Fugacities in Liquid Mixtures Activity Coefficients

For any component i in a liquid phase, the fugacity of f is most conveniently related to the mole fraction xt by use of the activity coefficient, y(, according to 

For liquid mixtures at low pressures, it is not important to specify with care the pressure of the standard state because at low pressures the thermodynamic properties of liquids, pure or mixed, are not sensitive to the pressure. However, at high pressures, liquid-phase properties are strong functions of pressure, and we cannot be careless about the pressure dependence of either the activity coefficient or the standard-state fugacity. 

The most frequently used standard state is the pure liquid (x = 1) at the system temperature and pressure. When this standard state is used for all components in the mixture, the activity coefficients are said to be symmetrically normalized, because in this case, for every component /, 

It is sometimes preferable to define the standard state as the pure liquid at the system temperature and at its own saturation pressure. For any component i for which this convention is used, the normalization is also given by Eq. (34). 

The difficulties engendered by a hypothetical liquid standard state can be eliminated by the use of unsymmetrically normalized activity coefficients. These have been used for many years in other areas of solution thermodynamics (e.g., for solutions of electrolytes or polymers in liquid solvents) but they have only recently been employed in high-pressure vapor-liquid equilibria (P7). 

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