The existence of phase transitions from liquid to gas phases in asymmetric
nuclear matter (ANM) is related with the instability regions which are limited
by the spinodals. In this work we investigate the instabilities in ANM
described within relativistic mean field hadron models, both with constant and
density dependent couplings at zero and finite temperatures. In calculating the
proton and neutron chemical potentials we have used an expansion in terms of
Bessel functions that is convenient at low densities. The role of the isovector
scalar δ-meson is also investigated in the framework of relativistic
mean field models and density dependent hadronic models. It is shown that the
main differences occur at finite temperature and large isospin asymmetry close
to the boundary of the instability regions.Comment: 13 pages, 5 figures; to appear in Phys. Rev.