Salt Crystals under the microscope

We all know what salt crystals look like. They are cubic structures made from Na+ and Cl-. Right? Turns out that the appearance of salt crystals is open for debate.

According to https://www.carolina.com/basic-science-microscope-slides/salt-crystals-wm-microscope-slides/291454.pr

this is a salt crystal:

I call BS! But then again this looks like a structure from a dried out vaxxine sample which I am now very familiar with. What could possibly cause the desire to normalise such images?

These are not salt crystals either well not entirely:

Darkfield x 100. From a drop of 0.9% saline manufactured in 1999. Fine oil added. Below images are at 200x and go anti-clockwise from the top.

What to make of this? Either the above represents salt crystal 2.0 and there is nothing to worry about OR we have a HUGE (emphasis all mine) fundamental problem….

Here are some images of salt crystals 1.0 just in case you had forgotten:

and for good measure some video:

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Here we have two salt crystals 1.0 and two salt crystals 2.0. No prizes for guessing which is which!

All these crystals come from the same sample of saline which had been in a friends first aid kit for over 20 years - since replaced I might add.

The presumption is that salt 2.0 is the result of doping the solution (with quantum dots??) and then this concentrates in salt 2.0 producing a fundamentally different structure. Consider, for instance, the 2019 paper by Taiwanese researchers titled, ‘The Photothermal Stability Study of Quantum Dots Embedded in Sodium Chlorides’. It describes “the use of ionic crystals like sodium chloride (NaCl)” to protect quantum dots from environmental degradation. Or the 2015 paper describing “a fast and versatile approach toward high quality mixed quantum dot-salt crystals”, also focussing on NaCl.

Consistent with this idea is the finding that the application of further saline to the sample results in dissolution of the structure and release of extra material. This can be seen in the following photos and video. I think the bright circle represents a stack of hydrophobic material that is gradually released from the exposed surface.

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Perhaps the moving “donuts” that Mat Taylor, Shimon Yanowitz and I have referred to in the past are actually hydrophobic material held in place electrostatically and moving due to the tension that inevitably results between these forces.

This video below is in real time. 200x darkfield of a saline crystal from the same sample i.e. manufactured in 1999. The “donut” on the left can be seen to move over time. Hopefully this is apparent from moving the slider - please do not try to watch it in real time!

A 2019 paper by Srivastava et al describes quantum dots forming ring-like structures. Clearly the scale is wrong but I think that a version of this is what is happening…

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I will endeavour to study this more closely and I encourage all of us, as Ryan Cole so eloquently put it “microscope wannabes” have a look with fresh eyes at salt.

What is the oldest saline product we can find? This has to start somewhere!

I look forward to my first zoom call with my paid subscribers next week and this will be the starting topic for discussion…

Have a great weekend and here’s to watching salt dry!

David

References

1. Adam, M., Wang, Z., Dubavik, A., Stachowski, G.M., Meerbach, C., Soran‐Erdem, Z., Rengers, C., Demir, H.V., Gaponik, N. and Eychmüller, A., 2015. Liquid–liquid diffusion‐assisted crystallization: A fast and versatile approach toward high quality mixed quantum dot‐salt crystals. Advanced Functional Materials, 25(18), pp.2638-2645. https://onlinelibrary.wiley.com/doi/10.1002/adfm.201500552

2. Huang, Y.M., Hsu, S.C., Li, N., Yu, C.P., Ke, L.A., Huang, C.P., Chang, S.H., Chueh, Y.L., Kuo, H.C. and Lin, C.C., 2019. The photothermal stability study of quantum dots embedded in sodium chlorides. Crystals, 10(1), pp.2-14. https://www.mdpi.com/2073-4352/10/1/2

3. I. Srivastava, J. Khamo, S. Pandit et al., Influence of electron acceptor and electron donor on the photophysical properties of carbon dots: A comparative investigation at the bulk‐state and single‐particle level. Adv. Funct. Mater. 29, 1902466 (2019). https://doi.org/10.1002/ adfm.201902466

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