Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: vahid on 20/07/2018 17:29:28
-
Hi Dear Scientists,
I hope everything is going well with you :).
Probably you know what is a Cosolvent.
Cosolvents are substances added to a primary solvent in small amounts to increase the solubility of a poorly-soluble compound.
So it seems to me that Cosolvent carries a positive meaning that helps solvation process. I was wondering if there is a situation that a Cosolvent Make things worse? I mean Instead of enhancing the solubility, Decrease it. Reduce solubility even more than pure primary solvent.
I know it might happen but I need you guys to share me some Information or published papers about this phenomena.
Thank you all
-
Can addition of a small amount of additive solvent reduce the solubility of a compound below that found in pure solvent?
Yes, absolutely! In these cases the additive can be called an "anti-solvent." I take advantage of this phenomenon fairly frequently for crystallizations.
For example, I can dissolve a polar compound in a polar solvent such as dichloromethane, and then by slowly* adding a non-polar solvent such as pentane, I can decrease the solubility of the solute to the point where the solution is saturated, and then even further to promote precipitation (crystallization) of the solute.
As far as the underlying reason for this: in most cases one has a polar (or even ionic) substance dissolved in a polar (high dielectric) solvent. By adding a nonpolar (low dielectric) liquid that is miscible with the polar solvent, reducing the effective polarity (dielectric). There are also some cases, though in which a more polar solvent can be the antisolvent. For instance, there are many organic compounds that are quite soluble in acetone (or ethanol), but hardly soluble in water. These compounds can be forced out of solution by adding water.
There are probably other cases in which there are more subtle effects involved (like pi stacking), but I think polarity is the dominant driver of most of these examples.
* By slow addition, I mean, over the course of hours, days, or even weeks. The more slowly the change occurs, the bigger and better the crystals can be (which is important for x-ray crystallography). I don't stand there and add the solvents over this whole time period, but usually can set up a system so that the antisolvent can slowly diffuse into the solution. When the antisolvent is less dense than the solution, one can very gently layer it on top of the solution, while trying not to mix it--and then just wait for entropy to take over and mix them. Or when the antisolvent has a higher vapor pressure than the solution, one can place an open container of the antisolvent and an open container of the solution in a sealed chamber together, and let the vapors of the antisolvent condense into the solution, precipitating the solute very, very slowly.
-
@chiralSPO as always, you are the First and the Best answer :)