  Using molecular simulations, we determine the locus of ideal compressibility,
or Zeno line, for a series of ionic compounds. We find that the shape of this
thermodynamic contour follows a linear law, leading to the determination of the
Boyle parameters. We also show that a similarity law, based on the Boyle
parameters, yields accurate critical data when compared to the experiment.
Furthermore, we show that the Boyle density scales linearly with the
size-asymmetry, providing a direct route to establish a correspondence between
the thermodynamic properties of different ionic compounds.
