Spinodal

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A phase diagram displaying spinodal curves, within the coexistence curves and two critical points: an upper and lower critical solution temperature.

In thermodynamics, the spinodal is the limit of stability of a solution,[1][2] denoting the boundary of absolute instability of a solution to decomposition into multiple phases. Within this curve, infinitesimally small fluctuations in composition and density will lead to phase separation via spinodal decomposition. Outside of the curve, the solution will be at least metastable with respect to fluctuations.[2] In other words, outside of the spinodal curve some careful process may at least in theory obtain a single phase system.[2] Inside of it, no process will. In general, the spinodal lies inside of a binodal (coexistence) curve, which denotes the minimum-energy equilibrium state of the system.

Criterion[edit]

For binary solutions, the thermodynamic criterion which defines the spinodal curve is that the second derivative of free energy with respect to density or some composition variable is zero.[2][3][4]

Critical point[edit]

Extrema of the spinodal in temperature with respect to the composition variable coincide with ones of the binodal curve and are known as critical points.[4]

References[edit]

  1. ^ Sandler S. I., Chemical and Engineering Thermodynamics. 1999 John Wiley & Sons, Inc., p 571.
  2. ^ a b c d Koningsveld K., Stockmayer W. H.,Nies, E., Polymer Phase Diagrams: A Textbook. 2001 Oxford, p 12.
  3. ^ Sandler S. I., Chemical and Engineering Thermodynamics. 1999 John Wiley & Sons, Inc., p 557.
  4. ^ a b Koningsveld K., Stockmayer W. H.,Nies, E., Polymer Phase Diagrams: A Textbook. 2001 Oxford, pp 46-47.