place, we use atomic-thick carbon nanohorns, one of our previous inventions. With the amazing 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 as evaporated water from a solution of sodium chloride. From the liquid chaos, induced by the shape of a vibrating carbon nanohorn that suppresses molecular diffusion, order emerged as dozens of salt molecules appeared and arranged into cube-shaped crystals.
These pre-crystallization aggregates have never been observed or characterized until now, the researchers said.
Nine times the researchers observed the process and nine times the molecules were arranged in a fluctuating group between the characterless 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 at which crystals formed, increased, and decreased. 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 had been theorized, but this is the first time it has been experimentally verified.
In general, their results showed that the size of the molecular assembly 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 in which it takes place. It could even control the size and shape of the crystal.
The next step of the research will be to try to study more complex crystallization, with wider practical applications.
“Salt is just our first model of substance to test the foundations of nucleation events,” said chemist Eiichi Nakamura of the University of Tokyo.
“Salt crystallizes in only one way. But other molecules, such as carbon, can crystallize in many ways, leading to graphite or diamond. This is called polymorphism, and no one has seen the early stages of nucleation that lead to it. I hope our study provides the first step in understanding the mechanism of polymorphism. “
The research was published in Journal of the American Chemical Society.