Date: Winter 2012-2013
I have two questions about supersaturated solutions:
1) What happens at the molecular level that certain amount of water dissolves more solute than its capability?
2) When a salt dissolves in water, salt ions are surrounded by certain number of water molecules to keep the ions separated. If a solution is unsaturated or saturated there is plenty of water molecules, and the ions are surrounded by sufficient number of water molecules. How does this happen for supersaturated solutions? In other words, if a solution is a supersaturated solution, what kind of arrangement/alignment occurs between the ions of solute and water molecules that plenty of solute stay dissolved in certain amount of water?
There are a number of theories that attempt to answer your questions. I will try to explain in terms of how the solute and solvent particles might be behave to form supersaturated solutions and then how the supersaturated solute particles come out of the solution. This explanation is a kind of summary of the different theories.
Let us start with how things are dissolved.
The solute has to break up into individual particles and be surrounded by the individual particles of the solute. These particles could be molecules or ions. The process of surrounding is called solvation and results from attractions between these particles. The surrounding process occurs because the solute particles are attracted to the solvent particles. This attraction and surrounding results in a kind of bubble or skin of solvent particles surrounding the solute particles.
Supersaturation may occur when the solute particles--surrounded with this bubble or skin--just don't have enough momentum to push through the skin or bubble. If you give the solute particles a "push" they start punch through that skin or bubble and at a certain point--when the particle size grows large enough--a cascade starts and the solute spontaneously comes out of solution. Why does particle size matter/ It may have to with how many solvent particles surround each solute particle. Smaller particles of solute make it possible for more solvent particles to surround more of the solute because smaller particles have more total surface area than larger ones. it may also have something to do with a kind of "anvil effect." The larger particles act as a kind of anvil against which smaller particles can knock off some of the solute particles can knock off the skin of solvent particles.
Thanks for the questions. For question 1, usually the solubility of a material increases with increasing temperature. When a solvent is warmed and the used to dissolve a solute, the solution will eventually cool and become supersaturated. This is a common way of forming super-saturated solutions. There is a more subtle effect (involving enthalpy of solutions) as a way to form super-saturated solutions. If you are interested, I can explain it to you.
For question 2, there are at least 12 water molecules shielding a cation and an anion. A cation is a positively charged ion while an anion is a negatively charged ion. There is around 6 or 7 water molecules directly interacting with the ion by ion-dipole attractions. That is the first solvation shell. There is a second solvation shell surround the first solvation shell. The exact arrangements of water molecules solvating an ion can be determined by X-ray absorption techniques such as EXAFS (Extended X-ray Absorption Fine Structure). The arrangements can be modeled using correlation function formalisms in statistical thermodynamics.
I hope this helps. Please let me know if you have any more questions.
The solubility of NaCl in water is about 360g salt per kilogram water. At about 58.5 g/mol (formula weight of NaCl), 360 g represents approximately 6.2 moles of NaCl. For water 1kg represents 55.6 moles. This means that for a saturated solution of NaCl in water, there are 55.6/6.2 = 9 molecules of water for every molecule of NaCl, if we split that into ions, there are approximately 4.5 molecules of water for every ion. Let us say we supersaturated the solution so that instead of 360 g/kg we have 400 g/kg, this would come to about 4 molecules of water per ion.
So from an ion's perspective, the amount of water solvating it, as the solution goes from saturated to supersaturated, is not all that different. This tells us that the ion-molecule interaction between saturated versus supersaturated solutions is not all that different.
The difference is in the energies involved. The difference between the energy (enthalpy and entropy) of a saturated solution versus a solution with a precipitate is what drives the amount of substance that can be dissolved in a solvent. When the energy lowering (forming stable solutions) is no longer as much as the energy lowering that can be gained by forming precipitate, then a solution can no longer hold additional substrate because there is more stability as a precipitate than as a solvated ion or molecule.
Greg (Roberto Gregorius)
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Update: November 2011