intrinsic solubility

Intrinsic solubility is the solubility of a compound's neutral, uncharged form alone, stripped of any help from ionization. Often written S0, it is the baseline figure for how soluble the molecule is when none of its acidic or basic groups are charged, the worst-case waterline beneath the boosts that charge can provide.

It matters because real solubility at a given pH is built on top of this baseline. An ionizable drug can be far more soluble than its S0 when the surrounding pH pushes it into a charged, water-loving form, but it can never dissolve less than its intrinsic solubility allows. For a neutral, non-ionizable molecule, intrinsic solubility simply is its aqueous solubility.

Because it isolates the contribution of the molecule itself from the contribution of pH, intrinsic solubility is the cleaner property to compare across compounds and to feed into models. A low S0 is a warning that no amount of pH adjustment or salt formation will fully rescue solubility, often pointing instead to changes in the structure or solid form.