  The heating of the ion-neutral (or ambipolar) diffusion may affect the
thermal phases of the molecular clouds. We present an investigation on the
effect of this heating mechanism in the thermal instability of the molecular
clouds. A weakly ionized one dimensional slab geometry, which is allowed for
self-gravity and ambipolar diffusion, is chosen to study its thermal phases. We
use the thermodynamic evolution of the slab to obtain the regions where slab
cloud becomes thermally unstable. We investigate this evolution using the model
of ambipolar diffusion with two-fluid smoothed particle hydrodynamics, as
outlined by Hosking & Whitworth. Firstly, some parts of the technique are
improved to test the pioneer works on behavior of the ambipolar diffusion in an
isothermal self-gravitating slab. Afterwards, the improved two-fluid technique
is used for thermal evolution of the slab. The results show that the thermal
instability may persist inhomogeneities with a large density contrast at the
intermediate parts of the cloud. We suggest that this feature may be
responsible for the planet formation in the intermediate regions of a
collapsing molecular cloud and/or may also be relevant to the formation of star
forming dense cores in the clumps.
