Non-ideal Mixtures

Consider a multicomponent system in the gaseous phase. For low pressures, the gaseous mixture can be considered as ideal with a Clapeyron equation of state \(PV=N\mathscr {R}T\) . In this state, the mean free path is very high, and the intermolecular electrostatic potentials do not intercept, each component in the mixture behaving as an ideal gas. When the pressure increases or the volume decreases the molecules become closer and intermolecular attraction forces begin to be appreciable, deviating the whole system from the ideal gas behavior. In these conditions, the several components in the mixture may stand as a homogeneous system or segregate into separate phases. In this chapter, we deal with homogeneous mixtures, when the attraction forces between different molecules predominate over the attraction forces between identical molecules and there is no segregation. After exposing the theoretical fundamentals of multicomponent systems, the concepts of fugacity and activity are introduced and used for calculating the bubble and dew points of homogeneous liquid solutions in equilibrium with vapor. The functional group semi-empirical method of Prausnitz and collaborators is also exposed and used in the estimation of the activity coefficients.