Nature of double critical points in binary solutions

The nature of the reentrant demixing transition in binary solutions with H bonds is studied in the framework of an Ising-like Hamiltonian with effective spin-spin interaction constant. It is taken into account that the internal variables describing the H-bond network are characterized by the spatial time scales essentially shorter than those for the spin variables. Due to this the contribution of H bonds to the effective spin-spin interaction constant is described by thermodynamic methods. With the help of the catastrophe theory the classification of possible types of phase diagrams leading to the double critical point is given. The influence of small quantities of third component (electrolytes, water, and molecules similar to CCl4) is discussed.