Chemistry - What makes some chemicals form steep density gradients in solution?

Solution 1:

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.

Solution 2:

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.