Deviations from ideal-gas mixtures

We start with the KB result, equation (4.73) from chapter 4, 

When there are no interactions in the system (theoretical ideal gas), we must have 

the chemical potential in the ideal-gas reference state is 

The excess chemical potential and the corresponding activity coefficient are now defined by 

When we turn off all the interactions, all the three terms on the rhs of (6.6) become zero. Hence, /i X,IG measures the deviations of the chemical potential in a real system from the corresponding chemical potential in an ideal-gas system due to turning on all the interactions in the system. The same is true when pt — 0. 

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First-order deviations from ideal-gas mixtures... 

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