The Evolution of Selectivity

In some of them near the diffusion limit, sodium just doesn't go through. And people who made the best measurements of the ratio, depending on how you measure it, it's a thousand or 10,000 times better for potassium than sodium. How does that work? I think probably chemists a long time ago figured out the principle by making crown ethers and cryptans that could selectively bind alkali metal cations like sodium and potassium and rubidium. So we finally got to see it because each little binding site in the potassium selectivity filter looked like one of those cryptan that chemists had made with the oxygen. You know, it's not me, not my work, it