A comprehensive description of chemical association effects on second derivative properties of alcohols through a SAFT-VR approach

A recently derived version of the statistical associating fluid theory (SAFT), denoted as SAFT-VR Mie, which incorporates the Mie potentials within the SAFT-VR framework to model the monomer segment interactions (Lafitte et al. J. Chem. Phys. 2006, 124, 024509), is used for the study of second-order derivative properties and phase equilibria of alcohols and 1-alcohol + n-alkane binary mixtures. For this purpose, a variable repulsive potential is used to induce nonconformal interactions in the reference nonbonded fluid. These features have a significant influence on the chain and association contributions through the contact value of the radial distribution function, and they enhance the SAFT theory performance in the application to associating substances. When dealing with pure alcohols and 1-alcohol + n-alkane binary mixtures, an accurate description of both phase equilibria and second-order derivatives is obtained with a single set of molecular parameters. To explore the predictive ability limit of the model we have particularly focused our attention on secondary derivative properties, which display singularities due to the formation of aggregates. With this approach, we have found that the model is able to reproduce accurately the complex behavior of the isobaric heat capacity of alcohols as, for instance, the maximum versus temperature in the compressed liquid region. Furthermore, in the case of 1-hexanol + n-hexane binary mixtures, the proposed equation is found to capture the association effects on the pressure and temperature dependence of the isobaric thermal expansivity. These two special features, which to our knowledge have never been described by a theoretical model, emphasize both the validity of the changes in the model proposed and the physical meaning of the molecular parameters obtained in this study.