Effect of liquid pressure on gas fugacity

If we vary the pressure of a liquid mixture at constant temperature and composition, there is a small effect on the fugacity of each volatile component in an equihbrated gas phase. One way to vary the pressure at essentially constant hquid composition is to change the partial pressure of a component of the gas phase that has negligible solubility in the liquid. 

Equation 12.8.1 shows that an increase in pressure, at constant temperature and liquid composition, causes an increase in the fugacity of each component in the gas phase. 

The exponential on the right side is called the Poynting factor. 

The integral in the Poynting factor is simplified if we make the approximation that Fi(l) is independent of pressure. Then we obtain the approximate relation 

As an application of these relations, consider the effect of the size of a liquid droplet on the equilibrium vapor pressure. The calculation of Prob. 12.8(b) shows that the fugacity of H2O in a gas phase equilibrated with liquid water in a small droplet is slightly greater than when the liquid is in a bulk phase. The smaller the radius of the droplet, the greater is the fugacity and the vapor pressure. 

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