Solutions – advanced and special cases - Nonideal mixtures
3 important questions on Solutions – advanced and special cases - Nonideal mixtures
Why can the gas phase, in general, be considered more often ideal than the liquid phase?
In a solution molecules are much closer to each other resulting in more interactions. More interactions means more substantial enthalpies of mixing.
How is the partition coefficient and the relative volatility calculated for a non ideal vapor-liquid equilibrium?
All Ki and αi have a composition dependence.
Considering a binary system in which the vapor phase acts ideal and the exes Gibbs free energy of the liquid phase can be described according to: 
what are K1 and K2?
To start with the expression of the total Gibbs free energy of the liquid is needed:
Then obtaining the chemical potential of species 1 in the liquid by taking the partial derivative of Gl:
Combining with the general non-ideal chemical potential expression:
Substituting the activity coefficient in the partition coefficient, and assuming that the fugacity coefficients are equal:
Integration to Nl2 results in:
Combining for the relative volatility:
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