Lecture 21: Phase Coexistence and Separation  Summary and Q&A
TL;DR
Phase coexistence is determined by the equilibrium condition where the Gibbs free energy is stationary, and common tangent construction is used to determine the compositions that can coexist at equilibrium.
Questions & Answers
Q: What is the equilibrium condition for phase coexistence?
The equilibrium condition for phase coexistence is that the Gibbs free energy is stationary, meaning that the chemical potentials of each component are equal in both phases.
Q: How is common tangent construction used to determine the compositions that can coexist at equilibrium?
Common tangent construction involves drawing tangents to the solution models of each phase on a free energy composition diagram. The compositions at the points of tangency represent the compositions that can coexist at equilibrium.
Q: What are tie lines on a phase diagram?
Tie lines are lines on a phase diagram that connect the compositions that can coexist at equilibrium. They are defined by the common tangents on the free energy composition diagram.
Q: Does common tangent construction provide information about the kinetics of phase segregation?
No, common tangent construction only provides information about the equilibrium configuration of the system. Kinetics of phase segregation depend on factors such as diffusion coefficients and temperature.
Summary & Key Takeaways

Phase coexistence in a system with two phases and two components requires that the Gibbs free energy is stationary at equilibrium.

Common tangent construction is used to determine the compositions that can coexist at equilibrium by drawing tangents to solution models of each phase.

Tie lines on a phase diagram are defined by the common tangents and represent the compositions that can coexist at equilibrium.