[Omega Glory]

Hello all --

Say I have a compound in water. The compound is a basic drug, and it has the form of a hydrochloride salt. The pKa lies at 9, so in water of pH 7, approximately 99% will remain in the acidified (salt) form.

Please, check my thinking:

If the compound remains solvated -- if it remains in aqueous media -- then the salt form will exhibit a substantially LOWER vapor pressure than the free base form, yes? Because the salt form has more ionic character, and will be "gripped" by the polar water molecules more tightly, inhibiting its release into the air. Whereas the opposite would be true of the free base form in organic solvent: there, the salt form of the drug would have greater vapor pressure, and the more organic free base retained preferentially in the organic solvent. Yes?

But are salts not non-volatile, in general? Is there such a thing as a volatile salt? What if it is not solvated, say if it is distributed on a deactivated substrate -- is there any circumstance in which that salt would be volatile?

[mjc123]

The salt itself might not be volatile, but it will have a dissociation pressure of HCl:
BH⁺Cl^-(s)  B(s) + HCl(g)
If the free base is volatile, it and the HCl could recombine, so you could effectively sublime the salt, as with ammonium chloride:
BHCl(s)  B(g) + HCl(g)  BHCl(s)