Theory of phase equilibria for model mixtures of n-alkanes, perfluoroalkanes and perfluoroalkylalkane diblock surfactants
Résumé
An extension of the SAFT-VR equation of state, the so-called hetero-SAFT approach is used to examine the phase equilibria exhibited by a number of model binary mixtures of n-alkanes, perfluoroalkanes and perfluoroalkylalkane diblock surfactants. In this work we use simple molecular models for n-alkanes, perfluoroalkanes and perfluoroalkylalkane diblock molecules to predict, from a molecular perspective, the phase behaviour of selected model mixtures of perfluoroalkylalkanes with n-alkanes and perfluoroalkanes. In particular, we focus our interest on the understanding of the microscopic conditions that control the liquid-liquid separation and the stabilisation of these mixtures. We have obtained the phase behaviour of different kind of mixtures and studied the effect of the molecular weight of n-alkanes and perfluoroalkanes on the type of phase behaviour observed in these mixtures. In addition, we have analyzed the effect of the number of the alkyl and perfluoroalkyl chemical groups in the surfactant molecule on the phase behaviour. As well as the usual vapour-liquid phase separation, liquid-liquid, positive azeotropes, and Bancroft points are found for different mixtures. This rich phase behaviour is a consequence of a delicate balance between the alkyl-alkyl, perfluoroalkyl-perfluoroalkyl, and alkyl-perfluoroalkyl interactions in different molecules. We have used the SAFT-VR microscopic description of chainlike systems to analyse the conditions that the n-alkane + and perfluoroalkane + perfluoroalkyalkane mixtures should posses in order to exhibit complete liquid miscibility.
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