alt Hacker News> Chong Liu and her colleagues knew that one of the differences between rare earth ions was the size of the water shell surrounding each one when they are dissolved in solution. Lighter rare earths like lanthanum have larger first water shell, while heavier rare earths like dysprosium have a smaller first shell. Taking advantage of that size difference, Chong Liu’s group engineered manganese oxide so that the gaps between its stacked layers were only a few water molecules wide. Then, they squeezed raw mixtures of rare earth elements inside.
Cool! Does this only work with a mixture of mostly rare earths, or can ores be "squeezed...inside"? Also, is the end product metal or intercalated manganese oxide?
Interestingly (well to me) this is also how some ion channels in cells work - the solvation shell (water cage) allows protein pores to distinguish between sodium and potassium.
Not sure this is the best paper, just a random pick :
https://www.sciencedirect.com/science/article/abs/pii/S09692...
Huh. More complex than I understood:
"...there are two groups of hypotheses explaining the selectivity on the basis of molecular dynamics (MD) simulations. The first group mainly considers flexibility/mobility of the carboxyl groups in the EEEE ring that provides a preferable space-charge environment for partly hydrated Na+ ions to pass and an unfavorable environment for K+.19,20,21 The second group proposes the “steric” selectivity mechanism, suggesting that the SF of Navs is not wide enough to let a fully hydrated K+ pass through, while fully hydrated Na+ traverses through the pore without a significant barrier"
Different paper about the 'selectivity filter' https://www.frontiersin.org/journals/physiology/articles/10....
"The backbone carbonyl oxygens plus the hydroxyl group of the threonine form four ion binding sites, called S1 to S4 from the extracellular side (see Figure 1A), and they perfectly mimic the hydration shell of potassium ions"
What I like about the selectivity filter is that it uses the backbone carbonyls.