Effects of mixing and molecular packing on the isobaric expansivity of mixtures of n-hexane and 1-hexanol as modelled by FT-EoS, PC-SAFT and SAFT-[formula omitted] Mie

Abstract

The behaviour of the isobaric coefficient of thermal expansion in liquids under high pressures is known as a long-standing challenge for equations of state especially with respect to the prediction of the isotherms’ crossing detected for this derivative thermodynamic quantity. Being based on a kind of universality revealed recently as relating the location of the crossing curve and the molecular packing fraction for pure non-polar liquids, here we explore more complicated situation of mixtures of a polar and a non-polar compound using prototypic compounds as an example. It is demonstrated that the Fluctuation Theory-based equation of State (FT-EoS), which uses the data along the ambient pressure conditions as a reference, has advantages in the predictive ability over the considered SAFT models and proposed a new approach connecting the parameters of the FT-EoS and the spinodal representation of the isobaric expansivity. At the same time, all three approaches argue in favour of the thermodynamic definition of the free volume that allows a microscopic interpretation of inputs of free space gaps and polar associations in the free volume and the packing fraction of complex mixtures.