  We report an accurate Monte Carlo calculation of the phase diagram and
clustering properties of the restricted primitive model with non-additive
hard-sphere diameters. At high density the positively non-additive fluid shows
more clustering than the additive model and the negatively non-additive fluid
shows less clustering than the additive model, at low density the reverse
scenario appears. A negative nonadditivity tends to favor the formation of
neutrally charged clusters starting from the dipole. A positive nonadditivity
favors the pairing of like ions at high density. The critical point of the
gas-liquid phase transition moves at higher temperatures and higher densities
for a negative nonadditivity and at lower temperatures and lower densities for
a positive nonadditivity. The law of corresponding states does not seem to hold
strictly. Our results can be used to interpret recent experimental works on
room temperature ionic liquids.
