Is there a direct and simple relationship between the solubility of a salt and the dielectric constant of the solvent or solvent mixture in which the salt is to be solubilised?
For example: the saturation molality of NaCl in pure water at 25°C is about 6.14 molal. The saturation molality of NaCl at 25°C, either in mixtures of water and formamide (the dielectric constant of formamide is much higher than that of water) or in mixtures of water and ethanol (the dielectric constant of ethanol is much lower than that of water) decreases in both cases.
I have not found any co-solvent that increases the solubility of NaCl compared to that of pure water. The same is true for all alkali halides!
The dielectric constant tells us how well a solvent can decrease the force between charged particles. Basically, a high dielectric constant means the solvent is really good at pulling ions apart, which usually makes it easier for salts to dissolve in it. Water is a prime example—it has a high dielectric constant, making it excellent for dissolving many salts.
But, the story doesn't end there. Several other factors play into how well a salt dissolves:
- The specific interactions between the solvent molecules and the ions (ion-dipole interactions) matter a lot. Even if a solvent has a high dielectric constant, its molecular structure and how it can form bonds or interactions (like hydrogen bonds) significantly affect its ability to dissolve salts.
- When you mix solvents (like water with formamide or ethanol), things get tricky. The mix affects solubility not just based on the combined dielectric constant, but also on how these solvents interact with each other and with the salt.
- In these mixes, how ions pair up or clump together can change, which also influences how much salt can dissolve. The mix can shift the balance between ions sticking together and being free in the solution, changing solubility.
For NaCl and similar salts, water's super high dielectric constant plus its knack for forming strong hydrogen bonds make it especially good at dissolving salts. Adding another solvent into the mix, whether it's formamide (which has a different way of bonding despite its high dielectric constant) or ethanol (which has a lower dielectric constant and isn't as good at stabilizing ions), tends to mess up the delicate balance that makes water so effective on its own, usually making the salt less soluble.
The fact that no added solvent makes NaCl more soluble than it is in pure water highlights how uniquely suited water is for dissolving ionic compounds like salts.
Thank you very much for taking the time to answer my question. I cannot say whether it is the best answer or not because it is the "only" one I have received.
I remember that on one occasion, Professor Torquato Mussini (may he rest in peace), of Milan University, told me on this question that it is not only the dielectric constant that is important (as you also say) but "the polarity of the medium". What do you think about this, and how do you calculate the polarity of a solvent mixture?
Thank you again for your answer.
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