Basic Thermodynamic Principles
Fugacity and Fugacity Coefficient
Fugacity can be thought of as the corrected partial pressure (\(p_i=u_i\cdot p\) where \(u_i\) is either \(z_i\), \(x_i\) or \(y_i\)) of a component in a mixture where the fugacity coefficient is the correction term to the partial pressure for non-ideal conditions. Based on this description, you would expect the fugacity coefficient (\(\phi_i\)) to tend to 1 as the fluid acts ideal, as is the case.
Gibbs Free Energy
where \(u_i\) can be either the liquid molar composition (\(x_i\)) or the vapor molar composition (\(y_i\)).
Equal Chemical Potential Criteria
Equal Fugacity Criteria