Binodal
The curve in a phase diagram from which a phase transition can take place and is thermodynamically more favorable is referred to as the binodal curve (also called coexistence curve) . It thus represents exactly the line of coexistence of both phases . The binodal encloses the spinodal , with which it can have common, so-called critical points (cf. illustration).
This concept is important for binary mixtures, for example . If one is in a suitable temperature range, a point can be reached by gradually increasing the concentration of one substance (movement on a line parallel to the composition axis, i.e. on a conode ) at which the two substances no longer mix homogeneously , but rather to separate into two phases which coexist side by side ( miscibility gap ).
For states within the Spinodal , two-phase states are the states of equilibrium (minimum of free enthalpy ), in addition they are thermodynamically stable (the second derivative of the free enthalpy e.g. after concentration is positive). For the states between spinodal and binodal, the two-phase state is also thermodynamically the most favorable (minimum of free enthalpy), but the state without demixing is only metastable (stable against small deflections), since the second derivative of the free enthalpy is positive.
Individual evidence
- ^ Günter Jakob Lauth, Jürgen Kowalczyk: Thermodynamics: An introduction . Springer-Verlag, 2015, ISBN 978-3-662-46229-4 , pp. 250 ( books.google.de ).
- ^ Hermann Weingärtner: Chemical Thermodynamics: Introduction for Chemists and Chemical Engineers . Springer-Verlag, 2013, ISBN 978-3-322-91221-3 , pp. 130 ( books.google.de ).