excess free energy of solid solution

[Riche Ma]

Dear all,

If I understand it correctly from the literature, the excess free energy (\Delta G ^{E}) of a solid solution is defined by:

\Delta G ^{E}=x1x2*RT*(a0+a1(x1-x2))

where

x1 and x2 are the fractions of two components, x1=1-x2.

a0 and a1 are Guggenheim coefficients.

In Phreeqc, one defines a0 and a1 to define a non ideal solid solution.

My question is that whether \Delta G ^{E} implicitly includes the entropy of mixing, i.e. -TS^{mix}=-TR(x1lnx1+x2lnx2).

My gut feeling is that it does not include it, but I cannot find anything where it is explicitly explained.

Can anyone help me with it? i.e., whether \Delta G ^{E} and -TS^{mix} are two separate free energy contributions to a solid solution, or \Delta G ^{E} is the only one which is considered in Phreeqc simulations.

Thank you very much in advance!

Riche Ma