Microbiologists deal with salt, sodium chloride, all the time. Most growth media for our cherished research contain at least "a pinch of salt." Or we need salt as a component of buffers for biochemical experiments that require defined concentrations of Na+ and Cl− ions. However, we usually only see crystalline salt when we take the NaCl box from the chemicals shelf to weigh it out. Or if a colleague insists on keeping the bottle with the 5 M NaCl stock solution in the 4°C cold room. Or when it has co‑precipitated with other media components, making for gorgeous patterns in desiccated Petri dishes. Perhaps we learned in chemistry courses that NaCl forms cubic crystals of a specific configuration (cF8). Yet, I have never before seen sodium chloride "in the act" of crystallizing. That is, until now in a recent paper by Takayuki Nakamuro and colleagues from the Dept. of Chemistry of Tokyo University, Japan.
The researchers studied NaCl crystallization by transmission electron microscopy (TEM) and report in situ atomic-resolution detection of crystal nuclei forming during nucleation and growth of NaCl nanocrystals, which took place at 298 K (24.85°C) in a few tens of nm3 vacuum space of a vibrating conical carbon nanotube (CNT) as recorded at 20−40 ms per frame, with a localization precision of <0.1 nm (Figure 1).
They describe in detail the experiment from their series shown here as Figure 2:"In the initial 5 s ..., featureless and semiordered morphology alternated before the formation of the 4×6 crystal at 5.04 s. At 0−0.72 s, we saw a cluster growing up to approximately 2×3 in size near the apex of the CNT that then disappeared quickly, and at 2.00−2.68 s, the appearance of a semiordered cluster made of several rows of NaCl along the graphitic wall, which also disappeared quickly. A sequence from 5.00 to 5.04 s is particularly noteworthy because a clear image of the 4×6 crystal nucleus suddenly emerged from a featureless object. Note that all of the semiordered prenucleation clusters before 5.04 s formed reversibly, while the crystals after 5.04 s grew irreversibly as found in the eight other events ... During the next 6 s, stochastic homoepitaxial growth of the 4×6 crystal occurred. At 5.48 s, we saw the formation of a step in contact with the graphitic wall (white arrow), its disappearance to regenerate the 4×6 crystal in the next frame, and further growth to 5×6. As a note of caution, we assume that the crystal growth took place three-dimensionally, although the TEM information was limited to 2-D."
I know, of course, that I can induce instantaneous crystallization from a (over)saturated salt solution in a glass beaker by scraping along the inner rim of the beaker with a glass rod. The fact that the actual crystallization is preceded by a reversible semi-ordered phase – mostly in contact with the wall of the nanotube – escaped me during my macroscale experiment. You may also want to see the snapshot series of Figure 2 as a movie. The Open-Access publication of the Nakamuro et al. (2021) paper links to it in the 'Supporting Information' section.
Observing salt crystals as they form and re-dissolve is just one next big step in the microscopy of molecules. Previously, researchers have observed complex aromatic molecules in their reactions, for example intramolecular rearrangements by atomic force microscopy (AFM). Figure 2 shows an example, and the authors, de Oteyza et al. (2013), explained:"We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways." Without diving into the reasonably complicated calculations that make such images possible we are left with the pleasant surprise that complex aromatic structures do indeed "look" the way we know them from textbooks.
An aside. Don't mind that these images look somewhat blurry. Apart from the technical difficulties of imaging such ultra-small structures as ions during crystallization or individual C−C or C−H bonds in aromatic molecules, there is also the fundamental problem that they oscillate thermally and simply do not "hold still." Except of course at absolute zero, 0 Kelvin, but there, well, no "camera shutter" works anymore either.