Question

Can somebody explain to me the miscibility of isopropanol alcohol and ethanol alcohol in a NaCl solution and why higher levels of concentration of salt solution make the alcohol less miscible (homogeneous)?

Answer 1

Alcohols are actually still quite polar, but the problem is the way the orient themselves. Nearly all solvation mechanics is based on this principal.

Alcohols line up so their polar heads are toward the ion, but as a result the non-polar tail is facing away. This creates problems for the counter ion since it also has an alcohol solvation shell. By the time you actually put the two ions and their shells together you find the ions can't pack that close to each other and as a result any stability given by the solvent is not enough to overcome the extra distance. The more bulky the alcohol is the more problematic this effect is.

What determines solubility is the solvation structure not just the polarity.

Isopropanol is a bulky alcohol which means it is even more difficult to arrange it around an ion in a way that is stable.

NaCl has strong ionic bonds, but will still dissolve partially in ethanol. When you move up to larger alcohols you find solubility drops dramatically not because the OH to ion bond is weaker, but because with the increased size of the alcohol chain the overall solvation shell is not as stable.

For any kind of mixing to occur you need to be able to form a stable solvation shell in addition to providing enough stability to the solute.