Binodal

Binodal is a term used in phase diagram terminology to describe the boundary between a single-phase region and a two-phase region in a binary mixture. The binodal line (or curve) marks the conditions—typically temperature and composition—under which the mixture becomes thermodynamically unstable enough to separate into two coexisting phases at equilibrium. At a given temperature below the critical point, the compositions of the two coexisting phases are found at the endpoints of a tie line that touches the binodal.

Thermodynamically, the binodal is determined by the equality of chemical potentials in the two phases; equivalently, it can be constructed from the Gibbs free energy of mixing by the common tangent method. The region enclosed by the binodal consists of mixtures that will phase-separate, while outside the binodal the mixture remains single-phase.

The spinodal curve lies inside the binodal boundary. The area between the binodal and the spinodal is metastable: phase separation can occur, but only via nucleation and growth. Inside the spinodal, the mixture is unstable and phase separation proceeds spontaneously through spinodal decomposition.

Binodal curves are central in polymer science, colloid chemistry, and materials engineering. They appear in polymer solutions, polymer blends, and aqueous two-phase systems, where control of the two-phase region enables separation, purification, or the creation of distinct material properties. In practice, binodal boundaries can be determined experimentally through cloud-point measurements, turbidimetry, or by thermodynamic modeling using theories such as Flory–Huggins for polymers.