Theoretical investigation of the phase behaviour of model ternary mixtures containing n-alkanes, perfluoro-n-alkanes, and perfluoroalkylalkane diblock surfactants
Résumé
We have used the hetero-SAFT-VR approach to investigate the phase equilibria of a number of binary and ternary mixtures of n -alkanes, perfluoro-n -alkanes, and perfluoroalkylalkane diblock surfactants. We focussed our work on the understanding of the microscopic conditions that control the phase behaviour of these mixtures, with a particular emphasis of the effect on the liquid-liquid separation and the stabilisation of n -alkane + perfluoro-n -alkane mixtures when a diblock surfactant is added. We selected the n -heptane + perfluoromethane binary mixture, and studied the changes on the phase behaviour when a symmetric (same number of alkyl and perfluoroalkyl chemical groups) or an asymmetric (different number of alkyl and perfluoroalkyl chemical groups) diblock surfactants is added to the binary mixture. We have obtained the phase diagrams of a wide range of binary and ternary mixtures at different thermodynamic conditions. We have found a variety of interesting behaviours as we modify the alkyl or/and the perfluoroalkyl chain-length of the diblock surfactants: the usual changes in the vapour-liquid phase separation, changes in the type of phase diagrams (typically from type I to type V phase behaviour according to the Scott and Konynenburg classification), azeotropy, and Bancroft points. We noted that the main effect of adding a symmetric or an asymmetric surfactant to the n -heptane + perfluoromethane mixture is to stabilise the system, i.e., to decrease the two-phase (liquid-liquid) immiscibility region of the ternary diagram as the surfactant concentration is increased.
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