Can homogeneous solutions of high molecular weight compounds settle over time?

Question

Note that I am not talking about precipitation, I am talking about an uneven distribution of the solute throughout the solvent. That is, you would find that the solution is more concentrated near the bottom than near the top. I'm thinking the reason for any such discrepancy would be gravity, but this puzzles me, so please answer! I don't know if it would affect the answer, but the solution in question is of a shellfish toxin, known as saxitoxin(s), dissolved in 0.003N hydrochloric acid.

The concentrations of the solutions range from 0.297 ug/ml (micrograms per millilitre) to 0.333 ug/ml. It does seem unlikely that a "true" solution of concentrations this low would experience any settling effects, but I have run into an interesting trend. I actually inject the various toxin levels into mice and measure their death time, and I find that if I do NOT shake the toxin before hand, and I take the aliquot from the top of the sample vial, then sometimes the mice don't even die. At first I thought it may have been a problem with the standard toxin solution I made up, but after shaking and re-injecting I was getting death times within the required interval. It is a curious thing to have happen...any thoughts?

Answer 1

It is absolutely possible and does happen! There is always an equilibrium that is reached between the gravitational pull on the solutes and the random forces exerted by the solvent molecules.

You may recall the famous Meselson-Stahl experiment. They proved the semi-conservative theory of DNA replication by separating strands of DNA according to density. They loaded the DNA (some of which was doped with heavy nitrogen) into centrifuge tubes filled with a cesium chloride (CsCl) solution. Cesium has an atomic mass of 132.9g/mol, its a very heavy atom. When they spun the tubes the CsCl would become more concentrated near the bottom of the tube and create a density gradient. The different strands of DNA would then migrate to the different regions of the tube where they were more or less neutrally buoyant. since the centrifugal force is equivalent to a gravitational force they would both create a density gradient.

In addition, when I took analytic chemistry we were always reminded to shake our standardized solutions so that the concentration would be truly uniform.

Answer 2

Finally....an interesting question. I don't believe there would be any settling of a true solution; by definition, a solution is an evenly distributed homogeneous mixture......BUT an experiment might show otherwise. What is the concentration of the toxin?? Shake the bottle and wait five days and see what happens.

Answer 3

i do believe that it is possible.... heres the rationale.

higher molecular weight means higher vanderWalls attraction. If the energy of solvation is (or enthalpy) is lesser (or more depending on the sign used) the vanderWalls attraction, then the ions may tend to aggregate. Probably particles or colloidal dimensions or greater could be possible. If the range exceeds colloidal dimension, i guess concentration gradient is also possible... but i would not call it a solution still... it is probably not pure precipitation also. but this is all so abscract cos im not really strong in physical chemsitry

Answer 4

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Answer 5

I didn't understand a word you just said.