Thermodynamic Basis for Phase Separation

Liquid-liquid immiscibility, or phase separation, is a common phenomenon in liquid systems. It should not be surprising to find that it is also a common phenomenon in melts. Actually, far more binary glass forming melts exhibit liquid-liquid immiscibility than exhibit homogeneous liquid behavior. Formation of homogeneous glasses from melts might therefore be considered to be rather unusual. 

Why do some liquids or melts separate into two liquid phases while others remain homogeneous The answer, as usual, is found in the behavior of the free energy of the system. If mixing of two components yields a lower free energy, the mixture will remain homogeneous. If, however, separation of the mixture into two components yields a lower free energy, separation will occur if allowed by kinetic considerations. 

A simple model for the thermodynamic basis for phase separation can be derived from consideration of the free energy of mixing, AG , which is given by the expression  

Figrue 4.1 Effect of composition on free energy for a binary immiscible system 

The relative concentrations of phases A and B can be determined by the familiar lever rule used in all phase diagrams. The fractional concentration, or relative proportion, of phase A is given by the distance between the bulk composition Y and the composition of B, divided by the distance from A to B. The fractional concentration of phase B is given by the distance between the bulk composition and the composition of A, divided by the bulk composition. Expressed in a different form  

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