  Thermodynamic perturbation theory for central-force (TPT-CF) type of
associating potential is used to study the phase behavior of symmetric binary
mixture of associating particles with spherically symmetric interaction. The
model is represented by the binary Yukawa hard-sphere mixture with additional
spherically symmetric square-well associative interaction located inside the
hard-core region and valid only between dissimilar species. To account for the
change of the system packing fraction due to association we propose an extended
version of the TPT-CF approach. In addition to the already known four types of
the phase diagram for binary mixtures we were able to identify the fifth type,
which is characterized by the absence of intersection of the $\lambda$-line
with the liquid-vapour binodals and by the appearance of the closed- loop
liquid-liquid immiscibility with upper and lower critical solution
temperatures.
