Effect of Pressure on Activity Coefficients Partial Molar Volumes

Effect of Pressure on Activity Coefficients Partial Molar Volumes 

In Section I, we indicated that significant progress in understanding high-pressure thermodynamics of mixtures requires a quantitative description of the variation of fugacity with pressure as given by Eq. (3). To obtain the effect of pressure on activity coefficient we substitute as follows  

If we define the standard-state fugacity f° at a fixed pressure, then the second term on the left side of Eq. (50) vanishes and we obtain Eq. (42). However, if we define f° at the total pressure of the system, we obtain Eq. (43). 

Equations (42), (43), and (50) are independent of the convention used for the normalization of activity coefficients they apply equally to yt and to y (see Section III). 

It is difficult to measure partial molar volumes, and, unfortunately, many experimental studies of high-pressure vapor-liquid equilibria report no volumetric data at all more often than not, experimental measurements are confined to total pressure, temperature, and phase compositions. Even in those cases where liquid densities are measured along the saturation curve, there is a fundamental difficulty in calculating partial molar volumes as indicated by 

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IV. Effect of Pressure on Activity Coefficients Partial Molar Volumes. 160... 

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