  In the framework of the theoretical model of the phase transition of binary
solutions into spatially inhomogeneous states proposed earlier by the autors
[1], which takes into account nonlinear effects, the role of the cubic in
concentration term in the expansion of free energy was studied. It is shown
that taking into account the cubic term contributions to the stabilization of a
homogeneous state. The existence of two inhomogeneous phases in an isotropic
medium, considered in [1], proves to be possible only at half the concentration
of the solution. The contribution of inhomogeneity effects to thermodynamic
quantities is calculated. Phase transitions from a homogeneous state and
between inhomogeneous phases are second-order phase transitions.
