# What makes some chemicals form steep density gradients in solution?

Caesium chloride and sucrose are both suitable for isopycnic centrifugation, and presumably not much else is. I'm unclear as to the mechanism by which they form useful density gradients. Wikipedia says,

The $$\ce{CsCl}$$ molecules become densely packed toward the bottom, so even layers of different densities form.

(No doubt $$\ce{CsCl}$$ is molecular, despite being the chloride of a group I element, because $$\ce{Cs}$$ has such low electropositivity, but I digress.) But what is it about these molecules, not to mention those of sucrose, that causes their packing to be so strongly isopycnic in solution?

• Wow, I've never heard of this technique, it sounds really interesting. I thought these kinds of gradients only happened appreciably in the gas phase. – Nicolau Saker Neto Apr 11 '20 at 12:52
• @NicolauSakerNeko It's probably better-known among biologists; biology is where I learned it. – J.G. Apr 11 '20 at 13:14

A determination was made that coordination number of cesium ions is 8 only in diluted solutions; in concentrated solutions it is near 4. The form of cation hydrate complexes depends largely on concentration and effect of counterions. https://inis.iaea.org/search/search.aspx?orig_q=RN:27004247

If the cesium hydrates are fairly stable, their volumes, and hence densities would vary with the amount of water bonded. It would seem that after centrifugation and separation, each portion with a uniform density would be a separate phase, with a different composition. Not a liquid crystal, or a crystalline liquid. Maybe a liquid glass? It should be possible to extract some of each layer and determine the total solids, or water content.

Sucrose presents a more complicated situation, but studies indicate degrees of hydration with relaxation times of a few picoseconds to 30 or so (https://www.sciencedirect.com/science/article/abs/pii/S0008621596910198), and so this could indicate hydrates of different stability that might be forced into a certain and constant composition - another liquid glass.

Another separation from a mixture that I have seen occasionally is when a mixture of different polymers in the same solvent evaporates. If the polymers are of sufficiently low molecular weight, or high mobility, or slow enough evaporation rate, they might separate into two layers. I suppose if the mobility is not high enough, the mixed polymer will evaporate with obvious inhomogeneity, or haze.

• But how is this used to create the density gradient? Are solutions of different concentrations separately created, so that when they're mixed they're like water and oil? Or does the centrifuge cause a concentration gradient, and hence a density gradient upon hydration? – J.G. Apr 11 '20 at 15:00
• We expect that waters of hydration exchange fairly rapidly with bulk water. But even if a cesium 4- or 8-hydrate is extremely stable, they would seem to be too small to be centrifuged out unless, perhaps the higher gravity forces a few together to coalesce into larger moieties which also include the anions. The Cs hydrates might be a special case, not held together by long chains, but by a structural fit not quite enough for solidity. These phases ought to have definite - and different - melting points. But the entropy of mixing prevents their existence at 1g. – James Gaidis Apr 12 '20 at 14:41

Gravity will make the density of a column of a gas greater at the bottom that the top. In the centrifuge, gravity is replaced by a much, much stronger centrifugal force by rapidly spinning the sample about a vertical axis. The centrifugal forces on the solute will slowly create a density gradient in the solution, the more massive the solute the better. For this type of separation the species to be separated (say a protein or DNA) must be less dense than the solution and eventually will come to equilibrium at some point along the tube when the densities are equal.

• All you've described is how the process works, not what makes certain solutions especially helpful for it. – J.G. Apr 11 '20 at 17:42
• You asked how density gradients are formed. – porphyrin Apr 12 '20 at 7:12
• No, I didn't. My title was, "What makes some chemicals form steep density gradients in solution?" (emphasis added). My post's last sentence was, "But what is it about these molecules, not to mention those of sucrose, that causes their packing to be so strongly isopycnic in solution?" – J.G. Apr 12 '20 at 7:13
• All solutes form a density gradient to some extent when centrifuged so it is quite general. Packing is not the correct word to use, the density gradient gradually varies along the centrifuge tube. Look up 'sedimentation' in a text book for the equations to use to calculate this. – porphyrin Apr 12 '20 at 9:29
• "To some extent"? The question is clearly about why a few substances do it to an especial extent. If caesium chloride & sucrose form especially gradated sediments, your answer should explain why. – J.G. Apr 12 '20 at 9:31