# Phase separation

Entry by Emily Redston, AP 225, Fall 2011

Figure 1 (from Haasen)
Figure 2 (from Haasen)

We typically talk about phase separation in terms of the regular solution model of liquids. The free energy of mixing can be written as,

$\frac{G_{mix}}{kT}=Nnv _{A}\ln v _{A}+x _{B}\ln v _{B}+\epsilon v _{A} v _{B}$
where $\epsilon = zN[V_{AB} - {1 \over 2}(V_{BB}+V_{BB})]$

$V$ represents bond energies, $z$ is the number of neighbors, $N$ is the total number of atoms, and $x$ is the mole fraction. The sign of $/epsilon$ is essential in determining the solution's behavior as a function of temperature. If $/epsilon$ then the change in free energy upon mixing is always negative, so the atoms will always want to be fully mixed. If $/epsilon$, then mixing or phase separation can occur depending on the temperature.