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We have used the hetero-SAFT-VR approach developed by McCabe and collaborators [Mol. Phys. 104, 571 (2006)] to investigate the phase equilibria of a number of binary and ternary mixtures of n-alkanes, perfluoro- n-alkanes, and perfluoroalkylalkane diblock surfactants. We focused 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-alkaneþperfluoro-n-alkane mixtures when a diblock surfactant is added. We used very simple molecular models for n-alkanes, and perfluoro-n-alkanes that describe the molecules as chains with tangentially bonded segments with molecular parameters taken from the literature. In the particular case of semifluorinated alkanes or SFA surfactants, we used an hetero-segmented diblock chain model where the parameters for the alkyl and perfluoroalkyl segments taken from the corresponding linear alkanes and perfluoroalkanes, as shown in our previous work [J. Phys. Chem. B 111, 2856 (2007)]. Our goal was to identify the main effects on the phase behaviour when different perfluoroalkylalkane surfactants are added to mixtures of n-alkanes and perfluoro-n-alkanes. We selected the n-hepta- neþperfluoromethane 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 þ 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. This effect becomes larger as the chain length of the surfactant is increased, which is consistent with a higher number of alkyl–alkyl and perfluoroalkyl–perfluoroalkyl favourable interactions in the mixture. ; M.C.dR. acknowledges the Programme Alban from European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a Fellowship. Authors also acknowledge financial support from project number FIS2007-66079-C02-02 of the Spanish Dirección General de Investigación, and additional support from Universidad de Huelva and Junta de Andalucía. ; M.C.dR. acknowledges the Programme Al beta an from European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a Fellowship. Authors also acknowledge financial support from project number FIS2007-66079-C02-02 of the Spanish Direccion General de Investigacion, and additional support from Universidad de Huelva and Junta de Andalucia.

An extension of the SAFT-VR equation of state, the so-called hetero-SAFT approach [Y. Peng, H. Zhao, and C. McCabe, Molec. Phys. 104, 571 (2006)], is used to examine the phase equilibria exhibited by a number of model binary mixtures of n-alkanes, perfluoroalkanes and perfluoroalkylalkane diblock surfactants. Despite the increasing recent interest in semifluori- nated alkanes (or perfluoroalkylalkane diblock molecules), the phase behaviour of mixtures involving these molecules with n-alkanes or perfluoroalkanes is practically unknown from the experimental point of view. 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 stabilization of these mixtures. n-Alkanes and perfluoroalkanes are modelled as tangentially bonded monomer segments with molecular parameters taken from the literature. The perfluoroalkylalkane diblock molecules are modelled as heterosegmented diblock chains, with parameters for the alkyl and perfluoroalkyl segments developed in earlier work. This simple approach, which was proposed in previous work [P. Morgado, H. Zhao, F. J. Blas, C. McCabe, L. P. N. Rebelo, and E. J. M. Filipe, J. Phys. Chem. B, 111, 2856], is now extended to describe model n-alkane (or perfluoroalkane) þ perfluroalkylalkane binary mixtures. We have obtained the phase behaviour of different mixtures and studied the effect of the molecular weight of n-alkanes and perfluoroalkanes on the type of phase behaviour observed in these mixtures. We have also analysed the effect of the number of alkyl and perfluoroalkyl chemical groups in the surfactant molecule on the phase behaviour. In addition to 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 used the SAFT-VR microscopic description of chain-like systems to analyse the conditions that n-alkane (or perfluoroalkane) þ perfluoroalkyalkane mixtures should posses in order to exhibit complete liquid miscibility. Although the model proposed here is chosen to reproduce most of the quantitative features of the phase equilibria of some pure perfluoroalkylalkane diblock surfactants and their mixtures with n-alkanes, this is the first time the SAFT approach has been used to predict the phase behaviour of the mixtures considered here. The lack of experimental data for these systems does not allow us to test the accuracy of our theoretical predictions directly. However, since SAFT has proven to be an excellent approach for the prediction of the phase behaviour of complex mixtures, we expect that the theory will reproduce the most important qualitative trends exhibited by real mixtures. ; M.C.dR. acknowledges the programme Alßan of the European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a fellowship. The authors also acknowledge financial support from project number FIS2004-06627-C02-01 of the Spanish Dirección General de Investigación. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged. We also thank Clare McCabe and Eduardo J. M. Filipe for useful discussions.

The high-pressure phase diagram and excess thermodynamic properties of the binary mixture of carbon dioxide and water are examined using the statistical associating fluid theory for potentials of variable range (SAFT-VR). The carbon dioxide molecule is modelled with two tangentially bonded spherical segments, while the water molecule is modelled as spherical with four associating sites to represent the hydrogen bonding. Dispersion interactions are modelled using square-well potentials. The optimised intermolecular parameters are taken from the works of Galindo and Blas [F.J. Blas, A. Galindo, Fluid Phase Equilib. 194–197 (2002) 501–509; A. Galindo, F.J. Blas, J. Phys. Chem. B 106 (2002) 4503–4515] and Clark et al. [G.N.I. Clark, A.J. Haslam, A. Galindo, G. Jackson, Mol. Phys. 104 (2006) 3561–3581] for carbon dioxide and water, respectively. The mixture exhibits type III phase behaviour in the classification of Scott and van Konynenburg, with the gas–liquid critical line continuously changing into a liquid–liquid line at high pressures. In this work one unlike intermolecular interaction parameter is fitted to give the best possible representation of the minimum temperature of the gas–liquid critical line of the mixture, and is then used in a transferable manner to study other thermodynamic conditions and properties. The phase diagrams predicted by the SAFT-VR approach are found to be in very good agreement with the experimental data at low and high pressures and temperatures. In addition, a good qualitative description of the excess molar volume and excess enthalpy and different temperatures and pressures is obtained. ; M.C.dR. acknowledges the Programme Alβan from the European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a Fellowship. The authors also acknowledge financial support from project number FIS2004-06627-C02-01 of the Spanish Dirección General de Investigación. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged. We also thank Manuel M. Piñeiro and Eduardo J.M. Filipe for useful discussions concerning to the excess enthalpy experimental results.

In a recent generalisation of the SAFT-VR equation of state the method was extended so as to deal with wide square-well ranges, namely, 1.2 ≤ λ ≤ 3.0 [B. H. Patel, H. Docherty, S. Varga, A. Galindo, and G. C. Maitland. Mol. Phys., 103(1), 129–139, 2005.]. In this work, this equation is used to revisit the adjustment of intermolecular model parameters, with special emphasis on substances where the upper boundary of the potential range (λ = 1.8) has been previously reported or may be expected on grounds of the polar nature of the molecules. For this purpose, we follow the work of Clark et al. [G. N. I. Clark, A. J. Haslam, A. Galindo, and G. Jackson. Mol. Phys., 104(22-24), 3561–3581, 2006] and study a relative least squares objective function and the percentage absolute average deviation (D) to determine the intermolecular model parameters (m, λ, σ, ε/kB, εhb/kB and rc) by comparison to experimental vapour-pressure and saturated liquid density data. In order to ensure in each case that the global minimum is identified, the dimensionality of the problem is reduced by discretising the parameter-space. Applying this method to the study of argon, nitrogen, benzene, carbon dioxide, carbon monoxide, n-alkanes, the refrigerant R1270, water, hydrogen chloride and hydrogen bromide, we find that the optimal models always present square-well ranges λ < 1.8, meaning that an upper bound value of λ = 1.8 (as set in the original approach) for the square-well range is sufficient to model real fluids. Accurate intermolecular potential models with ranges higher than 1.8 are also identified, but we find that these do not usually correspond to the global minimum of the objective function considered. ; M.C.dR. acknowledges the Programme Alßan from European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a Fellowship. Authors also acknowledge financial support from project number FIS2007- 66079-C02-02 of the Spanish Dirección General de Investigación. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged.